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About this Specification
This document at https://tc39.es/ecma262/ is the most
accurate up-to-date ECMAScript specification. It contains the content of
the most recent yearly snapshot plus any
finished proposals
(those that have reached Stage 4 in the
proposal process and
thus are implemented in several implementations and will be in the next
practical revision) since that snapshot was taken.
Introduction
This Ecma Standard defines the ECMAScript 2020 Language. It is the
eleventh edition of the ECMAScript Language Specification. Since
publication of the first edition in 1997, ECMAScript has grown to be one
of the world's most widely used general-purpose programming languages.
It is best known as the language embedded in web browsers but has also
been widely adopted for server and embedded applications.
ECMAScript is based on several originating technologies, the most
well-known being JavaScript (Netscape) and JScript (Microsoft). The
language was invented by Brendan Eich at Netscape and first appeared in
that company's Navigator 2.0 browser. It has appeared in all subsequent
browsers from Netscape and in all browsers from Microsoft starting with
Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in
November 1996. The first edition of this Ecma Standard was adopted by
the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the
fast-track procedure, and approved as international standard ISO/IEC
16262, in April 1998. The Ecma General Assembly of June 1998 approved
the second edition of ECMA-262 to keep it fully aligned with ISO/IEC
16262. Changes between the first and the second edition are editorial in
nature.
The third edition of the Standard introduced powerful regular
expressions, better string handling, new control statements, try/catch
exception handling, tighter definition of errors, formatting for numeric
output and minor changes in anticipation of future language growth. The
third edition of the ECMAScript standard was adopted by the Ecma General
Assembly of December 1999 and published as ISO/IEC 16262:2002 in June
2002.
After publication of the third edition, ECMAScript achieved massive
adoption in conjunction with the World Wide Web where it has become the
programming language that is supported by essentially all web browsers.
Significant work was done to develop a fourth edition of ECMAScript.
However, that work was not completed and not published as the fourth
edition of ECMAScript but some of it was incorporated into the
development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5th
edition) codified de facto interpretations of the language specification
that have become common among browser implementations and added support
for new features that had emerged since the publication of the third
edition. Such features include accessor properties, reflective creation
and inspection of objects, program control of property attributes,
additional array manipulation functions, support for the JSON object
encoding format, and a strict mode that provides enhanced error checking
and program security. The fifth edition was adopted by the Ecma General
Assembly of December 2009.
The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the
fast-track procedure, and approved as international standard ISO/IEC
16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor
corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition
was adopted by the Ecma General Assembly of June 2011.
Focused development of the sixth edition started in 2009, as the fifth
edition was being prepared for publication. However, this was preceded
by significant experimentation and language enhancement design efforts
dating to the publication of the third edition in 1999. In a very real
sense, the completion of the sixth edition is the culmination of a
fifteen year effort. The goals for this edition included providing
better support for large applications, library creation, and for use of
ECMAScript as a compilation target for other languages. Some of its
major enhancements included modules, class declarations, lexical block
scoping, iterators and generators, promises for asynchronous
programming, destructuring patterns, and proper tail calls. The
ECMAScript library of built-ins was expanded to support additional data
abstractions including maps, sets, and arrays of binary numeric values
as well as additional support for Unicode supplemental characters in
strings and regular expressions. The built-ins were also made extensible
via subclassing. The sixth edition provides the foundation for regular,
incremental language and library enhancements. The sixth edition was
adopted by the General Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under Ecma
TC39's new yearly release cadence and open development process. A
plain-text source document was built from the ECMAScript 2015 source
document to serve as the base for further development entirely on
GitHub. Over the year of this standard's development, hundreds of pull
requests and issues were filed representing thousands of bug fixes,
editorial fixes and other improvements. Additionally, numerous software
tools were developed to aid in this effort including Ecmarkup,
Ecmarkdown, and Grammarkdown. ES2016 also included support for a new
exponentiation operator and adds a new method to
Array.prototype called includes.
ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics
along with smaller language and library enhancements, bug fixes, and
editorial updates. Async functions improve the asynchronous programming
experience by providing syntax for promise-returning functions. Shared
Memory and Atomics introduce a new
memory model
that allows multi-agent
programs to communicate using atomic operations that ensure a
well-defined execution order even on parallel CPUs. It also included new
static methods on Object: Object.values,
Object.entries, and
Object.getOwnPropertyDescriptors.
ECMAScript 2018 introduced support for asynchronous iteration via the
AsyncIterator protocol and async generators. It also included four new
regular expression features: the dotAll flag, named capture
groups, Unicode property escapes, and look-behind assertions. Lastly it
included object rest and spread properties.
ECMAScript 2019 introduced a few new built-in functions:
flat and flatMap on
Array.prototype for flattening arrays,
Object.fromEntries for directly turning the return value of
Object.entries into a new Object, and
trimStart and trimEnd on
String.prototype as better-named alternatives to the widely
implemented but non-standard String.prototype.trimLeft and
trimRight built-ins. In addition, it included a few minor
updates to syntax and semantics. Updated syntax included optional catch
binding parameters and allowing U+2028 (LINE SEPARATOR) and U+2029
(PARAGRAPH SEPARATOR) in string literals to align with JSON. Other
updates included requiring that Array.prototype.sort be a
stable sort, requiring that JSON.stringify return
well-formed UTF-8 regardless of input, and clarifying
Function.prototype.toString by requiring that it either
return the corresponding original source text or a standard placeholder.
Dozens of individuals representing many organizations have made very
significant contributions within Ecma TC39 to the development of this
edition and to the prior editions. In addition, a vibrant community has
emerged supporting TC39's ECMAScript efforts. This community has
reviewed numerous drafts, filed thousands of bug reports, performed
implementation experiments, contributed test suites, and educated the
world-wide developer community about ECMAScript. Unfortunately, it is
impossible to identify and acknowledge every person and organization who
has contributed to this effort.
Allen Wirfs-Brock
ECMA-262, Project Editor, 6th Edition
Brian Terlson
ECMA-262, Project Editor, 7th through 10th
Editions
1Scope
This Standard defines the ECMAScript 2020 general-purpose programming
language.
2Conformance
A conforming implementation of ECMAScript must provide and support all
the types, values, objects, properties, functions, and program syntax
and semantics described in this specification.
A conforming implementation of ECMAScript must interpret source text
input in conformance with the latest version of the Unicode Standard and
ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an application
programming interface (API) that supports programs that need to adapt to
the linguistic and cultural conventions used by different human
languages and countries must implement the interface defined by the most
recent edition of ECMA-402 that is compatible with this specification.
A conforming implementation of ECMAScript may provide additional types,
values, objects, properties, and functions beyond those described in
this specification. In particular, a conforming implementation of
ECMAScript may provide properties not described in this specification,
and values for those properties, for objects that are described in this
specification.
A conforming implementation of ECMAScript may support program and
regular expression syntax not described in this specification. In
particular, a conforming implementation of ECMAScript may support
program syntax that makes use of any “future reserved words” noted in
subclause
11.6.2
of this specification.
A conforming implementation of ECMAScript must not implement any
extension that is listed as a Forbidden Extension in subclause
16.2.
3Normative References
The following referenced documents are indispensable for the application
of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document
(including any amendments) applies.
ISO/IEC 10646
Information Technology — Universal Multiple-Octet Coded Character Set
(UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and
Amendment 4:2008, plus additional amendments and corrigenda, or successor
This section contains a non-normative overview of the ECMAScript
language.
ECMAScript is an object-oriented programming language for performing
computations and manipulating computational objects within a host
environment. ECMAScript as defined here is not intended to be
computationally self-sufficient; indeed, there are no provisions in this
specification for input of external data or output of computed results.
Instead, it is expected that the computational environment of an
ECMAScript program will provide not only the objects and other
facilities described in this specification but also certain
environment-specific objects, whose description and behaviour are beyond
the scope of this specification except to indicate that they may provide
certain properties that can be accessed and certain functions that can
be called from an ECMAScript program.
ECMAScript was originally designed to be used as a scripting language,
but has become widely used as a general-purpose programming language. A
scripting language is a programming language that is used to
manipulate, customize, and automate the facilities of an existing
system. In such systems, useful functionality is already available
through a user interface, and the scripting language is a mechanism for
exposing that functionality to program control. In this way, the
existing system is said to provide a host environment of objects and
facilities, which completes the capabilities of the scripting language.
A scripting language is intended for use by both professional and
non-professional programmers.
ECMAScript was originally designed to be a
Web scripting language, providing a mechanism to enliven Web
pages in browsers and to perform server computation as part of a
Web-based client-server architecture. ECMAScript is now used to provide
core scripting capabilities for a variety of host environments.
Therefore the core language is specified in this document apart from any
particular host environment.
ECMAScript usage has moved beyond simple scripting and it is now used
for the full spectrum of programming tasks in many different
environments and scales. As the usage of ECMAScript has expanded, so
have the features and facilities it provides. ECMAScript is now a fully
featured general-purpose programming language.
Some of the facilities of ECMAScript are similar to those used in other
programming languages; in particular C, Java™, Self, and Scheme as
described in:
ISO/IEC 9899:1996, Programming Languages — C.
Gosling, James, Bill Joy and Guy Steele.
The Java™ Language Specification. Addison Wesley
Publishing Co., 1996.
Ungar, David, and Smith, Randall B. Self: The Power of Simplicity.
OOPSLA '87 Conference Proceedings, pp. 227-241, Orlando, FL,
October 1987.
IEEE Standard for the Scheme Programming Language. IEEE Std
1178-1990.
4.1Web Scripting
A web browser provides an ECMAScript host environment for client-side
computation including, for instance, objects that represent windows,
menus, pop-ups, dialog boxes, text areas, anchors, frames, history,
cookies, and input/output. Further, the host environment provides a
means to attach scripting code to events such as change of focus, page
and image loading, unloading, error and abort, selection, form
submission, and mouse actions. Scripting code appears within the HTML
and the displayed page is a combination of user interface elements and
fixed and computed text and images. The scripting code is reactive to
user interaction, and there is no need for a main program.
A web server provides a different host environment for server-side
computation including objects representing requests, clients, and
files; and mechanisms to lock and share data. By using browser-side
and server-side scripting together, it is possible to distribute
computation between the client and server while providing a customized
user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its own
host environment, completing the ECMAScript execution environment.
4.2ECMAScript Overview
The following is an informal overview of ECMAScript—not all parts of
the language are described. This overview is not part of the standard
proper.
ECMAScript is object-based: basic language and host facilities are
provided by objects, and an ECMAScript program is a cluster of
communicating objects. In ECMAScript, an object is a
collection of zero or more properties each with
attributes that determine how each property can be used—for
example, when the Writable attribute for a property is set to
false, any attempt by executed ECMAScript code to
assign a different value to the property fails. Properties are
containers that hold other objects, primitive values, or
functions. A primitive value is a member of one of the
following built-in types: Undefined, Null,
Boolean, Number, BigInt, String, and
Symbol; an object is a member of the built-in type
Object; and a function is a callable object. A function that is
associated with an object via a property is called a method.
ECMAScript defines a collection of built-in objects that
round out the definition of ECMAScript entities. These built-in
objects include the
global object; objects that are fundamental to the
runtime semantics
of the language including Object, Function,
Boolean, Symbol, and various
Error objects; objects that represent and manipulate
numeric values including Math, Number, and
Date; the text processing objects String and
RegExp; objects that are indexed collections of values
including Array and nine different kinds of Typed Arrays
whose elements all have a specific numeric data representation; keyed
collections including Map and Set objects;
objects supporting structured data including the
JSON object, ArrayBuffer,
SharedArrayBuffer, and DataView; objects
supporting control abstractions including generator functions and
Promise objects; and reflection objects including
Proxy and Reflect.
ECMAScript also defines a set of built-in operators.
ECMAScript operators include various unary operations, multiplicative
operators, additive operators, bitwise shift operators, relational
operators, equality operators, binary bitwise operators, binary
logical operators, assignment operators, and the comma operator.
Large ECMAScript programs are supported by modules which
allow a program to be divided into multiple sequences of statements
and declarations. Each module explicitly identifies declarations it
uses that need to be provided by other modules and which of its
declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript
syntax is relaxed to enable it to serve as an easy-to-use scripting
language. For example, a variable is not required to have its type
declared nor are types associated with properties, and defined
functions are not required to have their declarations appear textually
before calls to them.
4.2.1Objects
Even though ECMAScript includes syntax for class definitions,
ECMAScript objects are not fundamentally class-based such as those
in C++, Smalltalk, or Java. Instead objects may be created in
various ways including via a literal notation or via
constructors which create objects and then execute code
that initializes all or part of them by assigning initial values to
their properties. Each
constructor
is a function that has a property named
"prototype" that is used to implement
prototype-based inheritance and shared properties.
Objects are created by using constructors in new expressions;
for example, new Date(2009, 11) creates a new Date
object. Invoking a
constructor
without using new has consequences that depend on the
constructor. For example, Date() produces a string representation
of the current date and time rather than an object.
Every object created by a
constructor
has an implicit reference (called the object's prototype)
to the value of its
constructor's "prototype" property. Furthermore, a
prototype may have a non-null implicit reference to its prototype,
and so on; this is called the prototype chain. When a
reference is made to a property in an object, that reference is to
the property of that name in the first object in the prototype chain
that contains a property of that name. In other words, first the
object mentioned directly is examined for such a property; if that
object contains the named property, that is the property to which
the reference refers; if that object does not contain the named
property, the prototype for that object is examined next; and so on.
In a class-based object-oriented language, in general, state is
carried by instances, methods are carried by classes, and
inheritance is only of structure and behaviour. In ECMAScript, the
state and methods are carried by objects, while structure,
behaviour, and state are all inherited.
All objects that do not directly contain a particular property that
their prototype contains share that property and its value. Figure 1
illustrates this:
CF is a
constructor
(and also an object). Five objects have been created by using
new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named
"q1" and "q2". The dashed
lines represent the implicit prototype relationship; so, for
example, cf3's prototype is CFp. The
constructor, CF, has two properties itself, named
"P1" and "P2", which are not
visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named "CFP1" in
CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any
properties found in CFp's implicit prototype chain that are not named
"q1", "q2", or
"CFP1". Notice that there is no implicit
prototype link between CF and CFp.
Unlike most class-based object languages, properties can be added to
objects dynamically by assigning values to them. That is,
constructors are not required to name or assign values to all or any
of the constructed object's properties. In the above diagram, one
could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the
property in CFp.
Although ECMAScript objects are not inherently class-based, it is
often convenient to define class-like abstractions based upon a
common pattern of
constructor
functions, prototype objects, and methods. The ECMAScript built-in
objects themselves follow such a class-like pattern. Beginning with
ECMAScript 2015, the ECMAScript language includes syntactic class
definitions that permit programmers to concisely define objects that
conform to the same class-like abstraction pattern used by the
built-in objects.
4.2.2The Strict Variant of ECMAScript
The ECMAScript Language recognizes the possibility that some users
of the language may wish to restrict their usage of some features
available in the language. They might do so in the interests of
security, to avoid what they consider to be error-prone features, to
get enhanced error checking, or for other reasons of their choosing.
In support of this possibility, ECMAScript defines a strict variant
of the language. The strict variant of the language excludes some
specific syntactic and semantic features of the regular ECMAScript
language and modifies the detailed semantics of some features. The
strict variant also specifies additional error conditions that must
be reported by throwing error exceptions in situations that are not
specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the
strict mode of the language. Strict mode selection and use
of the strict mode syntax and semantics of ECMAScript is explicitly
made at the level of individual ECMAScript source text units.
Because strict mode is selected at the level of a syntactic source
text unit, strict mode only imposes restrictions that have local
effect within such a source text unit. Strict mode does not restrict
or modify any aspect of the ECMAScript semantics that must operate
consistently across multiple source text units. A complete
ECMAScript program may be composed of both strict mode and
non-strict mode ECMAScript source text units. In this case, strict
mode only applies when actually executing code that is defined
within a strict mode source text unit.
In order to conform to this specification, an ECMAScript
implementation must implement both the full unrestricted ECMAScript
language and the strict variant of the ECMAScript language as
defined by this specification. In addition, an implementation must
support the combination of unrestricted and strict mode source text
units into a single composite program.
4.3Terms and Definitions
For the purposes of this document, the following terms and definitions
apply.
4.3.1type
set of data values as defined in clause
6
of this specification
4.3.2primitive value
member of one of the types Undefined, Null, Boolean, Number, BigInt,
Symbol, or String as defined in clause
6
Note
A primitive value is a datum that is represented directly at the
lowest level of the language implementation.
4.3.3object
member of the type Object
Note
An object is a collection of properties and has a single
prototype object. The prototype may be the null value.
The value of a
constructor's "prototype" property is a prototype
object that is used to implement inheritance and shared
properties.
4.3.5prototype
object that provides shared properties for other objects
Note
When a
constructor
creates an object, that object implicitly references the
constructor's "prototype" property for the purpose of
resolving property references. The
constructor's "prototype" property can be referenced by
the program expression
constructor.prototype, and properties
added to an object's prototype are shared, through inheritance,
by all objects sharing the prototype. Alternatively, a new
object may be created with an explicitly specified prototype by
using the Object.create built-in function.
4.3.6ordinary object
object that has the default behaviour for the essential internal
methods that must be supported by all objects
4.3.7exotic object
object that does not have the default behaviour for one or more of
the essential internal methods
Note
Any object that is not an ordinary object is an
exotic object.
4.3.8standard object
object whose semantics are defined by this specification
4.3.9built-in object
object specified and supplied by an ECMAScript implementation
Note
Standard built-in objects are defined in this specification. An
ECMAScript implementation may specify and supply additional
kinds of built-in objects. A
built-in
constructor
is a built-in object that is also a
constructor.
4.3.10undefined value
primitive value used when a variable has not been assigned a value
4.3.11Undefined type
type whose sole value is the undefined value
4.3.12null value
primitive value that represents the intentional absence of any
object value
4.3.13Null type
type whose sole value is the null value
4.3.14Boolean value
member of the Boolean type
Note
There are only two Boolean values, true and
false.
4.3.15Boolean type
type consisting of the primitive values true and
false
4.3.16Boolean object
member of the Object type that is an instance of the standard
built-in Booleanconstructor
Note
A Boolean object is created by using the Booleanconstructor
in a new expression, supplying a Boolean value as
an argument. The resulting object has an internal slot whose
value is the Boolean value. A Boolean object can be coerced to a
Boolean value.
4.3.17String value
primitive value that is a finite ordered sequence of zero or more
16-bit unsigned
integer
values
Note
A String value is a member of the String type. Each
integer
value in the sequence usually represents a single 16-bit unit of
UTF-16 text. However, ECMAScript does not place any restrictions
or requirements on the values except that they must be 16-bit
unsigned integers.
4.3.18String type
set of all possible String values
4.3.19String object
member of the Object type that is an instance of the standard
built-in Stringconstructor
Note
A String object is created by using the Stringconstructor
in a new expression, supplying a String value as an
argument. The resulting object has an internal slot whose value
is the String value. A String object can be coerced to a String
value by calling the Stringconstructor
as a function (21.1.1.1).
4.3.20Number value
primitive value corresponding to a double-precision 64-bit binary
format IEEE 754-2008 value
Note
A
Number value
is a member of the Number type and is a direct representation of
a number.
4.3.21Number type
set of all possible Number values including the special
“Not-a-Number” (NaN) value, positive infinity, and negative infinity
4.3.22Number object
member of the Object type that is an instance of the standard
built-in Numberconstructor
Note
A Number object is created by using the Numberconstructor
in a new expression, supplying a number value as an
argument. The resulting object has an internal slot whose value
is the number value. A Number object can be coerced to a number
value by calling the Numberconstructor
as a function (20.1.1.1).
4.3.23Infinity
number value that is the positive infinite number value
4.3.24NaN
number value that is an IEEE 754-2008 “Not-a-Number” value
4.3.25BigInt value
primitive value corresponding to an arbitrary-precision
integer
value
4.3.26BigInt type
set of all possible BigInt values
4.3.27BigInt object
member of the Object type that is an instance of the standard
built-in BigIntconstructor
4.3.28Symbol value
primitive value that represents a unique, non-String Object property
key
4.3.29Symbol type
set of all possible Symbol values
4.3.30Symbol object
member of the Object type that is an instance of the standard
built-in Symbolconstructor
4.3.31function
member of the Object type that may be invoked as a subroutine
Note
In addition to its properties, a function contains executable
code and state that determine how it behaves when invoked. A
function's code may or may not be written in ECMAScript.
4.3.32built-in function
built-in object that is a function
Note
Examples of built-in functions include parseInt and
Math.exp. An implementation may provide
implementation-dependent built-in functions that are not
described in this specification.
4.3.33property
part of an object that associates a key (either a String value or a
Symbol value) and a value
Note
Depending upon the form of the property the value may be
represented either directly as a data value (a primitive value,
an object, or a
function object) or indirectly by a pair of accessor functions.
4.3.34method
function that is the value of a property
Note
When a function is called as a method of an object, the object
is passed to the function as its this value.
4.3.35built-in method
method that is a built-in function
Note
Standard built-in methods are defined in this specification, and
an ECMAScript implementation may specify and provide other
additional built-in methods.
4.3.36attribute
internal value that defines some characteristic of a property
4.3.37own property
property that is directly contained by its object
4.3.38inherited property
property of an object that is not an own property but is a property
(either own or inherited) of the object's prototype
4.4Organization of This Specification
The remainder of this specification is organized as follows:
Clause 5 defines the notational conventions used throughout the
specification.
Clauses 6-9 define the execution environment within which ECMAScript
programs operate.
Clauses 10-16 define the actual ECMAScript programming language
including its syntactic encoding and the execution semantics of all
language features.
Clauses 17-26 define the ECMAScript standard library. They include the
definitions of all of the standard objects that are available for use
by ECMAScript programs as they execute.
Clause 27 describes the memory consistency model of accesses on
SharedArrayBuffer-backed memory and methods of the Atomics object.
5Notational Conventions
5.1Syntactic and Lexical Grammars
5.1.1Context-Free Grammars
A context-free grammar consists of a number of
productions. Each production has an abstract symbol called
a nonterminal as its left-hand side, and a
sequence of zero or more nonterminal and terminal symbols
as its right-hand side. For each grammar, the terminal
symbols are drawn from a specified alphabet.
A chain production is a production that has exactly one
nonterminal symbol on its right-hand side along with zero or more
terminal symbols.
Starting from a sentence consisting of a single distinguished
nonterminal, called the goal symbol, a given context-free
grammar specifies a language, namely, the (perhaps
infinite) set of possible sequences of terminal symbols that can
result from repeatedly replacing any nonterminal in the sequence
with a right-hand side of a production for which the nonterminal is
the left-hand side.
Input elements other than white space and comments form the terminal
symbols for the syntactic grammar for ECMAScript and are called
ECMAScript tokens. These tokens are the reserved words,
identifiers, literals, and punctuators of the ECMAScript language.
Moreover, line terminators, although not considered to be tokens,
also become part of the stream of input elements and guide the
process of automatic semicolon insertion (11.9). Simple white space and single-line comments are discarded and do
not appear in the stream of input elements for the syntactic
grammar. A
MultiLineComment
(that is, a comment of the form /*…*/
regardless of whether it spans more than one line) is likewise
simply discarded if it contains no line terminator; but if a
MultiLineComment
contains one or more line terminators, then it is replaced by a
single line terminator, which becomes part of the stream of input
elements for the syntactic grammar.
A RegExp grammar for ECMAScript is given in
21.2.1. This grammar also has as its terminal symbols the code points as
defined by
SourceCharacter. It defines a set of productions, starting from the
goal symbolPattern, that describe how sequences of code points are translated into
regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by
having two colons “::” as separating punctuation. The lexical
and RegExp grammars share some productions.
5.1.3The Numeric String Grammar
Another grammar is used for translating Strings into numeric values.
This grammar is similar to the part of the lexical grammar having to
do with numeric literals and has as its terminal symbols
SourceCharacter. This grammar appears in
7.1.4.1.
Productions of the numeric string grammar are distinguished by
having three colons “:::” as punctuation.
5.1.4The Syntactic Grammar
The syntactic grammar for ECMAScript is given in clauses
11, 12, 13, 14, and 15. This grammar has ECMAScript tokens defined
by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from two alternative
goal symbols
Script
and Module, that describe how sequences of tokens form syntactically correct
independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScript
Script or
Module, it is first converted to a stream of input elements by repeated
application of the lexical grammar; this stream of input elements is
then parsed by a single application of the syntactic grammar. The
input stream is syntactically in error if the tokens in the stream
of input elements cannot be parsed as a single instance of the goal
nonterminal (Script
or Module), with no tokens left over.
When a parse is successful, it constructs a parse tree, a
rooted tree structure in which each node is a Parse Node.
Each Parse Node is an instance of a symbol in the grammar;
it represents a span of the source text that can be derived from
that symbol. The root node of the parse tree, representing the whole
of the source text, is an instance of the parse's
goal symbol. When a Parse Node is an instance of a nonterminal, it is also an
instance of some production that has that nonterminal as its
left-hand side. Moreover, it has zero or more children, one
for each symbol on the production's right-hand side: each child is a
Parse Node that is an instance of the corresponding symbol.
New Parse Nodes are instantiated for each invocation of the parser
and never reused between parses even of identical source text. Parse
Nodes are considered the same Parse Node if and only if
they represent the same span of source text, are instances of the
same grammar symbol, and resulted from the same parser invocation.
Note 1
Parsing the same String multiple times will lead to different
Parse Nodes. For example, consider:
let str = "1 + 1;";
eval(str);
eval(str);
Each call to eval converts the value of
str into an ECMAScript source text and performs an
independent parse that creates its own separate tree of Parse
Nodes. The trees are distinct even though each parse operates
upon a source text that was derived from the same String value.
Note 2
Parse Nodes are specification artefacts, and implementations are
not required to use an analogous data structure.
Productions of the syntactic grammar are distinguished by having
just one colon “:” as punctuation.
The syntactic grammar as presented in clauses 12, 13, 14 and 15 is
not a complete account of which token sequences are accepted as a
correct ECMAScript
Script or
Module. Certain additional token sequences are also accepted, namely,
those that would be described by the grammar if only semicolons were
added to the sequence in certain places (such as before line
terminator characters). Furthermore, certain token sequences that
are described by the grammar are not considered acceptable if a line
terminator character appears in certain “awkward” places.
In certain cases, in order to avoid ambiguities, the syntactic
grammar uses generalized productions that permit token sequences
that do not form a valid ECMAScript
Script or
Module. For example, this technique is used for object literals and
object destructuring patterns. In such cases a more restrictive
supplemental grammar is provided that further restricts the
acceptable token sequences. Typically, an
early error
rule will then define an error condition if "P is not
covering an N", where P is a Parse
Node (an instance of the generalized production) and N is
a nonterminal from the supplemental grammar. Here, the sequence of
tokens originally matched by P is parsed again using
N as the
goal symbol. (If N takes grammatical parameters, then they are set
to the same values used when P was originally parsed.) An
error occurs if the sequence of tokens cannot be parsed as a single
instance of N, with no tokens left over. Subsequently,
algorithms access the result of the parse using a phrase of the form
"the N that is covered by P". This
will always be a Parse Node (an instance of N, unique for
a given P), since any parsing failure would have been
detected by an
early error
rule.
5.1.5Grammar Notation
Terminal symbols are shown in fixed width font, both in
the productions of the grammars and throughout this specification
whenever the text directly refers to such a terminal symbol. These
are to appear in a script exactly as written. All terminal symbol
code points specified in this way are to be understood as the
appropriate Unicode code points from the Basic Latin range, as
opposed to any similar-looking code points from other Unicode
ranges. A code point in a terminal symbol cannot be expressed by a
\UnicodeEscapeSequence.
Nonterminal symbols are shown in italic type. The definition
of a nonterminal (also called a “production”) is introduced by the
name of the nonterminal being defined followed by one or more
colons. (The number of colons indicates to which grammar the
production belongs.) One or more alternative right-hand sides for
the nonterminal then follow on succeeding lines. For example, the
syntactic definition:
states that the nonterminal
WhileStatement
represents the token while, followed by a left
parenthesis token, followed by an
Expression, followed by a right parenthesis token, followed by a
Statement. The occurrences of
Expression
and
Statement
are themselves nonterminals. As another example, the syntactic
definition:
states that an
ArgumentList
may represent either a single
AssignmentExpression
or an
ArgumentList, followed by a comma, followed by an
AssignmentExpression. This definition of
ArgumentList
is recursive, that is, it is defined in terms of itself. The result
is that an
ArgumentList
may contain any positive number of arguments, separated by commas,
where each argument expression is an
AssignmentExpression. Such recursive definitions of nonterminals are common.
The subscripted suffix “opt”, which may appear after a
terminal or nonterminal, indicates an optional symbol. The
alternative containing the optional symbol actually specifies two
right-hand sides, one that omits the optional element and one that
includes it. This means that:
so, in this example, the nonterminal
IterationStatement
actually has four alternative right-hand sides.
A production may be parameterized by a subscripted annotation of the
form “[parameters]”, which may appear as a suffix to the
nonterminal symbol defined by the production.
“parameters” may be either a single name or a comma
separated list of names. A parameterized production is shorthand for
a set of productions defining all combinations of the parameter
names, preceded by an underscore, appended to the parameterized
nonterminal symbol. This means that:
Prefixing a parameter name with “?” on a right-hand side
nonterminal reference makes that parameter value dependent upon the
occurrence of the parameter name on the reference to the current
production's left-hand side symbol. For example:
If a right-hand side alternative is prefixed with “[+parameter]”
that alternative is only available if the named parameter was used
in referencing the production's nonterminal symbol. If a right-hand
side alternative is prefixed with “[~parameter]” that alternative is
only available if the named parameter was not used in
referencing the production's nonterminal symbol. This means that:
When the words “one of” follow the colon(s) in a grammar
definition, they signify that each of the terminal symbols on the
following line or lines is an alternative definition. For example,
the lexical grammar for ECMAScript contains the production:
If the phrase “[empty]” appears as the right-hand side of a
production, it indicates that the production's right-hand side
contains no terminals or nonterminals.
If the phrase “[lookahead ∉ set]” appears in the
right-hand side of a production, it indicates that the production
may not be used if the immediately following input token sequence is
a member of the given set. The set can be
written as a comma separated list of one or two element terminal
sequences enclosed in curly brackets. For convenience, the set can
also be written as a nonterminal, in which case it represents the
set of all terminals to which that nonterminal could expand. If the
set consists of a single terminal the phrase “[lookahead
≠ terminal]” may be used.
matches either the letter n followed by one or more
decimal digits the first of which is even, or a decimal digit not
followed by another decimal digit.
Similarly, if the phrase “[lookahead ∈ set]” appears in
the right-hand side of a production, it indicates that the
production may only be used if the immediately following input token
sequence is a member of the given set. If the
set consists of a single terminal the phrase “[lookahead
= terminal]” may be used.
If the phrase “[no
LineTerminator
here]” appears in the right-hand side of a production of the
syntactic grammar, it indicates that the production is
a restricted production: it may not be used if a
LineTerminator
occurs in the input stream at the indicated position. For example,
the production:
indicates that the production may not be used if a
LineTerminator
occurs in the script between the throw token and the
Expression.
Unless the presence of a
LineTerminator
is forbidden by a restricted production, any number of occurrences
of
LineTerminator
may appear between any two consecutive tokens in the stream of input
elements without affecting the syntactic acceptability of the
script.
When an alternative in a production of the lexical grammar or the
numeric string grammar appears to be a multi-code point token, it
represents the sequence of code points that would make up such a
token.
The right-hand side of a production may specify that certain
expansions are not permitted by using the phrase “but not”
and then indicating the expansions to be excluded. For example, the
production:
means that the nonterminal
Identifier
may be replaced by any sequence of code points that could replace
IdentifierName
provided that the same sequence of code points could not replace
ReservedWord.
Finally, a few nonterminal symbols are described by a descriptive
phrase in sans-serif type in cases where it would be impractical to
list all the alternatives:
The specification often uses a numbered list to specify steps in an
algorithm. These algorithms are used to precisely specify the required
semantics of ECMAScript language constructs. The algorithms are not
intended to imply the use of any specific implementation technique. In
practice, there may be more efficient algorithms available to
implement a given feature.
Algorithms may be explicitly parameterized, in which case the names
and usage of the parameters must be provided as part of the
algorithm's definition.
Algorithm steps may be subdivided into sequential substeps. Substeps
are indented and may themselves be further divided into indented
substeps. Outline numbering conventions are used to identify substeps
with the first level of substeps labelled with lower case alphabetic
characters and the second level of substeps labelled with lower case
roman numerals. If more than three levels are required these rules
repeat with the fourth level using numeric labels. For example:
Top-level step
Substep.
Substep.
Subsubstep.
Subsubsubstep
Subsubsubsubstep
Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that conditions
its substeps. In this case, the substeps are only applied if the
predicate is true. If a step or substep begins with the word “else”,
it is a predicate that is the negation of the preceding “if” predicate
step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an
invariant condition of its algorithm. Such assertions are used to make
explicit algorithmic invariants that would otherwise be implicit. Such
assertions add no additional semantic requirements and hence need not
be checked by an implementation. They are used simply to clarify
algorithms.
Algorithm steps may declare named aliases for any value using the form
“Let x be someValue”. These aliases are
reference-like in that both x and
someValue refer to the same underlying data and
modifications to either are visible to both. Algorithm steps that want
to avoid this reference-like behaviour should explicitly make a copy
of the right-hand side: “Let x be a copy of
someValue” creates a shallow copy of someValue.
Once declared, an alias may be referenced in any subsequent steps and
must not be referenced from steps prior to the alias's declaration.
Aliases may be modified using the form “Set x to
someOtherValue”.
5.2.1Abstract Operations
In order to facilitate their use in multiple parts of this
specification, some algorithms, called
abstract operations, are named and written in
parameterized functional form so that they may be referenced by name
from within other algorithms. Abstract operations are typically
referenced using a functional application style such as
OperationName(arg1, arg2). Some abstract
operations are treated as polymorphically dispatched methods of
class-like specification abstractions. Such method-like abstract
operations are typically referenced using a method application style
such as someValue.OperationName(arg1,
arg2).
5.2.2Syntax-Directed Operations
A syntax-directed operation is a named operation whose
definition consists of algorithms, each of which is associated with
one or more productions from one of the ECMAScript grammars. A
production that has multiple alternative definitions will typically
have a distinct algorithm for each alternative. When an algorithm is
associated with a grammar production, it may reference the terminal
and nonterminal symbols of the production alternative as if they
were parameters of the algorithm. When used in this manner,
nonterminal symbols refer to the actual alternative definition that
is matched when parsing the source text. The
source text matched by a grammar production is the
portion of the source text that starts at the beginning of the first
terminal that participated in the match and ends at the end of the
last terminal that participated in the match.
When an algorithm is associated with a production alternative, the
alternative is typically shown without any “[ ]” grammar
annotations. Such annotations should only affect the syntactic
recognition of the alternative and have no effect on the associated
semantics for the alternative.
Syntax-directed operations are invoked with a parse node and,
optionally, other parameters by using the conventions on steps 1, 3,
and 4 in the following algorithm:
Let status be SyntaxDirectedOperation of
SomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of someParseNode.
Perform SyntaxDirectedOperation of
someParseNode passing "value" as
the argument.
Unless explicitly specified otherwise, all chain productions have an
implicit definition for every operation that might be applied to
that production's left-hand side nonterminal. The implicit
definition simply reapplies the same operation with the same
parameters, if any, to the
chain production's sole right-hand side nonterminal and then returns the result.
For example, assume that some algorithm has a step of the form:
“Return the result of evaluating
Block” and that there is a production:
but the Evaluation operation does not associate an algorithm with
that production. In that case, the Evaluation operation implicitly
includes an association of the form:
Algorithms which specify semantics that must be called at runtime
are called runtime semantics. Runtime semantics are
defined by
abstract operations
or syntax-directed operations. Such algorithms always return a
completion record.
5.2.3.1Implicit Completion Values
The algorithms of this specification often implicitly return
Completion
Records whose [[Type]] is normal. Unless it
is otherwise obvious from the context, an algorithm statement that
returns a value that is not a
Completion Record, such as:
However, if the value expression of a “return” statement is a
Completion Record
construction literal, the resulting
Completion Record
is returned. If the value expression is a call to an abstract
operation, the “return” statement simply returns the
Completion Record
produced by the abstract operation.
The abstract operation
Completion(completionRecord) is used to emphasize that a
previously computed
Completion Record
is being returned. The
Completion
abstract operation takes a single argument,
completionRecord, and performs the following steps:
Similarly, prefix ! is used to indicate that the
following invocation of an abstract or syntax-directed operation
will never return an
abrupt completion
and that the resulting
Completion Record's [[Value]] field should be used in place of the return value of
the operation. For example, the step:
Syntax-directed operations for
runtime semantics
make use of this shorthand by placing ! or
? before the invocation of the operation:
Perform ! SyntaxDirectedOperation of
NonTerminal.
5.2.4Static Semantics
Context-free grammars are not sufficiently powerful to express all
the rules that define whether a stream of input elements form a
valid ECMAScript
Script or
Module
that may be evaluated. In some situations additional rules are
needed that may be expressed using either ECMAScript algorithm
conventions or prose requirements. Such rules are always associated
with a production of a grammar and are called the
static semantics of the production.
Static Semantic Rules have names and typically are defined using an
algorithm. Named Static Semantic Rules are associated with grammar
productions and a production that has multiple alternative
definitions will typically have for each alternative a distinct
algorithm for each applicable named static semantic rule.
Unless otherwise specified every grammar production alternative in
this specification implicitly has a definition for a static semantic
rule named Contains which takes an argument named
symbol whose value is a terminal or nonterminal of the
grammar that includes the associated production. The default
definition of Contains is:
Let contained be the result of
child Contains symbol.
If contained is true,
return true.
Return false.
The above definition is explicitly over-ridden for specific
productions.
A special kind of static semantic rule is an
Early Error Rule.
Early error
rules define
early error
conditions (see clause
16) that are associated with specific grammar productions. Evaluation
of most
early error
rules are not explicitly invoked within the algorithms of this
specification. A conforming implementation must, prior to the first
evaluation of a
Script or
Module, validate all of the
early error
rules of the productions used to parse that
Script or
Module. If any of the
early error
rules are violated the
Script or
Module is
invalid and cannot be evaluated.
5.2.5Mathematical Operations
This specification makes reference to two kinds of numeric values:
Number: IEEE 754-2008 double-precision floating point
values, used as the default numeric type.
Mathematical value: Arbitrary real numbers, used for specific situations.
In the language of this specification, numerical values and
operations (including addition, subtraction, negation,
multiplication, division, and comparison) are distinguished among
different numeric kinds using subscripts. The subscript
𝔽 refers to Numbers, and the subscript
ℝ refers to mathematical values. A
subscript is used following each numeric value and operation.
For brevity, the
𝔽
subscript can be omitted on Number values—a numeric value with no
subscript is interpreted to be a Number. An operation with no
subscript is interpreted to be a Number operation, unless one of the
parameters has a particular subscript, in which case the operation
adopts that subscript. For example, 1ℝ
+ 2ℝ
= 3ℝ
is a statement about mathematical values, and 1 + 2 = 3 is a
statement about Numbers.
In general, when this specification refers to a numerical value,
such as in the phrase, "the length of y" or "the
integer
represented by the four hexadecimal digits ...", without explicitly
specifying a numeric kind, the phrase refers to a Number. Phrases
which refer to a
mathematical value
are explicitly annotated as such; for example, "the
mathematical value
of the number of code points in ...".
It is not defined to mix Numbers and mathematical values in either
arithmetic or comparison operations, and any such undefined
operation would be an editorial error in this specification text.
The
Number value
0, alternatively written 0𝔽, is defined as the double-precision floating point positive zero
value. In certain contexts, it may also be written as
+0 for clarity.
This specification denotes most numeric values in base 10; it also
uses numeric values of the form 0x followed by digits 0-9 or A-F as
base-16 values.
In certain contexts, an operation is specified which is generic
between Numbers and mathematical values. In these cases, the
subscript can be a variable; t is often used for this
purpose, for example 5t × 10t
= 50t for any t ranging over
ℝ and
𝔽, since the values involved are within the range where the
semantics coincide.
Conversions between mathematical values and numbers are never
implicit, and always explicit in this document. A conversion from a
mathematical value
to a Number is denoted as "the
Number value
for x", and is defined in
6.1.6.1. A conversion from a Number to a
mathematical value
is denoted as "the
mathematical value of
x", or
ℝ(x). Note that the
mathematical value
of non-finite values is not defined, and the
mathematical value
of +0 and -0 is the
mathematical value
0ℝ.
When the term integer is used in this
specification, it refers to a
Number value
whose
mathematical value
is in the set of integers, unless otherwise stated: when the term
mathematical integer is used in
this specification, it refers to a
mathematical value
which is in the set of integers. As shorthand,
integert can be used to refer to either of the two,
as determined by t.
The mathematical function
abst(x)
produces the absolute value of x, which is
-tx
if x <t 0t
and otherwise is x itself.
The mathematical function
mint(x1, x2, … , xN)
produces the mathematically smallest of
x1 through
xN. The mathematical function
maxt(x1, x2, ..., xN)
produces the mathematically largest of
x1 through
xN. The domain and range of these mathematical functions include
+∞ and -∞.
The notation “x moduloty” (y must be finite and nonzero) computes a value
k of the same sign as y (or zero) such that
abst(k) <tabst(y) and x-tk = q ×ty
for some
integertq.
The mathematical function
floort(x)
produces the largest
integert (closest to positive infinity) that is not
larger than x.
In this specification, ECMAScript language values are displayed in
bold. Examples include null,
true, or "hello". These are
distinguished from longer ECMAScript code sequences such as
Function.prototype.apply or let n = 42;.
Values which are internal to the specification and not directly
observable from ECMAScript code are indicated with a
sans-serif typeface. For instance, a
Completion Record's [[Type]] field takes on values like
normal, return, or
throw.
6ECMAScript Data Types and Values
Algorithms within this specification manipulate values each of which has
an associated type. The possible value types are exactly those defined
in this clause. Types are further subclassified into ECMAScript language
types and specification types.
Within this specification, the notation “Type(x)” is used as
shorthand for “the type of x” where
“type” refers to the ECMAScript language and specification types defined
in this clause. When the term “empty” is used as if it was naming a
value, it is equivalent to saying “no value of any type”.
6.1ECMAScript Language Types
An ECMAScript language type corresponds to values that are
directly manipulated by an ECMAScript programmer using the ECMAScript
language. The ECMAScript language types are Undefined, Null, Boolean,
String, Symbol, Number, BigInt, and Object. An
ECMAScript language value is a value that is characterized
by an ECMAScript language type.
6.1.1The Undefined Type
The Undefined type has exactly one value, called
undefined. Any variable that has not been
assigned a value has the value undefined.
6.1.2The Null Type
The Null type has exactly one value, called null.
6.1.3The Boolean Type
The Boolean type represents a logical entity having two values,
called true and false.
6.1.4The String Type
The String type is the set of all ordered sequences of zero or more
16-bit unsigned
integer
values (“elements”) up to a maximum length of 253 - 1
elements. The String type is generally used to represent textual
data in a running ECMAScript program, in which case each element in
the String is treated as a UTF-16 code unit value. Each element is
regarded as occupying a position within the sequence. These
positions are indexed with nonnegative integers. The first element
(if any) is at index 0, the next element (if any) at index 1, and so
on. The length of a String is the number of elements (i.e., 16-bit
values) within it. The empty String has length zero and therefore
contains no elements.
ECMAScript operations that do not interpret String contents apply no
further semantics. Operations that do interpret String values treat
each element as a single UTF-16 code unit. However, ECMAScript does
not restrict the value of or relationships between these code units,
so operations that further interpret String contents as sequences of
Unicode code points encoded in UTF-16 must account for ill-formed
subsequences. Such operations apply special treatment to every code
unit with a numeric value in the inclusive range 0xD800 to 0xDBFF
(defined by the Unicode Standard as a
leading surrogate, or more
formally as a
high-surrogate code unit)
and every code unit with a numeric value in the inclusive range
0xDC00 to 0xDFFF (defined as a
trailing surrogate, or more
formally as a
low-surrogate code unit)
using the following rules:
A sequence of two code units, where the first code unit
c1 is a
leading surrogate
and the second code unit c2 a
trailing surrogate, is a surrogate pair and is
interpreted as a code point with the value (c1 -
0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See
10.1.2)
The function String.prototype.normalize (see
21.1.3.13) can be used to explicitly normalize a String value.
String.prototype.localeCompare (see
21.1.3.10) internally normalizes String values, but no other operations
implicitly normalize the strings upon which they operate. Only
operations that are explicitly specified to be language or locale
sensitive produce language-sensitive results.
Note
The rationale behind this design was to keep the implementation
of Strings as simple and high-performing as possible. If
ECMAScript source text is in Normalized Form C, string literals
are guaranteed to also be normalized, as long as they do not
contain any Unicode escape sequences.
In this specification, the phrase "the
string-concatenation of A, B, ..."
(where each argument is a String value, a code unit, or a sequence
of code units) denotes the String value whose sequence of code units
is the concatenation of the code units (in order) of each of the
arguments (in order).
6.1.5The Symbol Type
The Symbol type is the set of all non-String values that may be used
as the key of an Object property (6.1.7).
Each possible Symbol value is unique and immutable.
Each Symbol value immutably holds an associated value called
[[Description]] that is either undefined or a
String value.
6.1.5.1Well-Known Symbols
Well-known symbols are built-in Symbol values that are explicitly
referenced by algorithms of this specification. They are typically
used as the keys of properties whose values serve as extension
points of a specification algorithm. Unless otherwise specified,
well-known symbols values are shared by all realms (8.2).
Within this specification a well-known symbol is referred to by
using a notation of the form @@name, where “name” is one of the
values listed in
Table 1.
Table 1: Well-known Symbols
Specification Name
[[Description]]
Value and Purpose
@@asyncIterator
"Symbol.asyncIterator"
A method that returns the default AsyncIterator for an
object. Called by the semantics of the
for-await-of
statement.
@@hasInstance
"Symbol.hasInstance"
A method that determines if a
constructor
object recognizes an object as one of the
constructor's instances. Called by the semantics of the
instanceof operator.
@@isConcatSpreadable
"Symbol.isConcatSpreadable"
A Boolean valued property that if true indicates that an
object should be flattened to its array elements by
Array.prototype.concat.
@@iterator
"Symbol.iterator"
A method that returns the default Iterator for an
object. Called by the semantics of the for-of statement.
@@match
"Symbol.match"
A regular expression method that matches the regular
expression against a string. Called by the
String.prototype.match
method.
@@matchAll
"Symbol.matchAll"
A regular expression method that returns an iterator,
that yields matches of the regular expression against a
string. Called by the
String.prototype.matchAll
method.
@@replace
"Symbol.replace"
A regular expression method that replaces matched
substrings of a string. Called by the
String.prototype.replace
method.
@@search
"Symbol.search"
A regular expression method that returns the index
within a string that matches the regular expression.
Called by the
String.prototype.search
method.
@@species
"Symbol.species"
A function valued property that is the
constructor
function that is used to create derived objects.
@@split
"Symbol.split"
A regular expression method that splits a string at the
indices that match the regular expression. Called by the
String.prototype.split
method.
@@toPrimitive
"Symbol.toPrimitive"
A method that converts an object to a corresponding
primitive value. Called by the
ToPrimitive
abstract operation.
@@toStringTag
"Symbol.toStringTag"
A String valued property that is used in the creation of
the default string description of an object. Accessed by
the built-in method
Object.prototype.toString.
@@unscopables
"Symbol.unscopables"
An object valued property whose own and inherited
property names are property names that are excluded from
the with environment bindings of the
associated object.
6.1.6Numeric Types
ECMAScript has two built-in numeric types: Number and BigInt. In
this specification, every numeric type T contains a
multiplicative identity value denoted T::unit. The
specification types also have the following
abstract operations, likewise denoted T::op for a given operation
with specification name op. All argument types are
T. The "Result" column shows the return type, along with
an indication if it is possible for some invocations of the
operation to return an
abrupt completion.
Object internal methods, via
SameValue ( x, y ), to test exact value equality
Boolean
T::sameValueZero(x, y)
Array, Map, and Set methods, via
SameValueZero ( x, y ), to test value equality ignoring differences among
members of the zero cohort (e.g.,
-0 and +0)
The T::unit value and T::op
operations are not a part of the ECMAScript language; they are
defined here solely to aid the specification of the semantics of the
ECMAScript language. Other
abstract operations
are defined throughout this specification.
Because the numeric types are in general not convertible without
loss of precision or truncation, the ECMAScript language provides no
implicit conversion among these types. Programmers must explicitly
call Number and BigInt functions to
convert among types when calling a function which requires another
type.
Note
The first and subsequent editions of ECMAScript have provided,
for certain operators, implicit numeric conversions that could
lose precision or truncate. These legacy implicit conversions
are maintained for backward compatibility, but not provided for
BigInt in order to minimize opportunity for programmer error,
and to leave open the option of generalized
value types in a future edition.
6.1.6.1The Number Type
The Number type has exactly 18437736874454810627ℝ
(that is,
2ℝ64ℝ
- 2ℝ53ℝ
+ 3ℝ) values, representing the double-precision 64-bit format IEEE
754-2008 values as specified in the IEEE Standard for Binary
Floating-Point Arithmetic, except that the 9007199254740990ℝ
(that is,
2ℝ53ℝ
- 2ℝ) distinct “Not-a-Number” values of the IEEE Standard are
represented in ECMAScript as a single special
NaN value. (Note that the
NaN value is produced by the program expression
NaN.) In some implementations, external code might be
able to detect a difference between various Not-a-Number values,
but such behaviour is implementation-dependent; to ECMAScript
code, all NaN values are indistinguishable from
each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see
24.1) or a SharedArrayBuffer (see
24.2) after a
Number value
has been stored into it is not necessarily the same as the
internal representation of that
Number value
used by the ECMAScript implementation.
There are two other special values, called
positive Infinity and
negative Infinity. For brevity, these values
are also referred to for expository purposes by the symbols
+∞ and -∞, respectively.
(Note that these two infinite Number values are produced by the
program expressions +Infinity (or simply
Infinity) and -Infinity.)
The other 18437736874454810624ℝ
(that is,
2ℝ64ℝ
- 2ℝ53ℝ) values are called the finite numbers. Half of these are
positive numbers and half are negative numbers; for every finite
positive
Number value
there is a corresponding negative value having the same magnitude.
Note that there is both a positive zero and a
negative zero. For brevity, these values are
also referred to for expository purposes by the symbols
+0 and -0, respectively.
(Note that these two different zero Number values are produced by
the program expressions +0 (or simply 0)
and -0.)
The 18437736874454810622ℝ
(that is,
2ℝ64ℝ
- 2ℝ53ℝ
- 2ℝ) finite nonzero values are of two kinds:
18428729675200069632ℝ
(that is,
2ℝ64ℝ
- 2ℝ54ℝ) of them are normalized, having the form
Note that all the positive and negative mathematical integers
whose magnitude is no greater than 253 are
representable in the Number type (indeed, the
mathematical integer
0 has two representations, +0 and
-0).
A finite number has an odd significand if it is nonzero
and the
mathematical integerm used to express it (in one of the two forms shown
above) is odd. Otherwise, it has an even significand.
In this specification, the phrase “the
Number value for x” where
x represents an exact real mathematical quantity (which
might even be an irrational number such as π) means a
Number value
chosen in the following manner. Consider the set of all finite
values of the Number type, with -0 removed and
with two additional values added to it that are not representable
in the Number type, namely 2ℝ1024ℝ
(which is
+1ℝ
× 2ℝ53ℝ
× 2ℝ971ℝ) and
-2ℝ1024ℝ
(which is
-1ℝ
× 2ℝ53ℝ
× 2ℝ971ℝ). Choose the member of this set that is closest in value to
x. If two values of the set are equally close, then the
one with an even significand is chosen; for this purpose, the two
extra values 2ℝ1024ℝ
and
-2ℝ1024ℝ
are considered to have even significands. Finally, if 2ℝ1024ℝ
was chosen, replace it with +∞; if
-2ℝ1024ℝ
was chosen, replace it with -∞; if
+0 was chosen, replace it with
-0 if and only if x is less than
zero; any other chosen value is used unchanged. The result is the
Number value
for x. (This procedure corresponds exactly to the
behaviour of the IEEE 754-2008 roundTiesToEven mode.)
Some ECMAScript operators deal only with integers in specific
ranges such as
-231 through
231 - 1, inclusive,
or in the range 0 through
216 - 1, inclusive.
These operators accept any value of the Number type but first
convert each such value to an
integer
value in the expected range. See the descriptions of the numeric
conversion operations in
7.1.
The Number::unit value is 1.
6.1.6.1.1Number::unaryMinus (
x )
If x is NaN, return
NaN.
Return the result of negating x; that is, compute
a Number with the same magnitude but opposite sign.
Return the result of applying bitwise complement to
oldValue. The result is a signed 32-bit
integer.
6.1.6.1.3Number::exponentiate (
base, exponent )
Returns an implementation-dependent approximation of the result
of raising base to the power exponent.
If exponent is NaN, the result
is NaN.
If exponent is +0, the result is
1, even if base is NaN.
If exponent is -0, the result is
1, even if base is NaN.
If base is NaN and
exponent is nonzero, the result is
NaN.
If
abs(base) > 1 and exponent is
+∞, the result is +∞.
If
abs(base) > 1 and exponent is
-∞, the result is +0.
If
abs(base) is 1 and exponent is
+∞, the result is NaN.
If
abs(base) is 1 and exponent is
-∞, the result is NaN.
If
abs(base) < 1 and exponent is
+∞, the result is +0.
If
abs(base) < 1 and exponent is
-∞, the result is +∞.
If base is +∞ and
exponent > 0, the result is
+∞.
If base is +∞ and
exponent < 0, the result is
+0.
If base is -∞ and
exponent > 0 and exponent is an odd
integer, the result is -∞.
If base is -∞ and
exponent > 0 and exponent is not an
odd
integer, the result is +∞.
If base is -∞ and
exponent < 0 and exponent is an odd
integer, the result is -0.
If base is -∞ and
exponent < 0 and exponent is not an
odd
integer, the result is +0.
If base is +0 and
exponent > 0, the result is
+0.
If base is +0 and
exponent < 0, the result is
+∞.
If base is -0 and
exponent > 0 and exponent is an odd
integer, the result is -0.
If base is -0 and
exponent > 0 and exponent is not an
odd
integer, the result is +0.
If base is -0 and
exponent < 0 and exponent is an odd
integer, the result is -∞.
If base is -0 and
exponent < 0 and exponent is not an
odd
integer, the result is +∞.
If base < 0 and base is finite and
exponent is finite and exponent is not
an
integer, the result is NaN.
Note
The result of base**exponent when base is
1 or -1 and
exponent is +Infinity or
-Infinity differs from IEEE 754-2008. The
first edition of ECMAScript specified a result of
NaN for this operation, whereas later
versions of IEEE 754-2008 specified 1.
The historical ECMAScript behaviour is preserved for
compatibility reasons.
6.1.6.1.4Number::multiply (
x, y )
The *MultiplicativeOperator
performs multiplication, producing the product of
x and y. Multiplication is commutative.
Multiplication is not always associative in ECMAScript, because
of finite precision.
The result of a floating-point multiplication is governed by the
rules of IEEE 754-2008 binary double-precision arithmetic:
If either operand is NaN, the result is
NaN.
The sign of the result is positive if both operands have the
same sign, negative if the operands have different signs.
Multiplication of an infinity by a zero results in
NaN.
Multiplication of an infinity by an infinity results in an
infinity. The sign is determined by the rule already stated
above.
Multiplication of an infinity by a finite nonzero value
results in a signed infinity. The sign is determined by the
rule already stated above.
In the remaining cases, where neither an infinity nor
NaN is involved, the product is computed
and rounded to the nearest representable value using IEEE
754-2008 roundTiesToEven mode. If the magnitude is too large
to represent, the result is then an infinity of appropriate
sign. If the magnitude is too small to represent, the result
is then a zero of appropriate sign. The ECMAScript language
requires support of gradual underflow as defined by IEEE
754-2008.
6.1.6.1.5Number::divide (
x, y )
The /MultiplicativeOperator
performs division, producing the quotient of x and
y. x is the dividend and y is
the divisor. ECMAScript does not perform
integer
division. The operands and result of all division operations are
double-precision floating-point numbers. The result of division
is determined by the specification of IEEE 754-2008 arithmetic:
If either operand is NaN, the result is
NaN.
The sign of the result is positive if both operands have the
same sign, negative if the operands have different signs.
Division of an infinity by an infinity results in
NaN.
Division of an infinity by a zero results in an infinity. The
sign is determined by the rule already stated above.
Division of an infinity by a nonzero finite value results in a
signed infinity. The sign is determined by the rule already
stated above.
Division of a finite value by an infinity results in zero. The
sign is determined by the rule already stated above.
Division of a zero by a zero results in
NaN; division of zero by any other finite
value results in zero, with the sign determined by the rule
already stated above.
Division of a nonzero finite value by a zero results in a
signed infinity. The sign is determined by the rule already
stated above.
In the remaining cases, where neither an infinity, nor a zero,
nor NaN is involved, the quotient is
computed and rounded to the nearest representable value using
IEEE 754-2008 roundTiesToEven mode. If the magnitude is too
large to represent, the operation overflows; the result is
then an infinity of appropriate sign. If the magnitude is too
small to represent, the operation underflows and the result is
a zero of the appropriate sign. The ECMAScript language
requires support of gradual underflow as defined by IEEE
754-2008.
6.1.6.1.6Number::remainder (
n, d )
The %MultiplicativeOperator
yields the remainder of its operands from an implied division;
n is the dividend and d is the divisor.
Note
In C and C++, the remainder operator accepts only integral
operands; in ECMAScript, it also accepts floating-point
operands.
The result of a floating-point remainder operation as computed
by the % operator is not the same as the
“remainder” operation defined by IEEE 754-2008. The IEEE
754-2008 “remainder” operation computes the remainder from a
rounding division, not a truncating division, and so its
behaviour is not analogous to that of the usual
integer
remainder operator. Instead the ECMAScript language defines
% on floating-point operations to behave in a
manner analogous to that of the Java
integer
remainder operator; this may be compared with the C library
function fmod.
The result of an ECMAScript floating-point remainder operation
is determined by the rules of IEEE arithmetic:
If either operand is NaN, the result is
NaN.
The sign of the result equals the sign of the dividend.
If the dividend is an infinity, or the divisor is a zero, or
both, the result is NaN.
If the dividend is finite and the divisor is an infinity, the
result equals the dividend.
If the dividend is a zero and the divisor is nonzero and
finite, the result is the same as the dividend.
In the remaining cases, where neither an infinity, nor a zero,
nor NaN is involved, the floating-point
remainder r from a dividend n and a
divisor d is defined by the mathematical relation
r = n - (d × q)
where q is an
integer
that is negative only if n/d is negative
and positive only if n/d is positive,
and whose magnitude is as large as possible without exceeding
the magnitude of the true mathematical quotient of
n and d. r is computed and
rounded to the nearest representable value using IEEE 754-2008
roundTiesToEven mode.
6.1.6.1.7Number::add ( x,
y )
The + operator performs addition when applied to
x and y, producing the sum of the
operands.
Addition is a commutative operation, but not always associative.
The result of an addition is determined using the rules of IEEE
754-2008 binary double-precision arithmetic:
If either operand is NaN, the result is
NaN.
The sum of two infinities of opposite sign is
NaN.
The sum of two infinities of the same sign is the infinity of
that sign.
The sum of an infinity and a finite value is equal to the
infinite operand.
The sum of two negative zeroes is -0. The
sum of two positive zeroes, or of two zeroes of opposite sign,
is +0.
The sum of a zero and a nonzero finite value is equal to the
nonzero operand.
The sum of two nonzero finite values of the same magnitude and
opposite sign is +0.
In the remaining cases, where neither an infinity, nor a zero,
nor NaN is involved, and the operands have
the same sign or have different magnitudes, the sum is
computed and rounded to the nearest representable value using
IEEE 754-2008 roundTiesToEven mode. If the magnitude is too
large to represent, the operation overflows and the result is
then an infinity of appropriate sign. The ECMAScript language
requires support of gradual underflow as defined by IEEE
754-2008.
6.1.6.1.8Number::subtract (
x, y )
The - operator performs subtraction when applied to
two operands of numeric type, producing the difference of its
operands; x is the minuend and y is the
subtrahend. It is always the case that
x - y produces the same result as
x + (-y).
Let shiftCount be the result of masking out all
but the least significant 5 bits of rnum, that
is, compute rnum & 0x1F.
Return the result of performing a sign-extending right shift
of lnum by shiftCount bits. The most
significant bit is propagated. The result is a signed 32-bit
integer.
Let shiftCount be the result of masking out all
but the least significant 5 bits of rnum, that
is, compute rnum & 0x1F.
Return the result of performing a zero-filling right shift
of lnum by shiftCount bits. Vacated
bits are filled with zero. The result is an unsigned 32-bit
integer.
6.1.6.1.12Number::lessThan (
x, y )
If x is NaN, return
undefined.
If y is NaN, return
undefined.
If x and y are the same
Number value, return false.
If x is +0 and y is
-0, return false.
If x is -0 and y is
+0, return false.
If x is +∞, return
false.
If y is +∞, return
true.
If y is -∞, return
false.
If x is -∞, return
true.
If the
mathematical value
of x is less than the
mathematical value
of y—note that these mathematical values are both
finite and not both zero—return true.
Otherwise, return false.
Otherwise, let n, k, and
s be integers such that k ≥ 1, 10k - 1
≤ s < 10k, the
Number value
for
ℝ(s) × 10ℝℝ(n) -
ℝ(k)
is x, and k is as small as possible.
Note that k is the number of digits in the
decimal representation of s, that s is
not divisible by 10ℝ, and that the least significant digit of s is
not necessarily uniquely determined by these criteria.
The least significant digit of s is not always uniquely
determined by the requirements listed in step 5.
Note 2
For implementations that provide more accurate conversions
than required by the rules above, it is recommended that the
following alternative version of step 5 be used as a
guideline:
Otherwise, let n, k, and
s be integers such that k ≥ 1,
10k - 1 ≤ s < 10k, the
Number value
for
ℝ(s) × 10ℝℝ(n) -
ℝ(k)
is x, and k is as small as
possible. If there are multiple possibilities for
s, choose the value of s for which
ℝ(s) × 10ℝℝ(n) -
ℝ(k)
is closest in value to
ℝ(x). If there are two such possible values
of s, choose the one that is even. Note that
k is the number of digits in the decimal
representation of s and that s is
not divisible by 10ℝ.
Note 3
Implementers of ECMAScript may find useful the paper and
code written by David M. Gay for binary-to-decimal
conversion of floating-point numbers:
The BigInt type represents a
mathematical integer
value. The value may be any size and is not limited to a
particular bit-width. Generally, where not otherwise noted,
operations are designed to return exact mathematically-based
answers. For binary operations, BigInts act as two's complement
binary strings, with negative numbers treated as having bits set
infinitely to the left.
The BigInt::unit value is 1n.
6.1.6.2.1BigInt::unaryMinus (
x )
If x is 0n, return
0n.
Return the BigInt value that represents the
mathematical value
of negating x.
6.1.6.2.2BigInt::bitwiseNOT (
x )
The abstract operation BigInt::bitwiseNOT with an argument
x of type BigInt returns the one's complement of
x; that is, -x - 1.
6.1.6.2.3BigInt::exponentiate (
base, exponent )
If exponent < 0n, throw a
RangeError exception.
If base is 0n and
exponent is 0n, return
1n.
Return the BigInt value that represents the
mathematical value
of base raised to the power exponent.
6.1.6.2.4BigInt::multiply (
x, y )
The abstract operation BigInt::multiply with two arguments
x and y of type BigInt returns the BigInt
value that represents the result of multiplying x and
y.
Note
Even if the result has a much larger bit width than the input,
the exact mathematical answer is given.
Return the BigInt value that represents
quotient rounded towards 0 to the next integral
value.
6.1.6.2.6BigInt::remainder (
n, d )
If d is 0n, throw a
RangeError exception.
If n is 0n, return
0n.
Let r be the BigInt defined by the mathematical
relation r = n - (d ×
q) where q is a BigInt that is
negative only if n/d is negative and
positive only if n/d is positive, and
whose magnitude is as large as possible without exceeding
the magnitude of the true mathematical quotient of
n and d.
Return r.
Note
The sign of the result equals the sign of the dividend.
6.1.6.2.7BigInt::add ( x,
y )
The abstract operation BigInt::add with two arguments
x and y of type BigInt returns the BigInt
value that represents the sum of x and y.
6.1.6.2.8BigInt::subtract (
x, y )
The abstract operation BigInt::subtract with two arguments
x and y of type BigInt returns the BigInt
value that represents the difference x minus
y.
6.1.6.2.9BigInt::leftShift (
x, y )
The abstract operation BigInt::leftShift with two arguments
x and y of type BigInt performs the
following steps:
If y < 0n, then
Return the BigInt value that represents x ÷
2-y, rounding down to the nearest
integer, including for negative numbers.
Return the BigInt value that represents x × 2y.
Note
Semantics here should be equivalent to a bitwise shift,
treating the BigInt as an infinite length string of binary
two's complement digits.
6.1.6.2.10BigInt::signedRightShift (
x, y )
The abstract operation BigInt::signedRightShift with arguments
x and y of type BigInt performs the
following steps:
Return BigInt::leftShift(x, -y).
6.1.6.2.11BigInt::unsignedRightShift
( x, y )
The abstract operation BigInt::unsignedRightShift with two
arguments x and y of type BigInt performs
the following steps:
Throw a TypeError exception.
6.1.6.2.12BigInt::lessThan (
x, y )
The abstract operation BigInt::lessThan with two arguments
x and y of type BigInt returns
true if x is less than
y and false otherwise.
6.1.6.2.13BigInt::equal (
x, y )
The abstract operation BigInt::equal with two arguments
x and y of type BigInt returns
true if x and y have
the same
mathematical integer
value and false otherwise.
6.1.6.2.14BigInt::sameValue (
x, y )
The abstract operation BigInt::sameValue with two arguments
x and y of type BigInt performs the
following steps:
Return BigInt::equal(x, y).
6.1.6.2.15BigInt::sameValueZero (
x, y )
The abstract operation BigInt::sameValueZero with two arguments
x and y of type BigInt performs the
following steps:
Return the String value consisting of the code units of the
digits of the decimal representation of x.
6.1.7The Object Type
An Object is logically a collection of properties. Each property is
either a data property, or an accessor property:
A data property associates a key value with an
ECMAScript language value
and a set of Boolean attributes.
An accessor property associates a key value with one or
two accessor functions, and a set of Boolean attributes. The
accessor functions are used to store or retrieve an
ECMAScript language value
that is associated with the property.
Properties are identified using key values. A property key value is
either an ECMAScript String value or a Symbol value. All String and
Symbol values, including the empty string, are valid as property
keys. A property name is a property
key that is a String value.
An integer index is a String-valued
property key that is a canonical numeric String (see
7.1.21) and whose numeric value is either +0 or a
positive
integer
≤ 253 - 1. An array index is
an
integer index
whose numeric value i is in the range
+0 ≤ i < 232 - 1.
Property keys are used to access properties and their values. There
are two kinds of access for properties: get and
set, corresponding to value retrieval and assignment,
respectively. The properties accessible via get and set access
includes both own properties that are a direct part of an
object and inherited properties which are provided by
another associated object via a property inheritance relationship.
Inherited properties may be either own or inherited properties of
the associated object. Each own property of an object must each have
a key value that is distinct from the key values of the other own
properties of that object.
All objects are logically collections of properties, but there are
multiple forms of objects that differ in their semantics for
accessing and manipulating their properties.
Ordinary objects are the most
common form of objects and have the default object semantics. An
exotic object is any form of object
whose property semantics differ in any way from the default
semantics.
6.1.7.1Property Attributes
Attributes are used in this specification to define and explain
the state of Object properties. A
data property
associates a key value with the attributes listed in
Table 3.
The value retrieved by a get access of the property.
[[Writable]]
Boolean
If false, attempts by ECMAScript code
to change the property's [[Value]] attribute using
[[Set]] will not succeed.
[[Enumerable]]
Boolean
If true, the property will be
enumerated by a for-in enumeration (see
13.7.5). Otherwise, the property is said to be
non-enumerable.
[[Configurable]]
Boolean
If false, attempts to delete the
property, change the property to be an
accessor property, or change its attributes (other than [[Value]], or
changing [[Writable]] to false) will
fail.
If the value is an Object it must be a
function object. The function's [[Call]] internal method (Table 7) is called with an empty arguments list to retrieve
the property value each time a get access of the
property is performed.
[[Set]]
Object | Undefined
If the value is an Object it must be a
function object. The function's [[Call]] internal method (Table 7) is called with an arguments list containing the
assigned value as its sole argument each time a set
access of the property is performed. The effect of a
property's [[Set]] internal method may, but is not
required to, have an effect on the value returned by
subsequent calls to the property's [[Get]] internal
method.
[[Enumerable]]
Boolean
If true, the property is to be
enumerated by a for-in enumeration (see
13.7.5). Otherwise, the property is said to be
non-enumerable.
[[Configurable]]
Boolean
If false, attempts to delete the
property, change the property to be a
data property, or change its attributes will fail.
If the initial values of a property's attributes are not
explicitly specified by this specification, the default value
defined in
Table 5
is used.
Table 5: Default Attribute Values
Attribute Name
Default Value
[[Value]]
undefined
[[Get]]
undefined
[[Set]]
undefined
[[Writable]]
false
[[Enumerable]]
false
[[Configurable]]
false
6.1.7.2Object Internal Methods and
Internal Slots
The actual semantics of objects, in ECMAScript, are specified via
algorithms called internal methods. Each object in an
ECMAScript engine is associated with a set of internal methods
that defines its runtime behaviour. These internal methods are not
part of the ECMAScript language. They are defined by this
specification purely for expository purposes. However, each object
within an implementation of ECMAScript must behave as specified by
the internal methods associated with it. The exact manner in which
this is accomplished is determined by the implementation.
Internal method names are polymorphic. This means that different
object values may perform different algorithms when a common
internal method name is invoked upon them. That actual object upon
which an internal method is invoked is the “target” of the
invocation. If, at runtime, the implementation of an algorithm
attempts to use an internal method of an object that the object
does not support, a TypeError exception is
thrown.
Internal slots correspond to internal state that is associated
with objects and used by various ECMAScript specification
algorithms. Internal slots are not object properties and they are
not inherited. Depending upon the specific internal slot
specification, such state may consist of values of any
ECMAScript language type
or of specific ECMAScript specification type values. Unless
explicitly specified otherwise, internal slots are allocated as
part of the process of creating an object and may not be
dynamically added to an object. Unless specified otherwise, the
initial value of an internal slot is the value
undefined. Various algorithms within this
specification create objects that have internal slots. However,
the ECMAScript language provides no direct way to associate
internal slots with an object.
Internal methods and internal slots are identified within this
specification using names enclosed in double square brackets [[
]].
Table 6
summarizes the essential internal methods used by this
specification that are applicable to all objects created or
manipulated by ECMAScript code. Every object must have algorithms
for all of the essential internal methods. However, all objects do
not necessarily use the same algorithms for those methods.
The “Signature” column of
Table 6
and other similar tables describes the invocation pattern for each
internal method. The invocation pattern always includes a
parenthesized list of descriptive parameter names. If a parameter
name is the same as an ECMAScript type name then the name
describes the required type of the parameter value. If an internal
method explicitly returns a value, its parameter list is followed
by the symbol “→” and the type name of the returned value. The
type names used in signatures refer to the types defined in clause
6
augmented by the following additional names. “any” means
the value may be any
ECMAScript language type.
In addition to its parameters, an internal method always has
access to the object that is the target of the method invocation.
An internal method implicitly returns a
Completion Record, either a normal completion that wraps a value of the return
type shown in its invocation pattern, or a throw completion.
Table 6: Essential Internal Methods
Internal Method
Signature
Description
[[GetPrototypeOf]]
( ) → Object | Null
Determine the object that provides inherited properties
for this object. A null value
indicates that there are no inherited properties.
[[SetPrototypeOf]]
(Object | Null) → Boolean
Associate this object with another object that provides
inherited properties. Passing
null indicates that there are no
inherited properties. Returns
true indicating that the operation
was completed successfully or
false indicating that the operation
was not successful.
[[IsExtensible]]
( ) → Boolean
Determine whether it is permitted to add additional
properties to this object.
[[PreventExtensions]]
( ) → Boolean
Control whether new properties may be added to this
object. Returns true if the operation
was successful or false if the
operation was unsuccessful.
Return a
Property Descriptor
for the own property of this object whose key is
propertyKey, or
undefined if no such property exists.
[[DefineOwnProperty]]
(propertyKey, PropertyDescriptor)
→ Boolean
Create or alter the own property, whose key is
propertyKey, to have the state described by
PropertyDescriptor. Return
true if that property was
successfully created/updated or
false if the property could not be
created or updated.
[[HasProperty]]
(propertyKey) → Boolean
Return a Boolean value indicating whether this object
already has either an own or inherited property whose
key is propertyKey.
[[Get]]
(propertyKey, Receiver) →any
Return the value of the property whose key is
propertyKey from this object. If any
ECMAScript code must be executed to retrieve the
property value, Receiver is used as the
this value when evaluating the code.
[[Set]]
(propertyKey, value,
Receiver) → Boolean
Set the value of the property whose key is
propertyKey to value. If any
ECMAScript code must be executed to set the property
value, Receiver is used as the
this value when evaluating the code.
Returns true if the property value
was set or false if it could not be
set.
[[Delete]]
(propertyKey) → Boolean
Remove the own property whose key is
propertyKey from this object. Return
false if the property was not deleted
and is still present. Return true if
the property was deleted or is not present.
Return a
List
whose elements are all of the own property keys for the
object.
Table 7
summarizes additional essential internal methods that are
supported by objects that may be called as functions. A
function object is an object that
supports the [[Call]] internal method. A
constructor is an object that supports
the [[Construct]] internal method. Every object that supports
[[Construct]] must support [[Call]]; that is, every
constructor
must be a
function object. Therefore, a
constructor
may also be referred to as a
constructor
function
or
constructorfunction object.
Table 7: Additional Essential Internal Methods of Function
Objects
Executes code associated with this object. Invoked via a
function call expression. The arguments to the internal
method are a this value and a list
containing the arguments passed to the function by a
call expression. Objects that implement this internal
method are callable.
Creates an object. Invoked via the new or
super operators. The first argument to the
internal method is a list containing the arguments of
the operator. The second argument is the object to which
the new operator was initially applied.
Objects that implement this internal method are called
constructors. A
function object
is not necessarily a
constructor
and such non-constructor
function objects do not have a [[Construct]] internal
method.
The semantics of the essential internal methods for ordinary
objects and standard exotic objects are specified in clause
9. If any specified use of an internal method of an
exotic object
is not supported by an implementation, that usage must throw a
TypeError exception when attempted.
6.1.7.3Invariants of the Essential
Internal Methods
The Internal Methods of Objects of an ECMAScript engine must
conform to the list of invariants specified below. Ordinary
ECMAScript Objects as well as all standard exotic objects in this
specification maintain these invariants. ECMAScript Proxy objects
maintain these invariants by means of runtime checks on the result
of traps invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain
these invariants for those objects. Violation of these invariants
may cause ECMAScript code to have unpredictable behaviour and
create security issues. However, violation of these invariants
must never compromise the memory safety of an implementation.
An implementation must not allow these invariants to be
circumvented in any manner such as by providing alternative
interfaces that implement the functionality of the essential
internal methods without enforcing their invariants.
Definitions:
The target of an internal method is the object upon
which the internal method is called.
A target is non-extensible if it has been observed to
return false from its [[IsExtensible]]
internal method, or true from its
[[PreventExtensions]] internal method.
A non-existent property is a property that does not
exist as an own property on a non-extensible target.
All references to
SameValue
are according to the definition of the
SameValue
algorithm.
Return value:
The value returned by any internal method must be a
Completion Record
with either:
[[Type]] = normal, [[Target]] =
empty, and [[Value]] = a value of the
"normal return type" shown below for that internal method, or
An internal method must not return a completion with [[Type]]
= continue,
break, or
return.
[[GetPrototypeOf]] ( )
The normal return type is either Object or Null.
If target is non-extensible, and [[GetPrototypeOf]] returns a
value V, then any future calls to [[GetPrototypeOf]]
should return the
SameValue
as V.
Note 2
An object's prototype chain should have finite length (that
is, starting from any object, recursively applying the
[[GetPrototypeOf]] internal method to its result should
eventually lead to the value null).
However, this requirement is not enforceable as an object
level invariant if the prototype chain includes any exotic
objects that do not use the ordinary object definition of
[[GetPrototypeOf]]. Such a circular prototype chain may result
in infinite loops when accessing object properties.
[[SetPrototypeOf]] ( V )
The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return
false, unless V is the
SameValue
as the target's observed [[GetPrototypeOf]] value.
[[IsExtensible]] ( )
The normal return type is Boolean.
If [[IsExtensible]] returns false, all future
calls to [[IsExtensible]] on the target must return
false.
[[PreventExtensions]] ( )
The normal return type is Boolean.
If [[PreventExtensions]] returns true, all
future calls to [[IsExtensible]] on the target must return
false and the target is now considered
non-extensible.
If P is described as a non-configurable, non-writable
own
data property, all future calls to [[GetOwnProperty]] ( P ) must
return Property Descritor whose [[Value]] is
SameValue
as P's [[Value]] attribute.
If P's attributes other than [[Writable]] may change
over time or if the property might be deleted, then
P's [[Configurable]] attribute must be
true.
If the [[Writable]] attribute may change from
false to true, then the
[[Configurable]] attribute must be true.
If the target is non-extensible and P is
non-existent, then all future calls to [[GetOwnProperty]]
(P) on the target must describe P as
non-existent (i.e. [[GetOwnProperty]] (P) must return
undefined).
Note 3
As a consequence of the third invariant, if a property is
described as a
data property
and it may return different values over time, then either or
both of the [[Writable]] and [[Configurable]] attributes must
be true even if no mechanism to change the
value is exposed via the other internal methods.
[[DefineOwnProperty]] ( P, Desc )
The normal return type is Boolean.
[[DefineOwnProperty]] must return false if
P has previously been observed as a non-configurable
own property of the target, unless either:
All attributes of Desc are the
SameValue
as P's attributes.
[[DefineOwnProperty]] (P, Desc) must
return false if target is non-extensible and
P is a non-existent own property. That is, a
non-extensible target object cannot be extended with new
properties.
[[HasProperty]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable
own data or
accessor property
of the target, [[HasProperty]] must return
true.
If P was previously observed as a non-configurable,
non-writable own
data property
of the target with value V, then [[Get]] must return
the
SameValue
as V.
If P was previously observed as a non-configurable
own
accessor property
of the target whose [[Get]] attribute is
undefined, the [[Get]] operation must return
undefined.
[[Set]] ( P, V, Receiver )
The normal return type is Boolean.
If P was previously observed as a non-configurable,
non-writable own
data property
of the target, then [[Set]] must return
false unless V is the
SameValue
as P's [[Value]] attribute.
If P was previously observed as a non-configurable
own
accessor property
of the target whose [[Set]] attribute is
undefined, the [[Set]] operation must return
false.
[[Delete]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable
own data or
accessor property
of the target, [[Delete]] must return false.
The returned
List
must not contain any duplicate entries.
The Type of each element of the returned
List
is either String or Symbol.
The returned
List
must contain at least the keys of all non-configurable own
properties that have previously been observed.
If the object is non-extensible, the returned
List
must contain only the keys of all own properties of the object
that are observable using [[GetOwnProperty]].
Well-known intrinsics are built-in objects that are explicitly
referenced by the algorithms of this specification and which
usually have
realm-specific identities. Unless otherwise specified each intrinsic
object actually corresponds to a set of similar objects, one per
realm.
Within this specification a reference such as %name% means the
intrinsic object, associated with the current
realm, corresponding to the name. A reference such as %name.a.b%
means, as if the "b" property of the "a" property of the intrinsic
object %name% was accessed prior to any ECMAScript code being
evaluated. Determination of the current
realm
and its intrinsics is described in
8.3. The well-known intrinsics are listed in
Table 8.
The initial value of the "prototype"data property
of
%WeakSet%; i.e., %WeakSet.prototype%
6.2ECMAScript Specification Types
A specification type corresponds to meta-values that are used within
algorithms to describe the semantics of ECMAScript language constructs
and ECMAScript language types. The specification types include
Reference,
List,
Completion,
Property Descriptor,
Lexical Environment,
Environment Record, and
Data Block. Specification type values are specification artefacts that do not
necessarily correspond to any specific entity within an ECMAScript
implementation. Specification type values may be used to describe
intermediate results of ECMAScript expression evaluation but such
values cannot be stored as properties of objects or values of
ECMAScript language variables.
6.2.1The List and Record Specification
Types
The List type is used to explain the evaluation of
argument lists (see
12.3.8) in new expressions, in function calls, and in other
algorithms where a simple ordered list of values is needed. Values
of the List type are simply ordered sequences of list elements
containing the individual values. These sequences may be of any
length. The elements of a list may be randomly accessed using
0-origin indices. For notational convenience an array-like syntax
can be used to access List elements. For example,
arguments[2] is shorthand for saying the 3rd
element of the List arguments.
For notational convenience within this specification, a literal
syntax can be used to express a new List value. For example, « 1, 2
» defines a List value that has two elements each of which is
initialized to a specific value. A new empty List can be expressed
as « ».
The Record type is used to describe data aggregations
within the algorithms of this specification. A Record type value
consists of one or more named fields. The value of each field is
either an ECMAScript value or an abstract value represented by a
name associated with the Record type. Field names are always
enclosed in double brackets, for example [[Value]].
For notational convenience within this specification, an object
literal-like syntax can be used to express a Record value. For
example, { [[Field1]]: 42, [[Field2]]: false,
[[Field3]]: empty } defines a Record value
that has three fields, each of which is initialized to a specific
value. Field name order is not significant. Any fields that are not
explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used to
refer to a specific field of a Record value. For example, if R is
the record shown in the previous paragraph then R.[[Field2]] is
shorthand for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be named, and
that name may be used as a prefix to a literal Record value to
identify the specific kind of aggregations that is being described.
For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]:
false, [[Configurable]]:
true }.
6.2.2The Set and Relation Specification
Types
The Set type is used to explain a collection of unordered
elements for use in the
memory model. Values of the Set type are simple collections of elements, where
no element appears more than once. Elements may be added to and
removed from Sets. Sets may be unioned, intersected, or subtracted
from each other.
The Relation type is used to explain constraints on Sets.
Values of the Relation type are Sets of ordered pairs of values from
its value domain. For example, a Relation on events is a set of
ordered pairs of events. For a Relation R and two values
a and b in the value domain of R,
aRb is shorthand for saying the
ordered pair (a, b) is a member of
R. A Relation is least with respect to some conditions
when it is the smallest Relation that satisfies those conditions.
A strict partial order is a Relation value
R that satisfies the following.
For all a, b, and c in
R's domain:
It is not the case that aRa, and
If aRb and bRc, then aRc.
Note 1
The two properties above are called, in order, irreflexivity and
transitivity.
A strict total order is a Relation value
R that satisfies the following.
For all a, b, and c in
R's domain:
a is identical to b or aRb or bRa, and
It is not the case that aRa, and
If aRb and bRc, then aRc.
Note 2
The three properties above are called, in order, totality,
irreflexivity, and transitivity.
6.2.3The Completion Record Specification
Type
The Completion type is a
Record
used to explain the runtime propagation of values and control flow
such as the behaviour of statements (break,
continue, return and throw)
that perform nonlocal transfers of control.
Values of the Completion type are
Record
values whose fields are defined as by
Table 9. Such values are referred to as Completion Records.
Set the code evaluation state of asyncContext such
that when evaluation is resumed with a
Completioncompletion, the following steps of the algorithm
that invoked
Await
will be performed, with completion available.
Return.
NOTE: This returns to the evaluation of the operation that had
most previously resumed evaluation of asyncContext.
where all variables in the above steps, with the exception of
completion, are ephemeral and visible only in the steps
pertaining to Await.
Note
Await can be combined with the ? and
! prefixes, so that for example
An
Await
fulfilled function is an anonymous built-in function that is
used as part of the
Await
specification device to deliver the promise fulfillment value to
the caller as a normal completion. Each
Await
fulfilled function has an [[AsyncContext]] internal slot.
When an
Await
fulfilled function is called with argument value, the
following steps are taken:
The "length" property of an
Await
fulfilled function is 1.
6.2.3.1.2Await Rejected Functions
An
Await
rejected function is an anonymous built-in function that is used
as part of the
Await
specification device to deliver the promise rejection reason to
the caller as an abrupt throw completion. Each
Await
rejected function has an [[AsyncContext]] internal slot.
When an
Await
rejected function is called with argument reason, the
following steps are taken:
The Reference type is used to explain the behaviour of such
operators as delete, typeof, the
assignment operators, the superkeyword
and other language features. For example, the left-hand operand
of an assignment is expected to produce a reference.
A Reference is a resolved name or property binding. A
Reference consists of three components, the base value component,
the referenced name component, and the Boolean-valued strict
reference flag. The base value component is either
undefined, an Object, a Boolean, a String, a
Symbol, a Number, a BigInt, or an
Environment Record. A base value component of undefined indicates
that the Reference could not be resolved to a binding. The
referenced name component is a String or Symbol value.
A Super Reference is a Reference
that is used to represent a name binding that was expressed using
the super
keyword. A
Super Reference
has an additional thisValue component, and its base value component
will never be an
Environment Record.
The following
abstract operations
are used in this specification to operate on references:
The object that may be created in step 5.a.ii is not
accessible outside of the above abstract operation and the
ordinary object [[Get]] internal method. An implementation
might choose to avoid the actual creation of the object.
The object that may be created in step 6.a.ii is not
accessible outside of the above algorithm and the ordinary
object [[Set]] internal method. An implementation might choose
to avoid the actual creation of that object.
The Property Descriptor type is used to explain the
manipulation and reification of Object property attributes. Values
of the Property Descriptor type are Records. Each field's name is an
attribute name and its value is a corresponding attribute value as
specified in
6.1.7.1. In addition, any field may be present or absent. The schema name
used within this specification to tag literal descriptions of
Property Descriptor records is “PropertyDescriptor”.
Property Descriptor values may be further classified as data
Property Descriptors and accessor Property Descriptors based upon
the existence or use of certain fields. A data Property Descriptor
is one that includes any fields named either [[Value]] or
[[Writable]]. An accessor Property Descriptor is one that includes
any fields named either [[Get]] or [[Set]]. Any Property Descriptor
may have fields named [[Enumerable]] and [[Configurable]]. A
Property Descriptor value may not be both a data Property Descriptor
and an accessor Property Descriptor; however, it may be neither. A
generic Property Descriptor is a Property Descriptor value that is
neither a data Property Descriptor nor an accessor Property
Descriptor. A fully populated Property Descriptor is one that is
either an accessor Property Descriptor or a data Property Descriptor
and that has all of the fields that correspond to the property
attributes defined in either
Table 3
or
Table 4.
The following
abstract operations
are used in this specification to operate upon Property Descriptor
values:
6.2.5.1IsAccessorDescriptor (
Desc )
When the abstract operation IsAccessorDescriptor is called with
Property DescriptorDesc, the following steps are taken:
If Desc is undefined, return
false.
If both Desc.[[Get]] and Desc.[[Set]]
are absent, return false.
Return true.
6.2.5.2IsDataDescriptor (
Desc )
When the abstract operation IsDataDescriptor is called with
Property DescriptorDesc, the following steps are taken:
If Desc is undefined, return
false.
If both Desc.[[Value]] and
Desc.[[Writable]] are absent, return
false.
Return true.
6.2.5.3IsGenericDescriptor (
Desc )
When the abstract operation IsGenericDescriptor is called with
Property DescriptorDesc, the following steps are taken:
Let like be the
Record
{ [[Value]]: undefined, [[Writable]]:
false, [[Get]]:
undefined, [[Set]]:
undefined, [[Enumerable]]:
false, [[Configurable]]:
false }.
If Desc does not have a [[Value]] field, set
Desc.[[Value]] to like.[[Value]].
If Desc does not have a [[Writable]] field, set
Desc.[[Writable]] to
like.[[Writable]].
Else,
If Desc does not have a [[Get]] field, set
Desc.[[Get]] to like.[[Get]].
If Desc does not have a [[Set]] field, set
Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field, set
Desc.[[Enumerable]] to
like.[[Enumerable]].
If Desc does not have a [[Configurable]] field, set
Desc.[[Configurable]] to
like.[[Configurable]].
Return Desc.
6.2.6The Lexical Environment and
Environment Record Specification Types
The
Lexical Environment
and
Environment Record
types are used to explain the behaviour of name resolution in nested
functions and blocks. These types and the operations upon them are
defined in
8.1.
6.2.7Data Blocks
The Data Block specification type is used to describe a
distinct and mutable sequence of byte-sized (8 bit) numeric values.
A Data Block value is created with a fixed number of bytes that each
have the initial value 0.
For notational convenience within this specification, an array-like
syntax can be used to access the individual bytes of a Data Block
value. This notation presents a Data Block value as a 0-origined
integer-indexed sequence of bytes. For example, if db is a 5
byte Data Block value then db[2] can be used to access
its 3rd byte.
A data block that resides in memory that can be referenced from
multiple agents concurrently is designated a
Shared Data Block. A Shared Data Block has an identity
(for the purposes of equality testing Shared Data Block values) that
is address-free: it is tied not to the virtual addresses
the block is mapped to in any process, but to the set of locations
in memory that the block represents. Two data blocks are equal only
if the sets of the locations they contain are equal; otherwise, they
are not equal and the intersection of the sets of locations they
contain is empty. Finally, Shared Data Blocks can be distinguished
from Data Blocks.
The semantics of Shared Data Blocks is defined using Shared Data
Block events by the
memory model.
Abstract operations
below introduce Shared Data Block events and act as the interface
between evaluation semantics and the event semantics of the
memory model. The events form a
candidate execution, on which the
memory model
acts as a filter. Please consult the
memory model
for full semantics.
Shared Data Block events are modeled by Records, defined in the
memory model.
The following
abstract operations
are used in this specification to operate upon Data Block values:
6.2.7.1CreateByteDataBlock (
size )
When the abstract operation CreateByteDataBlock is called with
integer
argument size, the following steps are taken:
Let db be a new
Shared Data Block
value consisting of size bytes. If it is impossible
to create such a
Shared Data Block, throw a RangeError exception.
Let eventList be the [[EventList]] field of
the element in execution.[[EventsRecords]]
whose [[AgentSignifier]] is
AgentSignifier().
Let bytes be a
List
of length 1 that contains a nondeterministically
chosen byte value.
NOTE: In implementations, bytes is the
result of a non-atomic read instruction on the
underlying hardware. The nondeterminism is a semantic
prescription of the
memory model
to describe observable behaviour of hardware with weak
consistency.
Let readEvent be
ReadSharedMemory
{ [[Order]]: Unordered,
[[NoTear]]: true, [[Block]]:
fromBlock, [[ByteIndex]]:
fromIndex, [[ElementSize]]: 1 }.
Append readEvent to eventList.
Append
Chosen Value Record
{ [[Event]]: readEvent, [[ChosenValue]]:
bytes } to
execution.[[ChosenValues]].
These operations are not a part of the ECMAScript language; they are
defined here to solely to aid the specification of the semantics of the
ECMAScript language. Other, more specialized
abstract operations
are defined throughout this specification.
7.1Type Conversion
The ECMAScript language implicitly performs automatic type conversion
as needed. To clarify the semantics of certain constructs it is useful
to define a set of conversion
abstract operations. The conversion
abstract operations
are polymorphic; they can accept a value of any
ECMAScript language type. But no other specification types are used with these operations.
The BigInt type has no implicit conversions in the ECMAScript
language; programmers must call BigInt explicitly to convert values
from other types.
7.1.1ToPrimitive ( input [ ,
PreferredType ] )
The abstract operation ToPrimitive takes an
input argument and an optional argument
PreferredType. The abstract operation ToPrimitive
converts its input argument to a non-Object type. If an
object is capable of converting to more than one primitive type, it
may use the optional hint PreferredType to favour that
type. Conversion occurs according to the following algorithm:
When ToPrimitive is called with no hint, then it generally
behaves as if the hint were Number. However, objects may
over-ride this behaviour by defining a @@toPrimitive method. Of
the objects defined in this specification only Date objects (see
20.4.4.45) and Symbol objects (see
19.4.3.5) over-ride the default ToPrimitive behaviour. Date objects
treat no hint as if the hint were String.
7.1.1.1OrdinaryToPrimitive (
O, hint )
When the abstract operation OrdinaryToPrimitive is called with
arguments O and hint, the following steps
are taken:
ToNumber
applied to Strings applies the following grammar to the input
String interpreted as a sequence of UTF-16 encoded code points
(6.1.4). If the grammar cannot interpret the String as an expansion of
StringNumericLiteral, then the result of
ToNumber
is NaN.
Note 1
The terminal symbols of this grammar are all composed of
characters in the Unicode Basic Multilingual Plane (BMP).
Therefore, the result of
ToNumber
will be NaN if the string contains any
leading surrogate
or
trailing surrogate
code units, whether paired or unpaired.
The conversion of a String to a
Number value
is similar overall to the determination of the
Number value
for a numeric literal (see
11.8.3), but some of the details are different, so the process for
converting a String numeric literal to a value of Number type is
given here. This value is determined in two steps: first, a
mathematical value
(MV) is derived from the String numeric literal; second, this
mathematical value
is rounded as described below. The MV on any grammar symbol, not
provided below, is the MV for that symbol defined in
11.8.3.1.
Once the exact MV for a String numeric literal has been
determined, it is then rounded to a value of the Number type. If
the MV is 0, then the rounded value is
+0 unless the first non white space code
point in the String numeric literal is -, in which
case the rounded value is -0. Otherwise, the
rounded value must be the
Number value
for the MV (in the sense defined in
6.1.6.1), unless the literal includes a
StrUnsignedDecimalLiteral
and the literal has more than 20 significant digits, in which
case the
Number value
may be either the
Number value
for the MV of a literal produced by replacing each significant
digit after the 20th with a 0 digit or the
Number value
for the MV of a literal produced by replacing each significant
digit after the 20th with a 0 digit and then incrementing the
literal at the 20th digit position. A digit is significant if it
is not part of an
ExponentPart
and
it is not 0; or
there is a nonzero digit to its left and there is a nonzero
digit, not in the
ExponentPart, to its right.
7.1.5ToInteger ( argument )
The abstract operation ToInteger converts argument to an
integral
Number value. This abstract operation functions as follows:
Return the
Number value
that is the same sign as number and whose magnitude
is
floor(abs(number)).
7.1.6ToInt32 ( argument )
The abstract operation ToInt32 converts argument to one
of 232integer
values in the range
-231 through
231 - 1, inclusive.
This abstract operation functions as follows:
The ToInt32 abstract operation is idempotent: if applied to a
result that it produced, the second application leaves that
value unchanged.
ToInt32(ToUint32(x)) is equal to ToInt32(x) for all
values of x. (It is to preserve this latter
property that +∞ and
-∞ are mapped to +0.)
ToInt32 maps -0 to +0.
7.1.7ToUint32 ( argument )
The abstract operation ToUint32 converts argument to one
of 232integer
values in the range 0 through
232 - 1, inclusive.
This abstract operation functions as follows:
Step 5 is the only difference between ToUint32 and
ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a
result that it produced, the second application leaves that
value unchanged.
ToUint32(ToInt32(x)) is equal to ToUint32(x) for all
values of x. (It is to preserve this latter
property that +∞ and
-∞ are mapped to +0.)
ToUint32 maps -0 to +0.
7.1.8ToInt16 ( argument )
The abstract operation ToInt16 converts argument to one
of 216integer
values in the range -32768 through 32767, inclusive. This abstract
operation functions as follows:
The abstract operation ToUint16 converts argument to one
of 216integer
values in the range 0 through
216 - 1, inclusive.
This abstract operation functions as follows:
The substitution of 216 for 232 in step
4 is the only difference between
ToUint32
and ToUint16.
ToUint16 maps -0 to +0.
7.1.10ToInt8 ( argument )
The abstract operation ToInt8 converts argument to one of
28integer
values in the range -128 through 127, inclusive. This abstract
operation functions as follows:
The abstract operation ToUint8 converts argument to one
of 28integer
values in the range 0 through 255, inclusive. This abstract
operation functions as follows:
The abstract operation ToUint8Clamp converts argument to
one of 28integer
values in the range 0 through 255, inclusive. This abstract
operation functions as follows:
Unlike the other ECMAScript
integer
conversion abstract operation, ToUint8Clamp rounds rather than
truncates non-integer
values and does not convert +∞ to 0.
ToUint8Clamp does “round half to even” tie-breaking. This
differs from Math.round which does “round half up”
tie-breaking.
7.1.13ToBigInt ( argument )
The abstract operation ToBigInt converts its argument
argument to a BigInt value, or throws if an implicit
conversion from Number would be required.
If the MV is NaN, return
NaN, otherwise return the BigInt which exactly
corresponds to the MV, rather than rounding to a Number.
7.1.15ToBigInt64 ( argument )
The abstract operation ToBigInt64 converts argument to
one of 264integer
values in the range -263 through 263-1,
inclusive. This abstract operation functions as follows:
The abstract operation ToBigUint64 converts argument to
one of 264integer
values in the range 0 through 264-1, inclusive. This
abstract operation functions as follows:
The abstract operation ToLength converts argument to an
integer
suitable for use as the length of an
array-like object. It performs the following steps:
The abstract operation CanonicalNumericIndexString returns
argument converted to a
Number value
if it is a String representation of a Number that would be produced
by
ToString, or the string "-0". Otherwise, it returns
undefined. This abstract operation functions as
follows:
A canonical numeric string is any String value for which
the CanonicalNumericIndexString abstract operation does not return
undefined.
7.1.22ToIndex ( value )
The abstract operation ToIndex returns value argument
converted to a non-negative
integer
if it is a valid
integer index
value. This abstract operation functions as follows:
If
SameValueZero(integerIndex, index) is
false, throw a
RangeError exception.
Return index.
7.2Testing and Comparison Operations
7.2.1RequireObjectCoercible (
argument )
The abstract operation RequireObjectCoercible throws an error if
argument is a value that cannot be converted to an Object
using
ToObject. It is defined by
Table 15:
The abstract operation IsCallable determines if argument,
which must be an
ECMAScript language value, is a callable function with a [[Call]] internal method.
If argument has a [[Call]] internal method, return
true.
Return false.
7.2.4IsConstructor (
argument )
The abstract operation IsConstructor determines if
argument, which must be an
ECMAScript language value, is a
function object
with a [[Construct]] internal method.
If argument has a [[Construct]] internal method,
return true.
Return false.
7.2.5IsExtensible ( O )
The abstract operation IsExtensible is used to determine whether
additional properties can be added to the object that is
O. A Boolean value is returned. This abstract operation
performs the following steps:
If
floor(abs(argument)) ≠
abs(argument), return false.
Return true.
7.2.7IsNonNegativeInteger (
argument )
The abstract operation IsNonNegativeInteger determines if
argument is non-negative
integerNumber value.
If ! IsInteger(argument) is true and
argument ≥ 0, return true.
Otherwise, return false.
7.2.8IsPropertyKey (
argument )
The abstract operation IsPropertyKey determines if
argument, which must be an
ECMAScript language value, is a value that may be used as a property key.
If q can be the
string-concatenation
of p and some other String r, return
true. Otherwise, return
false.
NOTE: Any String is a prefix of itself, because r may
be the empty String.
7.2.11SameValue ( x,
y )
The internal comparison abstract operation SameValue(x,
y), where x and y are ECMAScript
language values, produces true or
false. Such a comparison is performed as follows:
If
Type(x) is different from
Type(y), return false.
This algorithm differs from the
Strict Equality Comparison
Algorithm in its treatment of signed zeroes and NaNs.
7.2.12SameValueZero ( x,
y )
The internal comparison abstract operation
SameValueZero(x, y), where x and
y are ECMAScript language values, produces
true or false. Such a
comparison is performed as follows:
If
Type(x) is different from
Type(y), return false.
SameValueZero differs from
SameValue
only in its treatment of +0 and
-0.
7.2.13SameValueNonNumeric (
x, y )
The internal comparison abstract operation
SameValueNonNumeric(x, y), where neither
x nor y are numeric type values, produces
true or false. Such a
comparison is performed as follows:
If x and y are exactly the same
sequence of code units (same length and same code units at
corresponding indices), return true;
otherwise, return false.
If x and y are both the same Symbol
value, return true; otherwise, return
false.
If x and y are the same Object value,
return true. Otherwise, return
false.
7.2.14Abstract Relational Comparison
The comparison x < y, where
x and y are values, produces
true, false, or
undefined (which indicates that at least one
operand is NaN). In addition to x and
y the algorithm takes a Boolean flag named
LeftFirst as a parameter. The flag is used to control the
order in which operations with potentially visible side-effects are
performed upon x and y. It is necessary
because ECMAScript specifies left to right evaluation of
expressions. The default value of LeftFirst is
true and indicates that the
x parameter corresponds to an expression that occurs to
the left of the y parameter's corresponding expression.
If LeftFirst is false, the reverse is
the case and operations must be performed upon y before
x. Such a comparison is performed as follows:
Let k be the smallest nonnegative
integer
such that the code unit at index k within
px is different from the code unit at index
k within py. (There must be such a
k, for neither String is a prefix of the other.)
Let m be the
integer
that is the numeric value of the code unit at index
k within px.
Let n be the
integer
that is the numeric value of the code unit at index
k within py.
If m < n, return
true. Otherwise, return
false.
Else,
If
Type(px) is BigInt and
Type(py) is String, then
Step 3 differs from step 7 in the algorithm for the addition
operator + (12.8.3) by using the logical-and operation instead of the logical-or
operation.
Note 2
The comparison of Strings uses a simple lexicographic ordering
on sequences of code unit values. There is no attempt to use the
more complex, semantically oriented definitions of character or
string equality and collating order defined in the Unicode
specification. Therefore String values that are canonically
equal according to the Unicode standard could test as unequal.
In effect this algorithm assumes that both Strings are already
in normalized form. Also, note that for strings containing
supplementary characters, lexicographic ordering on sequences of
UTF-16 code unit values differs from that on sequences of code
point values.
7.2.15Abstract Equality Comparison
The comparison x == y, where x and
y are values, produces true or
false. Such a comparison is performed as follows:
This algorithm differs from the
SameValue
Algorithm in its treatment of signed zeroes and NaNs.
7.3Operations on Objects
7.3.1Get ( O, P )
The abstract operation Get is used to retrieve the value of a
specific property of an object. The operation is called with
arguments O and P where O is the
object and P is the property key. This abstract operation
performs the following steps:
The abstract operation GetV is used to retrieve the value of a
specific property of an
ECMAScript language value. If the value is not an object, the property lookup is performed
using a wrapper object appropriate for the type of the value. The
operation is called with arguments V and
P where V is the value and P is the
property key. This abstract operation performs the following steps:
The abstract operation Set is used to set the value of a specific
property of an object. The operation is called with arguments
O, P, V, and Throw where
O is the object, P is the property key,
V is the new value for the property and
Throw is a Boolean flag. This abstract operation performs
the following steps:
If success is false and
Throw is true, throw a
TypeError exception.
Return success.
7.3.4CreateDataProperty ( O,
P, V )
The abstract operation CreateDataProperty is used to create a new
own property of an object. The operation is called with arguments
O, P, and V where O is
the object, P is the property key, and V is
the value for the property. This abstract operation performs the
following steps:
Let newDesc be the PropertyDescriptor { [[Value]]:
V, [[Writable]]: true,
[[Enumerable]]: true, [[Configurable]]:
true }.
Return ? O.[[DefineOwnProperty]](P,
newDesc).
Note
This abstract operation creates a property whose attributes are
set to the same defaults used for properties created by the
ECMAScript language assignment operator. Normally, the property
will not already exist. If it does exist and is not configurable
or if O is not extensible, [[DefineOwnProperty]] will
return false.
7.3.5CreateMethodProperty (
O, P, V )
The abstract operation CreateMethodProperty is used to create a new
own property of an object. The operation is called with arguments
O, P, and V where O is
the object, P is the property key, and V is
the value for the property. This abstract operation performs the
following steps:
Let newDesc be the PropertyDescriptor { [[Value]]:
V, [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]:
true }.
Return ? O.[[DefineOwnProperty]](P,
newDesc).
Note
This abstract operation creates a property whose attributes are
set to the same defaults used for built-in methods and methods
defined using class declaration syntax. Normally, the property
will not already exist. If it does exist and is not configurable
or if O is not extensible, [[DefineOwnProperty]] will
return false.
7.3.6CreateDataPropertyOrThrow (
O, P, V )
The abstract operation CreateDataPropertyOrThrow is used to create a
new own property of an object. It throws a
TypeError exception if the requested property
update cannot be performed. The operation is called with arguments
O, P, and V where O is
the object, P is the property key, and V is
the value for the property. This abstract operation performs the
following steps:
This abstract operation creates a property whose attributes are
set to the same defaults used for properties created by the
ECMAScript language assignment operator. Normally, the property
will not already exist. If it does exist and is not configurable
or if O is not extensible, [[DefineOwnProperty]] will
return false causing this operation to throw
a TypeError exception.
7.3.7DefinePropertyOrThrow (
O, P, desc )
The abstract operation DefinePropertyOrThrow is used to call the
[[DefineOwnProperty]] internal method of an object in a manner that
will throw a TypeError exception if the requested
property update cannot be performed. The operation is called with
arguments O, P, and desc where
O is the object, P is the property key, and
desc is the
Property Descriptor
for the property. This abstract operation performs the following
steps:
Let success be ?
O.[[DefineOwnProperty]](P,
desc).
If success is false, throw a
TypeError exception.
Return success.
7.3.8DeletePropertyOrThrow (
O, P )
The abstract operation DeletePropertyOrThrow is used to remove a
specific own property of an object. It throws an exception if the
property is not configurable. The operation is called with arguments
O and P where O is the object and
P is the property key. This abstract operation performs
the following steps:
The abstract operation GetMethod is used to get the value of a
specific property of an
ECMAScript language value
when the value of the property is expected to be a function. The
operation is called with arguments V and
P where V is the
ECMAScript language value, P is the property key. This abstract operation
performs the following steps:
If func is either undefined or
null, return undefined.
If
IsCallable(func) is false, throw a
TypeError exception.
Return func.
7.3.10HasProperty ( O,
P )
The abstract operation HasProperty is used to determine whether an
object has a property with the specified property key. The property
may be either an own or inherited. A Boolean value is returned. The
operation is called with arguments O and
P where O is the object and P is
the property key. This abstract operation performs the following
steps:
The abstract operation HasOwnProperty is used to determine whether
an object has an own property with the specified property key. A
Boolean value is returned. The operation is called with arguments
O and P where O is the object and
P is the property key. This abstract operation performs
the following steps:
The abstract operation Call is used to call the [[Call]] internal
method of a
function object. The operation is called with arguments F,
V, and optionally argumentsList where
F is the
function object, V is an
ECMAScript language value
that is the this value of the [[Call]], and
argumentsList is the value passed to the corresponding
argument of the internal method. If argumentsList is not
present, a new empty
List
is used as its value. This abstract operation performs the following
steps:
If argumentsList is not present, set
argumentsList to a new empty
List.
If
IsCallable(F) is false, throw a
TypeError exception.
The abstract operation Construct is used to call the [[Construct]]
internal method of a
function object. The operation is called with arguments F, and
optionally argumentsList, and newTarget where
F is the
function object. argumentsList and newTarget are the values
to be passed as the corresponding arguments of the internal method.
If argumentsList is not present, a new empty
List
is used as its value. If newTarget is not present,
F is used as its value. This abstract operation performs
the following steps:
If newTarget is not present, set
newTarget to F.
If argumentsList is not present, set
argumentsList to a new empty
List.
The abstract operation TestIntegrityLevel is used to determine if
the set of own properties of an object are fixed. This abstract
operation performs the following steps:
NOTE: If the object is extensible, none of its properties are
examined.
Let keys be ? O.[[OwnPropertyKeys]]().
For each element k of keys, do
Let currentDesc be ?
O.[[GetOwnProperty]](k).
If currentDesc is not
undefined, then
If currentDesc.[[Configurable]] is
true, return
false.
If level is frozen and
IsDataDescriptor(currentDesc) is true,
then
If currentDesc.[[Writable]] is
true, return
false.
Return true.
7.3.16CreateArrayFromList (
elements )
The abstract operation CreateArrayFromList is used to create an
Array object whose elements are provided by a
List. This abstract operation performs the following steps:
Assert: elements is a
List
whose elements are all ECMAScript language values.
The abstract operation CreateListFromArrayLike is used to create a
List
value whose elements are provided by the indexed properties of an
array-like object, obj. The optional argument elementTypes is
a
List
containing the names of ECMAScript Language Types that are allowed
for element values of the
List
that is created. This abstract operation performs the following
steps:
If elementTypes is not present, set
elementTypes to « Undefined, Null, Boolean, String,
Symbol, Number, BigInt, Object ».
If
Type(obj) is not Object, throw a
TypeError exception.
If
Type(next) is not an element of
elementTypes, throw a
TypeError exception.
Append next as the last element of
list.
Set index to index + 1.
Return list.
7.3.19Invoke ( V,
P [ , argumentsList ] )
The abstract operation Invoke is used to call a method property of
an
ECMAScript language value. The operation is called with arguments V,
P, and optionally argumentsList where
V serves as both the lookup point for the property and
the this value of the call, P is the
property key, and argumentsList is the list of arguments
values passed to the method. If argumentsList is not
present, a new empty
List
is used as its value. This abstract operation performs the following
steps:
The abstract operation OrdinaryHasInstance implements the default
algorithm for determining if an object O inherits from
the instance object inheritance path provided by
constructorC. This abstract operation performs the following steps:
7.3.21SpeciesConstructor ( O,
defaultConstructor )
The abstract operation SpeciesConstructor is used to retrieve the
constructor
that should be used to create new objects that are derived from the
argument object O. The
defaultConstructor argument is the
constructor
to use if a
constructor
@@species property cannot be found starting from O. This
abstract operation performs the following steps:
When the abstract operation EnumerableOwnPropertyNames is called
with an Object O and kind which is one of
(key, value,
key+value), the following steps are taken:
The abstract operation IteratorStep with argument
iteratorRecord requests the next value from
iteratorRecord.[[Iterator]] by calling
iteratorRecord.[[NextMethod]] and returns either
false indicating that the iterator has reached
its end or the IteratorResult object if a next value is available.
IteratorStep performs the following steps:
The abstract operation IteratorClose with arguments
iteratorRecord and completion is used to
notify an iterator that it should perform any actions it would
normally perform when it has reached its completed state:
Assert:
Type(iteratorRecord.[[Iterator]]) is Object.
The abstract operation AsyncIteratorClose with arguments
iteratorRecord and completion is used to
notify an async iterator that it should perform any actions it would
normally perform when it has reached its completed state:
Assert:
Type(iteratorRecord.[[Iterator]]) is Object.
The abstract operation CreateIterResultObject with arguments
value and done creates an object that supports
the IteratorResult interface by performing the following steps:
The abstract operation CreateListIteratorRecord with argument
list creates an Iterator (25.1.1.2) object record whose next method returns the successive elements
of list. It performs the following steps:
The "length" property of a ListIteratorNext
function is 0.
8Executable Code and Execution Contexts
8.1Lexical Environments
A Lexical Environment is a specification type used to
define the association of
Identifiers to specific variables and functions based upon the lexical nesting
structure of ECMAScript code. A Lexical Environment consists of an
Environment Record
and a possibly null reference to an outer Lexical
Environment. Usually a Lexical Environment is associated with some
specific syntactic structure of ECMAScript code such as a
FunctionDeclaration, a
BlockStatement, or a
Catch clause
of a
TryStatement
and a new Lexical Environment is created each time such code is
evaluated.
An
Environment Record
records the identifier bindings that are created within the scope of
its associated Lexical Environment. It is referred to as the Lexical
Environment's EnvironmentRecord.
The outer environment reference is used to model the logical nesting
of Lexical Environment values. The outer reference of a (inner)
Lexical Environment is a reference to the Lexical Environment that
logically surrounds the inner Lexical Environment. An outer Lexical
Environment may, of course, have its own outer Lexical Environment. A
Lexical Environment may serve as the outer environment for multiple
inner Lexical Environments. For example, if a
FunctionDeclaration
contains two nested
FunctionDeclarations then the Lexical Environments of each of the nested functions will
have as their outer Lexical Environment the Lexical Environment of the
current evaluation of the surrounding function.
A global environment is a Lexical
Environment which does not have an outer environment. The
global environment's outer environment reference is null. A
global environment's EnvironmentRecord may be prepopulated with identifier bindings and
includes an associated
global object
whose properties provide some of the
global environment's identifier bindings. As ECMAScript code is executed, additional
properties may be added to the
global object
and the initial properties may be modified.
A module environment is a Lexical
Environment that contains the bindings for the top level declarations
of a Module. It also contains the bindings that are explicitly imported by the
Module. The outer environment of a
module environment
is a
global environment.
A function environment is a
Lexical Environment that corresponds to the invocation of an
ECMAScript
function object. A
function environment
may establish a new this binding. A
function environment
also captures the state necessary to support super method
invocations.
Lexical Environments and
Environment Record
values are purely specification mechanisms and need not correspond to
any specific artefact of an ECMAScript implementation. It is
impossible for an ECMAScript program to directly access or manipulate
such values.
8.1.1Environment Records
There are two primary kinds of Environment Record values
used in this specification:
declarative Environment Records and
object Environment Records. Declarative Environment Records
are used to define the effect of ECMAScript language syntactic
elements such as
FunctionDeclarations,
VariableDeclarations, and
Catch
clauses that directly associate identifier bindings with ECMAScript
language values. Object Environment Records are used to define the
effect of ECMAScript elements such as
WithStatement
that associate identifier bindings with the properties of some
object. Global Environment Records and function Environment Records
are specializations that are used for specifically for
Script
global declarations and for top-level declarations within functions.
For specification purposes Environment Record values are values of
the
Record
specification type and can be thought of as existing in a simple
object-oriented hierarchy where Environment Record is an abstract
class with three concrete subclasses, declarative Environment
Record, object Environment Record, and global Environment Record.
Function Environment Records and module Environment Records are
subclasses of declarative Environment Record. The abstract class
includes the abstract specification methods defined in
Table 16. These abstract methods have distinct concrete algorithms for each
of the concrete subclasses.
Table 16: Abstract Methods of Environment Records
Method
Purpose
HasBinding(N)
Determine if an
Environment Record
has a binding for the String value N. Return
true if it does and
false if it does not.
CreateMutableBinding(N, D)
Create a new but uninitialized mutable binding in an
Environment Record. The String value N is the text of the bound
name. If the Boolean argument D is
true the binding may be subsequently
deleted.
CreateImmutableBinding(N, S)
Create a new but uninitialized immutable binding in an
Environment Record. The String value N is the text of the bound
name. If S is true then
attempts to set it after it has been initialized will
always throw an exception, regardless of the strict mode
setting of operations that reference that binding.
InitializeBinding(N, V)
Set the value of an already existing but uninitialized
binding in an
Environment Record. The String value N is the text of the bound
name. V is the value for the binding and is a
value of any
ECMAScript language type.
SetMutableBinding(N, V, S)
Set the value of an already existing mutable binding in an
Environment Record. The String value N is the text of the bound
name. V is the value for the binding and may be
a value of any
ECMAScript language type. S is a Boolean flag. If S is
true and the binding cannot be set
throw a TypeError exception.
GetBindingValue(N, S)
Returns the value of an already existing binding from an
Environment Record. The String value N is the text of the bound
name. S is used to identify references
originating in
strict mode code
or that otherwise require strict mode reference semantics.
If S is true and the binding
does not exist throw a
ReferenceError exception. If the
binding exists but is uninitialized a
ReferenceError is thrown, regardless of
the value of S.
DeleteBinding(N)
Delete a binding from an
Environment Record. The String value N is the text of the bound
name. If a binding for N exists, remove the
binding and return true. If the binding
exists but cannot be removed return
false. If the binding does not exist
return true.
HasThisBinding()
Determine if an
Environment Record
establishes a this binding. Return
true if it does and
false if it does not.
HasSuperBinding()
Determine if an
Environment Record
establishes a super method binding. Return
true if it does and
false if it does not.
WithBaseObject()
If this
Environment Record
is associated with a with statement, return
the with object. Otherwise, return
undefined.
8.1.1.1Declarative Environment Records
Each declarative
Environment Record
is associated with an ECMAScript program scope containing
variable, constant, let, class, module, import, and/or function
declarations. A declarative
Environment Record
binds the set of identifiers defined by the declarations contained
within its scope.
The behaviour of the concrete specification methods for
declarative Environment Records is defined by the following
algorithms.
8.1.1.1.1HasBinding ( N )
The concrete
Environment Record
method HasBinding for declarative Environment Records simply
determines if the argument identifier is one of the identifiers
bound by the record:
Let envRec be the declarative
Environment Record
for which the method was invoked.
If envRec has a binding for the name that is the
value of N, return true.
Return false.
8.1.1.1.2CreateMutableBinding (
N, D )
The concrete
Environment Record
method CreateMutableBinding for declarative Environment Records
creates a new mutable binding for the name N that is
uninitialized. A binding must not already exist in this
Environment Record
for N. If Boolean argument D has the value
true the new binding is marked as being
subject to deletion.
Let envRec be the declarative
Environment Record
for which the method was invoked.
Assert: envRec does not already have a binding for
N.
Create a mutable binding in envRec for
N and record that it is uninitialized. If
D is true, record that the
newly created binding may be deleted by a subsequent
DeleteBinding call.
The concrete
Environment Record
method CreateImmutableBinding for declarative Environment
Records creates a new immutable binding for the name
N that is uninitialized. A binding must not already
exist in this
Environment Record
for N. If the Boolean argument S has the
value true the new binding is marked as a
strict binding.
Let envRec be the declarative
Environment Record
for which the method was invoked.
Assert: envRec does not already have a binding for
N.
Create an immutable binding in envRec for
N and record that it is uninitialized. If
S is true, record that the
newly created binding is a strict binding.
The concrete
Environment Record
method InitializeBinding for declarative Environment Records is
used to set the bound value of the current binding of the
identifier whose name is the value of the argument
N to the value of argument V. An
uninitialized binding for N must already exist.
Let envRec be the declarative
Environment Record
for which the method was invoked.
Assert: envRec must have an uninitialized binding for
N.
Set the bound value for N in envRec to
V.
Record
that the binding for N in envRec has
been initialized.
The concrete
Environment Record
method SetMutableBinding for declarative Environment Records
attempts to change the bound value of the current binding of the
identifier whose name is the value of the argument
N to the value of argument V. A binding
for N normally already exists, but in rare cases it
may not. If the binding is an immutable binding, a
TypeError is thrown if S is
true.
Let envRec be the declarative
Environment Record
for which the method was invoked.
An example of ECMAScript code that results in a missing
binding at step 2 is:
functionf() { eval("var x; x = (delete x, 0);"); }
8.1.1.1.6GetBindingValue (
N, S )
The concrete
Environment Record
method GetBindingValue for declarative Environment Records
simply returns the value of its bound identifier whose name is
the value of the argument N. If the binding exists
but is uninitialized a ReferenceError is
thrown, regardless of the value of S.
Let envRec be the declarative
Environment Record
for which the method was invoked.
If the binding for N in envRec is an
uninitialized binding, throw a
ReferenceError exception.
Return the value currently bound to N in
envRec.
8.1.1.1.7DeleteBinding (
N )
The concrete
Environment Record
method DeleteBinding for declarative Environment Records can
only delete bindings that have been explicitly designated as
being subject to deletion.
Let envRec be the declarative
Environment Record
for which the method was invoked.
Assert: envRec has a binding for the name that is the
value of N.
If the binding for N in envRec cannot
be deleted, return false.
Remove the binding for N from envRec.
Return true.
8.1.1.1.8HasThisBinding ( )
Regular declarative Environment Records do not provide a
this binding.
Return false.
8.1.1.1.9HasSuperBinding ( )
Regular declarative Environment Records do not provide a
super binding.
Return false.
8.1.1.1.10WithBaseObject ( )
Declarative Environment Records always return
undefined as their WithBaseObject.
Return undefined.
8.1.1.2Object Environment Records
Each object
Environment Record
is associated with an object called its binding object.
An object
Environment Record
binds the set of string identifier names that directly correspond
to the property names of its binding object. Property keys that
are not strings in the form of an
IdentifierName
are not included in the set of bound identifiers. Both own and
inherited properties are included in the set regardless of the
setting of their [[Enumerable]] attribute. Because properties can
be dynamically added and deleted from objects, the set of
identifiers bound by an object
Environment Record
may potentially change as a side-effect of any operation that adds
or deletes properties. Any bindings that are created as a result
of such a side-effect are considered to be a mutable binding even
if the Writable attribute of the corresponding property has the
value false. Immutable bindings do not exist
for object Environment Records.
Object Environment Records created for
with statements (13.11) can provide their binding object as an implicit
this value for use in function calls. The
capability is controlled by a withEnvironment Boolean
value that is associated with each object
Environment Record. By default, the value of withEnvironment is
false for any object
Environment Record.
The behaviour of the concrete specification methods for object
Environment Records is defined by the following algorithms.
8.1.1.2.1HasBinding ( N )
The concrete
Environment Record
method HasBinding for object Environment Records determines if
its associated binding object has a property whose name is the
value of the argument N:
Let envRec be the object
Environment Record
for which the method was invoked.
Let blocked be ! ToBoolean(?
Get(unscopables, N)).
If blocked is true, return
false.
Return true.
8.1.1.2.2CreateMutableBinding (
N, D )
The concrete
Environment Record
method CreateMutableBinding for object Environment Records
creates in an
Environment Record's associated binding object a property whose name is the
String value and initializes it to the value
undefined. If Boolean argument
D has the value true the new
property's [[Configurable]] attribute is set to
true; otherwise it is set to
false.
Let envRec be the object
Environment Record
for which the method was invoked.
Let bindings be the binding object for
envRec.
Return ? DefinePropertyOrThrow(bindings, N, PropertyDescriptor {
[[Value]]: undefined, [[Writable]]:
true, [[Enumerable]]:
true, [[Configurable]]: D }).
Note
Normally envRec will not have a binding for
N but if it does, the semantics of
DefinePropertyOrThrow
may result in an existing binding being replaced or shadowed
or cause an
abrupt completion
to be returned.
8.1.1.2.3CreateImmutableBinding (
N, S )
The concrete
Environment Record
method CreateImmutableBinding is never used within this
specification in association with object Environment Records.
8.1.1.2.4InitializeBinding (
N, V )
The concrete
Environment Record
method InitializeBinding for object Environment Records is used
to set the bound value of the current binding of the identifier
whose name is the value of the argument N to the
value of argument V. An uninitialized binding for
N must already exist.
Let envRec be the object
Environment Record
for which the method was invoked.
Assert: envRec must have an uninitialized binding for
N.
Record
that the binding for N in envRec has
been initialized.
Return ? envRec.SetMutableBinding(N,
V, false).
Note
In this specification, all uses of CreateMutableBinding for
object Environment Records are immediately followed by a
call to InitializeBinding for the same name. Hence,
implementations do not need to explicitly track the
initialization state of individual object
Environment Record
bindings.
8.1.1.2.5SetMutableBinding (
N, V, S )
The concrete
Environment Record
method SetMutableBinding for object Environment Records attempts
to set the value of the
Environment Record's associated binding object's property whose name is the value
of the argument N to the value of argument
V. A property named N normally already
exists but if it does not or is not currently writable, error
handling is determined by the value of the Boolean argument
S.
Let envRec be the object
Environment Record
for which the method was invoked.
The concrete
Environment Record
method GetBindingValue for object Environment Records returns
the value of its associated binding object's property whose name
is the String value of the argument identifier N. The
property should already exist but if it does not the result
depends upon the value of the S argument:
Let envRec be the object
Environment Record
for which the method was invoked.
The concrete
Environment Record
method DeleteBinding for object Environment Records can only
delete bindings that correspond to properties of the environment
object whose [[Configurable]] attribute have the value
true.
Let envRec be the object
Environment Record
for which the method was invoked.
Let bindings be the binding object for
envRec.
Return ? bindings.[[Delete]](N).
8.1.1.2.8HasThisBinding ( )
Regular object Environment Records do not provide a
this binding.
Return false.
8.1.1.2.9HasSuperBinding ( )
Regular object Environment Records do not provide a
super binding.
Return false.
8.1.1.2.10WithBaseObject ( )
Object Environment Records return
undefined as their WithBaseObject unless
their withEnvironment flag is
true.
Let envRec be the object
Environment Record
for which the method was invoked.
If the withEnvironment flag of
envRec is true, return the
binding object for envRec.
Otherwise, return undefined.
8.1.1.3Function Environment Records
A function Environment Record is a declarative
Environment Record
that is used to represent the top-level scope of a function and,
if the function is not an
ArrowFunction, provides a this binding. If a function is not an
ArrowFunction
function and references super, its function
Environment Record also contains the state that is used to perform
super method invocations from within the function.
Function Environment Records have the additional state fields
listed in
Table 17.
Table 17: Additional Fields of Function Environment Records
Field Name
Value
Meaning
[[ThisValue]]
Any
This is the this value used for this
invocation of the function.
[[ThisBindingStatus]]
lexical |
initialized |
uninitialized
If the value is lexical, this is
an
ArrowFunction
and does not have a local this value.
If the associated function has
super property accesses and is not an
ArrowFunction, [[HomeObject]] is the object that the function is
bound to as a method. The default value for
[[HomeObject]] is undefined.
[[NewTarget]]
Object | undefined
If this
Environment Record
was created by the [[Construct]] internal method,
[[NewTarget]] is the value of the [[Construct]]
newTarget parameter. Otherwise, its value is
undefined.
Function Environment Records support all of the declarative
Environment Record
methods listed in
Table 16
and share the same specifications for all of those methods except
for HasThisBinding and HasSuperBinding. In addition, function
Environment Records support the methods listed in
Table 18:
Table 18: Additional Methods of Function Environment Records
Method
Purpose
BindThisValue(V)
Set the [[ThisValue]] and record that it has been
initialized.
GetThisBinding()
Return the value of this
Environment Record's this binding. Throws a
ReferenceError if the
this binding has not been initialized.
GetSuperBase()
Return the object that is the base for
super property accesses bound in this
Environment Record. The object is derived from this
Environment Record's [[HomeObject]] field. The value
undefined indicates that
super property accesses will produce
runtime errors.
The behaviour of the additional concrete specification methods for
function Environment Records is defined by the following
algorithms:
A global
Environment Record
is used to represent the outer most scope that is shared by all of
the ECMAScript
Script
elements that are processed in a common
realm. A global
Environment Record
provides the bindings for built-in globals (clause
18), properties of the
global object, and for all top-level declarations (13.2.8,
13.2.10) that occur within a
Script.
Determines if the argument is the name of a
global object
property that may not be shadowed by a global lexical
binding.
CanDeclareGlobalVar (N)
Determines if a corresponding CreateGlobalVarBinding
call would succeed if called for the same argument
N.
CanDeclareGlobalFunction (N)
Determines if a corresponding
CreateGlobalFunctionBinding call would succeed if called
for the same argument N.
CreateGlobalVarBinding(N, D)
Used to create and initialize to
undefined a global
var binding in the [[ObjectRecord]]
component of a global
Environment Record. The binding will be a mutable binding. The
corresponding
global object
property will have attribute values appropriate for a
var. The String value N is the
bound name. If D is
true the binding may be deleted.
Logically equivalent to CreateMutableBinding followed by
a SetMutableBinding but it allows var declarations to
receive special treatment.
CreateGlobalFunctionBinding(N, V, D)
Create and initialize a global
function binding in the [[ObjectRecord]]
component of a global
Environment Record. The binding will be a mutable binding. The
corresponding
global object
property will have attribute values appropriate for a
function. The String value N is
the bound name. V is the initialization
value. If the Boolean argument D is
true the binding may be deleted.
Logically equivalent to CreateMutableBinding followed by
a SetMutableBinding but it allows function declarations
to receive special treatment.
The behaviour of the concrete specification methods for global
Environment Records is defined by the following algorithms.
8.1.1.4.1HasBinding ( N )
The concrete
Environment Record
method HasBinding for global Environment Records simply
determines if the argument identifier is one of the identifiers
bound by the record:
Let envRec be the global
Environment Record
for which the method was invoked.
Let DclRec be
envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is
true, return true.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.HasBinding(N).
8.1.1.4.2CreateMutableBinding (
N, D )
The concrete
Environment Record
method CreateMutableBinding for global Environment Records
creates a new mutable binding for the name N that is
uninitialized. The binding is created in the associated
DeclarativeRecord. A binding for N must not already
exist in the DeclarativeRecord. If Boolean argument
D has the value true the new
binding is marked as being subject to deletion.
Let envRec be the global
Environment Record
for which the method was invoked.
Let DclRec be
envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is
true, throw a
TypeError exception.
Return DclRec.CreateMutableBinding(N,
D).
8.1.1.4.3CreateImmutableBinding (
N, S )
The concrete
Environment Record
method CreateImmutableBinding for global Environment Records
creates a new immutable binding for the name N that
is uninitialized. A binding must not already exist in this
Environment Record
for N. If the Boolean argument S has the
value true the new binding is marked as a
strict binding.
Let envRec be the global
Environment Record
for which the method was invoked.
Let DclRec be
envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is
true, throw a
TypeError exception.
Return
DclRec.CreateImmutableBinding(N,
S).
8.1.1.4.4InitializeBinding (
N, V )
The concrete
Environment Record
method InitializeBinding for global Environment Records is used
to set the bound value of the current binding of the identifier
whose name is the value of the argument N to the
value of argument V. An uninitialized binding for
N must already exist.
Let envRec be the global
Environment Record
for which the method was invoked.
The concrete
Environment Record
method SetMutableBinding for global Environment Records attempts
to change the bound value of the current binding of the
identifier whose name is the value of the argument
N to the value of argument V. If the
binding is an immutable binding, a
TypeError is thrown if S is
true. A property named N normally
already exists but if it does not or is not currently writable,
error handling is determined by the value of the Boolean
argument S.
Let envRec be the global
Environment Record
for which the method was invoked.
Let DclRec be
envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is
true, then
Return DclRec.SetMutableBinding(N,
V, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.SetMutableBinding(N,
V, S).
8.1.1.4.6GetBindingValue (
N, S )
The concrete
Environment Record
method GetBindingValue for global Environment Records returns
the value of its bound identifier whose name is the value of the
argument N. If the binding is an uninitialized
binding throw a ReferenceError exception. A
property named N normally already exists but if it
does not or is not currently writable, error handling is
determined by the value of the Boolean argument S.
Let envRec be the global
Environment Record
for which the method was invoked.
Let DclRec be
envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is
true, then
Return DclRec.GetBindingValue(N,
S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.GetBindingValue(N,
S).
8.1.1.4.7DeleteBinding (
N )
The concrete
Environment Record
method DeleteBinding for global Environment Records can only
delete bindings that have been explicitly designated as being
subject to deletion.
Let envRec be the global
Environment Record
for which the method was invoked.
Let DclRec be
envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is
true, then
Return DclRec.DeleteBinding(N).
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for
ObjRec.
If N is an element of
varNames, remove that element from the
varNames.
Return status.
Return true.
8.1.1.4.8HasThisBinding ( )
Return true.
8.1.1.4.9HasSuperBinding ( )
Return false.
8.1.1.4.10WithBaseObject ( )
Global Environment Records always return
undefined as their WithBaseObject.
Return undefined.
8.1.1.4.11GetThisBinding ( )
Let envRec be the global
Environment Record
for which the method was invoked.
Return envRec.[[GlobalThisValue]].
8.1.1.4.12HasVarDeclaration (
N )
The concrete
Environment Record
method HasVarDeclaration for global Environment Records
determines if the argument identifier has a binding in this
record that was created using a
VariableStatement
or a
FunctionDeclaration:
Let envRec be the global
Environment Record
for which the method was invoked.
Let varDeclaredNames be
envRec.[[VarNames]].
If varDeclaredNames contains N, return
true.
Return false.
8.1.1.4.13HasLexicalDeclaration (
N )
The concrete
Environment Record
method HasLexicalDeclaration for global Environment Records
determines if the argument identifier has a binding in this
record that was created using a lexical declaration such as a
LexicalDeclaration
or a
ClassDeclaration:
Let envRec be the global
Environment Record
for which the method was invoked.
Let DclRec be
envRec.[[DeclarativeRecord]].
Return DclRec.HasBinding(N).
8.1.1.4.14HasRestrictedGlobalProperty ( N )
The concrete
Environment Record
method HasRestrictedGlobalProperty for global Environment
Records determines if the argument identifier is the name of a
property of the
global object
that must not be shadowed by a global lexical binding:
Let envRec be the global
Environment Record
for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for
ObjRec.
Let existingProp be ?
globalObject.[[GetOwnProperty]](N).
If existingProp is undefined,
return false.
If existingProp.[[Configurable]] is
true, return false.
Return true.
Note
Properties may exist upon a
global object
that were directly created rather than being declared using
a var or function declaration. A global lexical binding may
not be created that has the same name as a non-configurable
property of the
global object. The global property "undefined" is an
example of such a property.
8.1.1.4.15CanDeclareGlobalVar (
N )
The concrete
Environment Record
method CanDeclareGlobalVar for global Environment Records
determines if a corresponding CreateGlobalVarBinding call would
succeed if called for the same argument N. Redundant
var declarations and var declarations for pre-existing
global object
properties are allowed.
Let envRec be the global
Environment Record
for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for
ObjRec.
The concrete
Environment Record
method CanDeclareGlobalFunction for global Environment Records
determines if a corresponding CreateGlobalFunctionBinding call
would succeed if called for the same argument N.
Let envRec be the global
Environment Record
for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for
ObjRec.
Let existingProp be ?
globalObject.[[GetOwnProperty]](N).
If existingProp is undefined,
return ? IsExtensible(globalObject).
If existingProp.[[Configurable]] is
true, return true.
If
IsDataDescriptor(existingProp) is true and
existingProp has attribute values { [[Writable]]:
true, [[Enumerable]]:
true }, return true.
Return false.
8.1.1.4.17CreateGlobalVarBinding (
N, D )
The concrete
Environment Record
method CreateGlobalVarBinding for global Environment Records
creates and initializes a mutable binding in the associated
object
Environment Record
and records the bound name in the associated [[VarNames]]
List. If a binding already exists, it is reused and assumed to be
initialized.
Let envRec be the global
Environment Record
for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for
ObjRec.
The concrete
Environment Record
method CreateGlobalFunctionBinding for global Environment
Records creates and initializes a mutable binding in the
associated object
Environment Record
and records the bound name in the associated [[VarNames]]
List. If a binding already exists, it is replaced.
Let envRec be the global
Environment Record
for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for
ObjRec.
Let existingProp be ?
globalObject.[[GetOwnProperty]](N).
If existingProp is
undefined or
existingProp.[[Configurable]] is
true, then
Let desc be the PropertyDescriptor {
[[Value]]: V, [[Writable]]:
true, [[Enumerable]]:
true, [[Configurable]]:
D }.
Else,
Let desc be the PropertyDescriptor {
[[Value]]: V }.
Global function declarations are always represented as own
properties of the
global object. If possible, an existing own property is reconfigured to
have a standard set of attribute values. Steps 8-9 are
equivalent to what calling the InitializeBinding concrete
method would do and if globalObject is a Proxy
will produce the same sequence of Proxy trap calls.
8.1.1.5Module Environment Records
A module
Environment Record
is a declarative
Environment Record
that is used to represent the outer scope of an ECMAScript
Module. In additional to normal mutable and immutable bindings, module
Environment Records also provide immutable import bindings which
are bindings that provide indirect access to a target binding that
exists in another
Environment Record.
Module Environment Records support all of the declarative
Environment Record
methods listed in
Table 16
and share the same specifications for all of those methods except
for GetBindingValue, DeleteBinding, HasThisBinding and
GetThisBinding. In addition, module Environment Records support
the methods listed in
Table 21:
Table 21: Additional Methods of Module Environment Records
Method
Purpose
CreateImportBinding(N, M, N2)
Create an immutable indirect binding in a module
Environment Record. The String value N is the text of the
bound name. M is a
Module Record, and N2 is a binding that exists in
M's module
Environment Record.
The behaviour of the additional concrete specification methods for
module Environment Records are defined by the following
algorithms:
8.1.1.5.1GetBindingValue (
N, S )
The concrete
Environment Record
method GetBindingValue for module Environment Records returns
the value of its bound identifier whose name is the value of the
argument N. However, if the binding is an indirect
binding the value of the target binding is returned. If the
binding exists but is uninitialized a
ReferenceError is thrown.
Module Environment Records are only used within strict code
and an
early error
rule prevents the delete operator, in strict code, from
being applied to a
Reference
that would resolve to a module
Environment Record
binding. See
12.5.3.1.
8.1.1.5.3HasThisBinding ( )
Module Environment Records provide a this binding.
Return true.
8.1.1.5.4GetThisBinding ( )
Return undefined.
8.1.1.5.5CreateImportBinding (
N, M, N2 )
The concrete
Environment Record
method CreateImportBinding for module Environment Records
creates a new initialized immutable indirect binding for the
name N. A binding must not already exist in this
Environment Record
for N. M is a
Module Record, and N2 is the name of a binding that exists in
M's module
Environment Record. Accesses to the value of the new binding will indirectly
access the bound value of the target binding.
Let envRec be the module
Environment Record
for which the method was invoked.
Assert: envRec does not already have a binding for
N.
The abstract operation GetIdentifierReference is called with a
Lexical Environmentlex, a String name, and a Boolean flag
strict. The value of lex may be
null. When called, the following steps are
performed:
If lex is the value null, then
Return a value of type
Reference
whose base value component is
undefined, whose referenced name
component is name, and whose strict reference
flag is strict.
Return a value of type
Reference
whose base value component is envRec, whose
referenced name component is name, and whose
strict reference flag is strict.
Else,
Let outer be the value of lex's
outer environment reference.
Set the outer lexical environment reference of
env to E.
Return env.
8.1.2.3NewObjectEnvironment (
O, E )
When the abstract operation NewObjectEnvironment is called with an
Object O and a
Lexical EnvironmentE as arguments, the following steps are performed:
Set the outer lexical environment reference of
env to E.
Return env.
8.2Realms
Before it is evaluated, all ECMAScript code must be associated with a
realm. Conceptually, a
realm
consists of a set of intrinsic objects, an ECMAScript
global environment, all of the ECMAScript code that is loaded within the scope of that
global environment, and other associated state and resources.
A
realm
is represented in this specification as a
Realm Record with the fields specified in
Table 22:
Template objects are canonicalized separately for each
realm
using its
Realm Record's [[TemplateMap]]. Each [[Site]] value is a
Parse Node
that is a
TemplateLiteral. The associated [[Array]] value is the corresponding
template object that is passed to a tag function.
Note
Once a
Parse Node
becomes unreachable, the corresponding [[Array]] is also
unreachable, and it would be unobservable if an
implementation removed the pair from the [[TemplateMap]]
list.
[[HostDefined]]
Any, default value is undefined.
Field reserved for use by host environments that need to
associate additional information with a
Realm Record.
8.2.1CreateRealm ( )
The abstract operation CreateRealm with no arguments performs the
following steps:
Set fields of intrinsics with the values listed in
Table 8. The field names are the names listed in column one of the
table. The value of each field is a new object value fully and
recursively populated with property values as defined by the
specification of each object in clauses 18-26. All object
property values are newly created object values. All values that
are built-in function objects are created by performing
CreateBuiltinFunction(<steps>, <slots>, realmRec,
<prototype>) where <steps> is the definition of that
function provided by this specification, <slots> is a list
of the names, if any, of the function's specified internal
slots, and <prototype> is the specified value of the
function's [[Prototype]] internal slot. The creation of the
intrinsics and their properties must be ordered to avoid any
dependencies upon objects that have not yet been created.
Let desc be the fully populated
data property
descriptor for the property containing the specified
attributes for the property. For properties listed in
18.2,
18.3, or
18.4
the value of the [[Value]] attribute is the corresponding
intrinsic object from realmRec.
An execution context is a specification device that is used
to track the runtime evaluation of code by an ECMAScript
implementation. At any point in time, there is at most one execution
context per
agent
that is actually executing code. This is known as the
agent's
running execution context.
All references to the
running execution context
in this specification denote the
running execution context
of the
surrounding agent.
The execution context stack is
used to track execution contexts. The
running execution context
is always the top element of this stack. A new execution context is
created whenever control is transferred from the executable code
associated with the currently
running execution context
to executable code that is not associated with that execution context.
The newly created execution context is pushed onto the stack and
becomes the
running execution context.
An execution context contains whatever implementation specific state
is necessary to track the execution progress of its associated code.
Each execution context has at least the state components listed in
Table 23.
Table 23: State Components for All Execution Contexts
Component
Purpose
code evaluation state
Any state needed to perform, suspend, and resume evaluation
of the code associated with this
execution context.
Evaluation of code by the
running execution context
may be suspended at various points defined within this specification.
Once the
running execution context
has been suspended a different execution context may become the
running execution context
and commence evaluating its code. At some later time a suspended
execution context may again become the
running execution context
and continue evaluating its code at the point where it had previously
been suspended. Transition of the
running execution context
status among execution contexts usually occurs in stack-like
last-in/first-out manner. However, some ECMAScript features require
non-LIFO transitions of the
running execution context.
In most situations only the
running execution context
(the top of the
execution context stack) is directly manipulated by algorithms within this specification.
Hence when the terms “LexicalEnvironment”, and “VariableEnvironment”
are used without qualification they are in reference to those
components of the
running execution context.
An execution context is purely a specification mechanism and need not
correspond to any particular artefact of an ECMAScript implementation.
It is impossible for ECMAScript code to directly access or observe an
execution context.
8.3.1GetActiveScriptOrModule ( )
The GetActiveScriptOrModule abstract operation is used to determine
the running script or module, based on the
running execution context. GetActiveScriptOrModule performs the following steps:
If no such
execution context
exists, return null. Otherwise, return
ec's ScriptOrModule.
8.3.2ResolveBinding ( name [
, env ] )
The ResolveBinding abstract operation is used to determine the
binding of name passed as a String value. The optional
argument env can be used to explicitly provide the
Lexical Environment
that is to be searched for the binding. During execution of
ECMAScript code, ResolveBinding is performed using the following
algorithm:
If env is not present or if env is
undefined, then
The result of ResolveBinding is always a
Reference
value with its referenced name component equal to the
name argument.
8.3.3GetThisEnvironment ( )
The abstract operation GetThisEnvironment finds the
Environment Record
that currently supplies the binding of the
keywordthis. GetThisEnvironment performs the following steps:
The loop in step 2 will always terminate because the list of
environments always ends with the
global environment
which has a this binding.
8.3.4ResolveThisBinding ( )
The abstract operation ResolveThisBinding determines the binding of
the
keywordthis using the LexicalEnvironment of the
running execution context. ResolveThisBinding performs the following steps:
The abstract operation GetNewTarget determines the NewTarget value
using the LexicalEnvironment of the
running execution context. GetNewTarget performs the following steps:
The abstract operation GetGlobalObject returns the
global object
used by the currently
running execution context. GetGlobalObject performs the following steps:
A Job is an abstract operation that initiates an
ECMAScript computation when no other ECMAScript computation is
currently in progress. A
Job
abstract operation may be defined to accept an arbitrary set of job
parameters.
Execution of a
Job
can be initiated only when there is no
running execution context
and the
execution context stack
is empty. A PendingJob is a request for the future execution of a
Job. A PendingJob is an internal
Record
whose fields are specified in
Table 26. Once execution of a
Job
is initiated, the
Job
always executes to completion. No other
Job
may be initiated until the currently running
Job
completes. However, the currently running
Job
or external events may cause the enqueuing of additional PendingJobs
that may be initiated sometime after completion of the currently
running
Job.
The script or module for the initial
execution context
when this PendingJob is initiated.
[[HostDefined]]
Any, default value is undefined.
Field reserved for use by host environments that need to
associate additional information with a pending
Job.
A Job Queue is a FIFO queue of PendingJob
records. Each
Job Queue
has a name and the full set of available
Job
Queues are defined by an ECMAScript implementation. Every ECMAScript
implementation has at least the
Job
Queues defined in
Table 27.
Each
agent
has its own set of named
Job
Queues. All references to a named job queue in this specification
denote the named job queue of the
surrounding agent.
Jobs that validate and evaluate ECMAScript
Script
and
Module
source text. See clauses 10 and 15.
PromiseJobs
Jobs that are responses to the settlement of a Promise (see
25.6).
A request for the future execution of a
Job
is made by enqueueing, on a
Job Queue, a PendingJob record that includes a
Job
abstract operation name and any necessary argument values. When there
is no
running execution context
and the
execution context stack
is empty, the ECMAScript implementation removes the first PendingJob
from a
Job Queue
and uses the information contained in it to create an
execution context
and starts execution of the associated
Job
abstract operation.
The PendingJob records from a single
Job Queue
are always initiated in FIFO order. This specification does not define
the order in which multiple
Job
Queues are serviced. An ECMAScript implementation may interweave the
FIFO evaluation of the PendingJob records of a
Job Queue
with the evaluation of the PendingJob records of one or more other
Job
Queues. An implementation must define what occurs when there are no
running execution context
and all
Job
Queues are empty.
Note
Typically an ECMAScript implementation will have its
Job
Queues pre-initialized with at least one PendingJob and one of
those Jobs will be the first to be executed. An implementation
might choose to free all resources and terminate if the current
Job
completes and all
Job
Queues are empty. Alternatively, it might choose to wait for a
some implementation specific
agent
or mechanism to enqueue new PendingJob requests.
Let callerScriptOrModule be
callerContext's ScriptOrModule.
Let pending be PendingJob { [[Job]]: job,
[[Arguments]]: arguments, [[Realm]]:
callerRealm, [[ScriptOrModule]]:
callerScriptOrModule, [[HostDefined]]:
undefined }.
Perform any implementation or host environment defined
processing of pending. This may include modifying the
[[HostDefined]] field or any other field of pending.
Add pending at the back of the
Job Queue
named by queueName.
If the host requires use of an
exotic object
to serve as realm's
global object, let global be such an object created in an
implementation-defined manner. Otherwise, let global be
undefined, indicating that an ordinary object
should be created as the
global object.
If the host requires that the this binding in
realm's global scope return an object other than the
global object, let thisValue be such an object created in an
implementation-defined manner. Otherwise, let
thisValue be undefined, indicating
that realm's global this binding should be
the
global object.
In an implementation-dependent manner, obtain the ECMAScript
source texts (see clause
10) and any associated host-defined values for zero or more
ECMAScript scripts and/or ECMAScript modules. For each such
sourceText and hostDefined, do
If sourceText is the source code of a script, then
Perform any implementation or host environment defined job
initialization using nextPending.
Let result be the result of performing the abstract
operation named by nextPending.[[Job]] using the
elements of nextPending.[[Arguments]] as its
arguments.
An
agent's
executing thread
executes the jobs in the
agent's job queues on the
agent's execution contexts independently of other agents, except that an
executing thread
may be used as the
executing thread
by multiple agents, provided none of the agents sharing the thread
have an
Agent Record
whose [[CanBlock]] property is true.
Note 1
Some web browsers share a single
executing thread
across multiple unrelated tabs of a browser window, for example.
The default value computed for the
isLittleEndian parameter when it is needed by the
algorithms
GetValueFromBuffer
and
SetValueInBuffer. The choice is implementation-dependent and should be the
alternative that is most efficient for the implementation.
Once the value has been observed it cannot change.
Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]]
have been observed by any
agent
in the
agent cluster
they cannot change.
Note 2
The values of [[IsLockFree1]] and [[IsLockFree2]] are not
necessarily determined by the hardware, but may also reflect
implementation choices that can vary over time and between
ECMAScript implementations.
There is no [[IsLockFree4]] property: 4-byte atomic operations are
always lock-free.
In practice, if an atomic operation is implemented with any type
of lock the operation is not lock-free. Lock-free does not imply
wait-free: there is no upper bound on how many machine steps may
be required to complete a lock-free atomic operation.
That an atomic access of size n is lock-free does not
imply anything about the (perceived) atomicity of non-atomic
accesses of size n, specifically, non-atomic accesses may
still be performed as a sequence of several separate memory
accesses. See
ReadSharedMemory
and
WriteSharedMemory
for details.
Note 3
An
agent
is a specification mechanism and need not correspond to any
particular artefact of an ECMAScript implementation.
8.7.1AgentSignifier ( )
The abstract operation AgentSignifier takes no arguments. It
performs the following steps:
In some environments it may not be reasonable for a given
agent
to suspend. For example, in a web browser environment, it may be
reasonable to disallow suspending a document's main event
handling thread, while still allowing workers' event handling
threads to suspend.
8.8Agent Clusters
An agent cluster is a maximal set of agents that can
communicate by operating on shared memory.
Note 1
Programs within different agents may share memory by unspecified
means. At a minimum, the backing memory for SharedArrayBuffer
objects can be shared among the agents in the cluster.
There may be agents that can communicate by message passing that
cannot share memory; they are never in the same agent cluster.
The agents in a cluster need not all be alive at some particular
point in time. If
agentA creates another
agentB, after which A terminates and B creates
agentC, the three agents are in the same cluster if
A could share some memory with B and B could
share some memory with C.
All agents within a cluster must have the same value for the
[[LittleEndian]] property in their respective
Agent
Records.
Note 3
If different agents within an agent cluster have different values
of [[LittleEndian]] it becomes hard to use shared memory for
multi-byte data.
All agents within a cluster must have the same values for the
[[IsLockFree1]] property in their respective
Agent
Records; similarly for the [[IsLockFree2]] property.
All agents within a cluster must have different values for the
[[Signifier]] property in their respective
Agent
Records.
An embedding may deactivate (stop forward progress) or activate
(resume forward progress) an
agent
without the
agent's knowledge or cooperation. If the embedding does so, it must not
leave some agents in the cluster active while other agents in the
cluster are deactivated indefinitely.
Note 4
The purpose of the preceding restriction is to avoid a situation
where an
agent
deadlocks or starves because another
agent
has been deactivated. For example, if an HTML shared worker that
has a lifetime independent of documents in any windows were
allowed to share memory with the dedicated worker of such an
independent document, and the document and its dedicated worker
were to be deactivated while the dedicated worker holds a lock
(say, the document is pushed into its window's history), and the
shared worker then tries to acquire the lock, then the shared
worker will be blocked until the dedicated worker is activated
again, if ever. Meanwhile other workers trying to access the
shared worker from other windows will starve.
The implication of the restriction is that it will not be possible
to share memory between agents that don't belong to the same
suspend/wake collective within the embedding.
An embedding may terminate an
agent
without any of the
agent's cluster's other agents' prior knowledge or cooperation. If an
agent
is terminated not by programmatic action of its own or of another
agent
in the cluster but by forces external to the cluster, then the
embedding must choose one of two strategies: Either terminate all the
agents in the cluster, or provide reliable APIs that allow the agents
in the cluster to coordinate so that at least one remaining member of
the cluster will be able to detect the termination, with the
termination data containing enough information to identify the
agent
that was terminated.
Note 5
Examples of that type of termination are: operating systems or
users terminating agents that are running in separate processes;
the embedding itself terminating an
agent
that is running in-process with the other agents when
per-agent
resource accounting indicates that the
agent
is runaway.
Prior to any evaluation of any ECMAScript code by any
agent
in a cluster, the [[CandidateExecution]] field of the
Agent Record
for all agents in the cluster is set to the initial
candidate execution. The initial
candidate execution
is an
empty candidate execution
whose [[EventsRecords]] field is a
List
containing, for each
agent, an
Agent Events Record
whose [[AgentSignifier]] field is that
agent's signifier, and whose [[EventList]] and [[AgentSynchronizesWith]]
fields are empty Lists.
An agent cluster is a specification mechanism and need not
correspond to any particular artefact of an ECMAScript
implementation.
8.9Forward Progress
For an
agent
to make forward progress is for it to perform an evaluation
step according to this specification.
An
agent
becomes blocked when its
running execution context
waits synchronously and indefinitely for an external event. Only
agents whose
Agent Record's [[CanBlock]] property is true can become
blocked in this sense. An unblockedagent
is one that is not blocked.
Implementations must ensure that:
every unblocked
agent
with a dedicated
executing thread
eventually makes forward progress
in a set of agents that share an
executing thread, one
agent
eventually makes forward progress
an
agent
does not cause another
agent
to become blocked except via explicit APIs that provide blocking.
Note
This, along with the liveness guarantee in the
memory model, ensures that all SeqCst writes
eventually become observable to all agents.
9Ordinary and Exotic Objects Behaviours
9.1Ordinary Object Internal Methods and
Internal Slots
All ordinary objects have an internal slot called [[Prototype]]. The
value of this internal slot is either null or an
object and is used for implementing inheritance. Data properties of
the [[Prototype]] object are inherited (and visible as properties of
the child object) for the purposes of get access, but not for set
access. Accessor properties are inherited for both get access and set
access.
Every ordinary object has a Boolean-valued [[Extensible]] internal
slot which is used to fulfill the extensibility-related internal
method invariants specified in
6.1.7.3. Namely, once the value of an object's [[Extensible]] internal slot
has been set to false, it is no longer possible to
add properties to the object, to modify the value of the object's
[[Prototype]] internal slot, or to subsequently change the value of
[[Extensible]] to true.
In the following algorithm descriptions, assume O is an
ordinary object, P is a property key value, V is
any
ECMAScript language value, and Desc is a
Property Descriptor
record.
Each ordinary object internal method delegates to a similarly-named
abstract operation. If such an abstract operation depends on another
internal method, then the internal method is invoked on
O rather than calling the similarly-named abstract
operation directly. These semantics ensure that exotic objects have
their overridden internal methods invoked when ordinary object
internal methods are applied to them.
9.1.1[[GetPrototypeOf]] ( )
When the [[GetPrototypeOf]] internal method of O is
called, the following steps are taken:
If p.[[GetPrototypeOf]] is not the ordinary
object internal method defined in
9.1.1, set done to true.
Else, set p to p.[[Prototype]].
Set O.[[Prototype]] to V.
Return true.
Note
The loop in step 8 guarantees that there will be no
circularities in any prototype chain that only includes
objects that use the ordinary object definitions for
[[GetPrototypeOf]] and [[SetPrototypeOf]].
9.1.3[[IsExtensible]] ( )
When the [[IsExtensible]] internal method of O is called,
the following steps are taken:
When the abstract operation OrdinaryDefineOwnProperty is called
with Object O, property key P, and
Property DescriptorDesc, the following steps are taken:
9.1.6.2IsCompatiblePropertyDescriptor
( Extensible, Desc, Current )
When the abstract operation IsCompatiblePropertyDescriptor is
called with Boolean value Extensible, and Property
Descriptors Desc, and Current, the following
steps are taken:
9.1.6.3ValidateAndApplyPropertyDescriptor ( O, P,
extensible, Desc, current )
When the abstract operation ValidateAndApplyPropertyDescriptor is
called with Object O, property key P,
Boolean value extensible, and Property Descriptors
Desc, and current, the following steps are
taken:
Note
If undefined is passed as O,
only validation is performed and no object updates are
performed.
If O is not undefined,
create an own
data property
named P of object O whose
[[Value]], [[Writable]], [[Enumerable]], and
[[Configurable]] attribute values are described by
Desc. If the value of an attribute field of
Desc is absent, the attribute of the newly
created property is set to its
default value.
If O is not undefined,
create an own
accessor property
named P of object O whose
[[Get]], [[Set]], [[Enumerable]], and [[Configurable]]
attribute values are described by Desc. If
the value of an attribute field of Desc is
absent, the attribute of the newly created property is
set to its
default value.
Return true.
If every field in Desc is absent, return
true.
If current.[[Configurable]] is
false, then
If Desc.[[Configurable]] is present and its
value is true, return
false.
If Desc.[[Enumerable]] is present and
! SameValue(Desc.[[Enumerable]],
current.[[Enumerable]]) is
false, return false.
If O is not undefined,
convert the property named P of object
O from a
data property
to an
accessor property. Preserve the existing values of the converted
property's [[Configurable]] and [[Enumerable]]
attributes and set the rest of the property's
attributes to their
default values.
Else,
If O is not undefined,
convert the property named P of object
O from an
accessor property
to a
data property. Preserve the existing values of the converted
property's [[Configurable]] and [[Enumerable]]
attributes and set the rest of the property's
attributes to their
default values.
When the abstract operation OrdinarySet is called with Object
O, property key P, value V, and
ECMAScript language valueReceiver, the following steps are taken:
9.1.9.2OrdinarySetWithOwnDescriptor (
O, P, V, Receiver,
ownDesc )
When the abstract operation OrdinarySetWithOwnDescriptor is called
with Object O, property key P, value
V,
ECMAScript language valueReceiver, and
Property Descriptor
(or undefined) ownDesc, the
following steps are taken:
For each own property key P of O that is
an
array index, in ascending numeric index order, do
Add P as the last element of keys.
For each own property key P of O that is
a String but is not an
array index, in ascending chronological order of property creation, do
Add P as the last element of keys.
For each own property key P of O that is
a Symbol, in ascending chronological order of property
creation, do
Add P as the last element of keys.
Return keys.
9.1.12ObjectCreate ( proto [
, internalSlotsList ] )
The abstract operation ObjectCreate with argument
proto (an object or null) is used to specify the runtime
creation of new ordinary objects. The optional argument
internalSlotsList is a
List
of the names of additional internal slots that must be defined as
part of the object. If the list is not provided, a new empty
List
is used. This abstract operation performs the following steps:
If internalSlotsList is not present, set
internalSlotsList to a new empty
List.
Let obj be a newly created object with an internal
slot for each name in internalSlotsList.
Set obj's essential internal methods to the default
ordinary object definitions specified in
9.1.
The abstract operation OrdinaryCreateFromConstructor creates an
ordinary object whose [[Prototype]] value is retrieved from a
constructor's "prototype" property, if it exists. Otherwise
the intrinsic named by intrinsicDefaultProto is used for
[[Prototype]]. The optional internalSlotsList is a
List
of the names of additional internal slots that must be defined as
part of the object. If the list is not provided, a new empty
List
is used. This abstract operation performs the following steps:
Assert: intrinsicDefaultProto is a String value that is
this specification's name of an intrinsic object. The
corresponding object must be an intrinsic that is intended to be
used as the [[Prototype]] value of an object.
The abstract operation GetPrototypeFromConstructor determines the
[[Prototype]] value that should be used to create an object
corresponding to a specific
constructor. The value is retrieved from the
constructor's "prototype" property, if it exists. Otherwise
the intrinsic named by intrinsicDefaultProto is used for
[[Prototype]]. This abstract operation performs the following steps:
Assert: intrinsicDefaultProto is a String value that is
this specification's name of an intrinsic object. The
corresponding object must be an intrinsic that is intended to be
used as the [[Prototype]] value of an object.
Set proto to realm's intrinsic object
named intrinsicDefaultProto.
Return proto.
Note
If constructor does not supply a [[Prototype]] value,
the default value that is used is obtained from the
realm
of the constructor function rather than from the
running execution context.
9.1.15RequireInternalSlot (
O, internalSlot )
The abstract operation RequireInternalSlot throws an exception
unless O is an Object and has the given internal slot.
If
Type(O) is not Object, throw a
TypeError exception.
If O does not have an
internalSlot internal slot, throw a
TypeError exception.
9.2ECMAScript Function Objects
ECMAScript function objects encapsulate parameterized ECMAScript code
closed over a lexical environment and support the dynamic evaluation
of that code. An ECMAScript
function object
is an ordinary object and has the same internal slots and the same
internal methods as other ordinary objects. The code of an ECMAScript
function object
may be either
strict mode code
(10.2.1) or
non-strict code. An ECMAScript
function object
whose code is
strict mode code
is called a strict function. One whose
code is not
strict mode code
is called a non-strict function.
ECMAScript function objects have the additional internal slots listed
in
Table 29.
Table 29: Internal Slots of ECMAScript Function Objects
The script or module in which the function was created.
[[ThisMode]]
lexical |
strict |
global
Defines how this references are interpreted
within the formal parameters and code body of the function.
lexical means that
this refers to the
this value of a lexically enclosing
function. strict means that the
this value is used exactly as provided by
an invocation of the function.
global means that a
this value of
undefined is interpreted as a reference
to the
global object.
Indicates whether the function is a class
constructor. (If true, invoking the function's
[[Call]] will immediately throw a
TypeError exception.)
All ECMAScript function objects have the [[Call]] internal method
defined here. ECMAScript functions that are also constructors in
addition have the [[Construct]] internal method.
9.2.1[[Call]] ( thisArgument,
argumentsList )
The [[Call]] internal method for an ECMAScript
function objectF is called with parameters thisArgument and
argumentsList, a
List
of ECMAScript language values. The following steps are taken:
When calleeContext is removed from the
execution context stack
in step 8 it must not be destroyed if it is suspended and
retained for later resumption by an accessible generator object.
When the abstract operation OrdinaryCallBindThis is called with
function objectF,
execution contextcalleeContext, and ECMAScript value
thisArgument, the following steps are taken:
When the abstract operation OrdinaryCallEvaluateBody is called
with
function objectF and
ListargumentsList, the following steps are taken:
Return the result of EvaluateBody of the parsed code that is
F.[[ECMAScriptCode]] passing F and
argumentsList as the arguments.
9.2.2[[Construct]] (
argumentsList, newTarget )
The [[Construct]] internal method for an ECMAScript
function objectF is called with parameters argumentsList and
newTarget. argumentsList is a possibly empty
List
of ECMAScript language values. The following steps are taken:
The abstract operation OrdinaryFunctionCreate requires the
arguments: an object functionPrototype, a parameter list
Parse Node
specified by ParameterList, a body
Parse Node
specified by Body, thisMode which is either
lexical-this or
non-lexical-this, and a
Lexical Environment
specified by Scope. OrdinaryFunctionCreate performs the
following steps:
The abstract operation AddRestrictedFunctionProperties is called
with a
function objectF and
Realm Recordrealm as its argument. It performs the following steps:
The %ThrowTypeError% intrinsic is an anonymous built-in
function object
that is defined once for each
realm. When %ThrowTypeError% is called it performs the following
steps:
Throw a TypeError exception.
The value of the [[Extensible]] internal slot of a
%ThrowTypeError% function is false.
The "length" property of a %ThrowTypeError%
function has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
The abstract operation MakeConstructor requires a Function argument
F and optionally, a Boolean
writablePrototype and an object prototype. If
prototype is provided it is assumed to already contain,
if needed, a "constructor" property whose value
is F. This operation converts F into a
constructor
by performing the following steps:
The abstract operation SetFunctionName requires a Function argument
F, a String or Symbol argument name and
optionally a String argument prefix. This operation adds
a "name" property to F by performing
the following steps:
Assert: F is an extensible object that does not have a
"name" own property.
The abstract operation SetFunctionLength requires a Function
argument F and a Number argument length. This
operation adds a "length" property to
F by performing the following steps:
Assert: F is an extensible object that does not have a
"length" own property.
When an
execution context
is established for evaluating an ECMAScript function a new
function Environment Record
is created and bindings for each formal parameter are
instantiated in that
Environment Record. Each declaration in the function body is also instantiated.
If the function's formal parameters do not include any default
value initializers then the body declarations are instantiated
in the same
Environment Record
as the parameters. If default value parameter initializers
exist, a second
Environment Record
is created for the body declarations. Formal parameters and
functions are initialized as part of
FunctionDeclarationInstantiation. All other bindings are
initialized during evaluation of the function body.
FunctionDeclarationInstantiation is performed as follows using
arguments func and argumentsList.
func is the
function object
for which the
execution context
is being established.
NOTE: A separate
Environment Record
is needed to ensure that bindings created by
direct eval
calls in the formal parameter list are outside the
environment where parameters are declared.
Let calleeEnv be the LexicalEnvironment of
calleeContext.
Assert: The VariableEnvironment of calleeContext is
calleeEnv.
Set the LexicalEnvironment of calleeContext to
env.
For each String paramName in
parameterNames, do
Let alreadyDeclared be
envRec.HasBinding(paramName).
NOTE: Early errors ensure that duplicate parameter names can
only occur in non-strict functions that do not have
parameter default values or rest parameters.
NOTE: mapped argument object is only provided for
non-strict functions that don't have a rest parameter,
any parameter default value initializers, or any
destructured parameters.
Perform ? IteratorBindingInitialization for
formals with iteratorRecord and
undefined as arguments.
Else,
Perform ? IteratorBindingInitialization for
formals with iteratorRecord and
env as arguments.
If hasParameterExpressions is
false, then
NOTE: Only a single lexical environment is needed for the
parameters and top-level vars.
Let instantiatedVarNames be a copy of the
ListparameterBindings.
For each n in varNames, do
If n is not an element of
instantiatedVarNames, then
Append n to
instantiatedVarNames.
Perform !
envRec.CreateMutableBinding(n,
false).
Call
envRec.InitializeBinding(n,
undefined).
Let varEnv be env.
Let varEnvRec be envRec.
Else,
NOTE: A separate
Environment Record
is needed to ensure that closures created by expressions in
the formal parameter list do not have visibility of
declarations in the function body.
NOTE: Non-strict functions use a separate lexical
Environment Record
for top-level lexical declarations so that a
direct eval
can determine whether any var scoped declarations introduced
by the eval code conflict with pre-existing top-level
lexically scoped declarations. This is not needed for strict
functions because a strict
direct eval
always places all declarations into a new
Environment Record.
Set the LexicalEnvironment of calleeContext to
lexEnv.
Let lexDeclarations be the
LexicallyScopedDeclarations of code.
For each element d in lexDeclarations, do
NOTE: A lexically declared name cannot be the same as a
function/generator declaration, formal parameter, or a var
name. Lexically declared names are only instantiated here
but not initialized.
B.3.3
provides an extension to the above algorithm that is necessary
for backwards compatibility with web browser implementations of
ECMAScript that predate ECMAScript 2015.
Note 3
Parameter
Initializers may contain
direct eval
expressions. Any top level declarations of such evals are only
visible to the eval code (10.2). The creation of the environment for such declarations is
described in
14.1.19.
9.3Built-in Function Objects
The built-in function objects defined in this specification may be
implemented as either ECMAScript function objects (9.2) whose behaviour is provided using ECMAScript code or as
implementation provided function exotic objects whose behaviour is
provided in some other manner. In either case, the effect of calling
such functions must conform to their specifications. An implementation
may also provide additional built-in function objects that are not
defined in this specification.
If a built-in
function object
is implemented as an
exotic object
it must have the ordinary object behaviour specified in
9.1. All such function exotic objects also have [[Prototype]],
[[Extensible]], [[Realm]], and [[ScriptOrModule]] internal slots.
Unless otherwise specified every built-in
function object
has the
%Function.prototype%
object as the initial value of its [[Prototype]] internal slot.
The behaviour specified for each built-in function via algorithm steps
or other means is the specification of the function body behaviour for
both [[Call]] and [[Construct]] invocations of the function. However,
[[Construct]] invocation is not supported by all built-in functions.
For each built-in function, when invoked with [[Call]], the [[Call]]
thisArgument provides the this value,
the [[Call]] argumentsList provides the named parameters,
and the NewTarget value is undefined. When invoked
with [[Construct]], the this value is
uninitialized, the [[Construct]] argumentsList provides the
named parameters, and the [[Construct]] newTarget parameter
provides the NewTarget value. If the built-in function is implemented
as an ECMAScript
function object
then this specified behaviour must be implemented by the ECMAScript
code that is the body of the function. Built-in functions that are
ECMAScript function objects must be strict functions. If a built-in
constructor
has any [[Call]] behaviour other than throwing a
TypeError exception, an ECMAScript implementation
of the function must be done in a manner that does not cause the
function's [[IsClassConstructor]] internal slot to have the value
true.
Built-in function objects that are not identified as constructors do
not implement the [[Construct]] internal method unless otherwise
specified in the description of a particular function. When a built-in
constructor
is called as part of a new expression the
argumentsList parameter of the invoked [[Construct]]
internal method provides the values for the built-in
constructor's named parameters.
Built-in functions that are not constructors do not have a
"prototype" property unless otherwise specified in
the description of a particular function.
If a built-in
function object
is not implemented as an ECMAScript function it must provide [[Call]]
and [[Construct]] internal methods that conform to the following
definitions:
9.3.1[[Call]] ( thisArgument,
argumentsList )
The [[Call]] internal method for a built-in
function objectF is called with parameters thisArgument and
argumentsList, a
List
of ECMAScript language values. The following steps are taken:
Let result be the
Completion Record
that is the result of evaluating F in a manner that
conforms to the specification of F.
thisArgument is the this value,
argumentsList provides the named parameters, and the
NewTarget value is undefined.
When calleeContext is removed from the
execution context stack
it must not be destroyed if it has been suspended and retained
by an accessible generator object for later resumption.
9.3.2[[Construct]] (
argumentsList, newTarget )
The [[Construct]] internal method for built-in
function objectF is called with parameters argumentsList and
newTarget. The steps performed are the same as [[Call]]
(see
9.3.1) except that step 10 is replaced by:
Let result be the
Completion Record
that is the result of evaluating F in a manner that
conforms to the specification of F. The
this value is uninitialized,
argumentsList provides the named parameters, and
newTarget provides the NewTarget value.
The abstract operation CreateBuiltinFunction takes arguments
steps, internalSlotsList, realm,
and prototype. The argument
internalSlotsList is a
List
of the names of additional internal slots that must be defined as
part of the object. CreateBuiltinFunction returns a built-in
function object
created by the following steps:
Assert: steps is either a set of algorithm steps or other
definition of a function's behaviour provided in this
specification.
If prototype is not present, set
prototype to
realm.[[Intrinsics]].[[%Function.prototype%]].
Let func be a new built-in
function object
that when called performs the action described by
steps. The new
function object
has internal slots whose names are the elements of
internalSlotsList.
Set func.[[Realm]] to realm.
Set func.[[Prototype]] to prototype.
Set func.[[Extensible]] to true.
Set func.[[ScriptOrModule]] to
null.
Return func.
Each built-in function defined in this specification is created by
calling the CreateBuiltinFunction abstract operation.
9.4Built-in Exotic Object Internal Methods
and Slots
This specification defines several kinds of built-in exotic objects.
These objects generally behave similar to ordinary objects except for
a few specific situations. The following exotic objects use the
ordinary object internal methods except where it is explicitly
specified otherwise below:
9.4.1Bound Function Exotic Objects
A bound function is an
exotic object
that wraps another
function object. A bound function is callable (it has a [[Call]] internal method
and may have a [[Construct]] internal method). Calling a bound
function generally results in a call of its wrapped function.
Bound function objects do not have the internal slots of ECMAScript
function objects defined in
Table 29. Instead they have the internal slots defined in
Table 30.
Table 30: Internal Slots of Bound Function Exotic Objects
A list of values whose elements are used as the first
arguments to any call to the wrapped function.
Bound function objects provide all of the essential internal methods
as specified in
9.1. However, they use the following definitions for the essential
internal methods of function objects.
9.4.1.1[[Call]] (
thisArgument, argumentsList )
When the [[Call]] internal method of a
bound functionexotic object, F, which was created using the bind function is
called with parameters thisArgument and
argumentsList, a
List
of ECMAScript language values, the following steps are taken:
Let target be F.[[BoundTargetFunction]].
Let boundThis be F.[[BoundThis]].
Let boundArgs be F.[[BoundArguments]].
Let args be a new list containing the same values
as the list boundArgs in the same order followed by
the same values as the list argumentsList in the
same order.
When the [[Construct]] internal method of a
bound functionexotic object, F that was created using the bind function is called
with a list of arguments argumentsList and
newTarget, the following steps are taken:
Let args be a new list containing the same values
as the list boundArgs in the same order followed by
the same values as the list argumentsList in the
same order.
If
SameValue(F, newTarget) is
true, set newTarget to
target.
The abstract operation BoundFunctionCreate with arguments
targetFunction, boundThis and
boundArgs is used to specify the creation of new Bound
Function exotic objects. It performs the following steps:
Set obj.[[BoundTargetFunction]] to
targetFunction.
Set obj.[[BoundThis]] to boundThis.
Set obj.[[BoundArguments]] to boundArgs.
Return obj.
9.4.2Array Exotic Objects
An Array object is an
exotic object
that gives special treatment to
array index
property keys (see
6.1.7). A property whose
property name
is an
array index
is also called an element. Every Array object has a
non-configurable "length" property whose value is
always a nonnegative
integer
less than 232. The value of the
"length" property is numerically greater than the
name of every own property whose name is an
array index; whenever an own property of an Array object is created or
changed, other properties are adjusted as necessary to maintain this
invariant. Specifically, whenever an own property is added whose
name is an
array index, the value of the "length" property is changed,
if necessary, to be one more than the numeric value of that
array index; and whenever the value of the
"length" property is changed, every own property
whose name is an
array index
whose value is not smaller than the new length is deleted. This
constraint applies only to own properties of an Array object and is
unaffected by "length" or
array index
properties that may be inherited from its prototypes.
Array exotic objects provide an alternative definition for the
[[DefineOwnProperty]] internal method. Except for that internal
method, Array exotic objects provide all of the other essential
internal methods as specified in
9.1.
9.4.2.1[[DefineOwnProperty]] (
P, Desc )
When the [[DefineOwnProperty]] internal method of an Array
exotic objectA is called with property key P, and
Property DescriptorDesc, the following steps are taken:
Assert: oldLenDesc will never be
undefined or an accessor descriptor
because Array objects are created with a length
data property
that cannot be deleted or reconfigured.
The abstract operation ArrayCreate with argument
length (either 0 or a positive
integer) and optional argument proto is used to specify the
creation of new Array exotic objects. It performs the following
steps:
The abstract operation ArraySpeciesCreate with arguments
originalArray and length is used to specify
the creation of a new Array object using a
constructor
function that is derived from originalArray. It
performs the following steps:
If originalArray was created using the standard
built-in Array
constructor
for a
realm
that is not the
realm
of the
running execution context, then a new Array is created using the
realm
of the
running execution context. This maintains compatibility with Web browsers that have
historically had that behaviour for the
Array.prototype methods that now are defined
using ArraySpeciesCreate.
9.4.2.4ArraySetLength ( A,
Desc )
When the abstract operation ArraySetLength is called with an Array
exotic objectA, and
Property DescriptorDesc, the following steps are taken:
Assert: oldLenDesc will never be
undefined or an accessor descriptor because
Array objects are created with a length
data property
that cannot be deleted or reconfigured.
Return
OrdinaryDefineOwnProperty(A, "length",
PropertyDescriptor { [[Writable]]:
false }). This call will always return
true.
Return true.
Note
In steps 3 and 4, if Desc.[[Value]] is an object
then its valueOf method is called twice. This is
legacy behaviour that was specified with this effect starting
with the 2nd Edition of this specification.
9.4.3String Exotic Objects
A String object is an
exotic object
that encapsulates a String value and exposes virtual
integer-indexed data properties corresponding to the individual code unit
elements of the String value. String exotic objects always have a
data property
named "length" whose value is the number of code
unit elements in the encapsulated String value. Both the code unit
data properties and the "length" property are
non-writable and non-configurable.
String exotic objects have the same internal slots as ordinary
objects. They also have a [[StringData]] internal slot.
String exotic objects provide alternative definitions for the
following internal methods. All of the other String
exotic object
essential internal methods that are not defined below are as
specified in
9.1.
9.4.3.1[[GetOwnProperty]] (
P )
When the [[GetOwnProperty]] internal method of a String
exotic objectS is called with property key P, the
following steps are taken:
When the [[DefineOwnProperty]] internal method of a String
exotic objectS is called with property key P, and
Property DescriptorDesc, the following steps are taken:
For each own property key P of O such
that P is an
array index
and
ToInteger(P) ≥ len, in ascending numeric index
order, do
Add P as the last element of keys.
For each own property key P of O such
that
Type(P) is String and P is not an
array index, in ascending chronological order of property creation, do
Add P as the last element of keys.
For each own property key P of O such
that
Type(P) is Symbol, in ascending chronological order of
property creation, do
Add P as the last element of keys.
Return keys.
9.4.3.4StringCreate (
value, prototype )
The abstract operation StringCreate with arguments
value and prototype is used to specify the
creation of new String exotic objects. It performs the following
steps:
Most ECMAScript functions make an arguments object available to
their code. Depending upon the characteristics of the function
definition, its arguments object is either an ordinary object or an
arguments
exotic object. An arguments
exotic object
is an
exotic object
whose
array index
properties map to the formal parameters bindings of an invocation of
its associated ECMAScript function.
Arguments exotic objects have the same internal slots as ordinary
objects. They also have a [[ParameterMap]] internal slot. Ordinary
arguments objects also have a [[ParameterMap]] internal slot whose
value is always undefined. For ordinary argument objects the
[[ParameterMap]] internal slot is only used by
Object.prototype.toString (19.1.3.6) to identify them as such.
Arguments exotic objects provide alternative definitions for the
following internal methods. All of the other arguments
exotic object
essential internal methods that are not defined below are as
specified in
9.1
Note 1
The
integer-indexed data properties of an arguments
exotic object
whose numeric name values are less than the number of formal
parameters of the corresponding
function object
initially share their values with the corresponding argument
bindings in the function's
execution context. This means that changing the property changes the
corresponding value of the argument binding and vice-versa. This
correspondence is broken if such a property is deleted and then
redefined or if the property is changed into an
accessor property. If the arguments object is an ordinary object, the values of
its properties are simply a copy of the arguments passed to the
function and there is no dynamic linkage between the property
values and the formal parameter values.
Note 2
The ParameterMap object and its property values are used as a
device for specifying the arguments object correspondence to
argument bindings. The ParameterMap object and the objects that
are the values of its properties are not directly observable
from ECMAScript code. An ECMAScript implementation does not need
to actually create or use such objects to implement the
specified semantics.
Note 3
Ordinary arguments objects define a non-configurable
accessor property
named "callee" which throws a
TypeError exception on access. The
"callee" property has a more specific meaning
for arguments exotic objects, which are created only for some
class of non-strict functions. The definition of this property
in the ordinary variant exists to ensure that it is not defined
in any other manner by conforming ECMAScript implementations.
Note 4
ECMAScript implementations of arguments exotic objects have
historically contained an
accessor property
named "caller". Prior to ECMAScript 2017,
this specification included the definition of a throwing
"caller" property on ordinary arguments
objects. Since implementations do not contain this extension any
longer, ECMAScript 2017 dropped the requirement for a throwing
"caller" accessor.
9.4.4.1[[GetOwnProperty]] (
P )
The [[GetOwnProperty]] internal method of an arguments
exotic object
when called with a property key P performs the
following steps:
The [[DefineOwnProperty]] internal method of an arguments
exotic object
when called with a property key P and
Property DescriptorDesc performs the following steps:
Let setStatus be
Set(map, P,
Desc.[[Value]],
false).
Assert: setStatus is
true because formal parameters
mapped by argument objects are always writable.
If Desc.[[Writable]] is present and its
value is false, then
Call map.[[Delete]](P).
Return true.
9.4.4.3[[Get]] ( P,
Receiver )
The [[Get]] internal method of an arguments
exotic object
when called with a property key P and
ECMAScript language valueReceiver performs the following steps:
The [[Set]] internal method of an arguments
exotic object
when called with property key P, value V,
and
ECMAScript language valueReceiver performs the following steps:
The abstract operation CreateMappedArgumentsObject is called with
object func,
Parse Nodeformals,
ListargumentsList, and
Environment Recordenv. The following steps are performed:
Assert: formals does not contain a rest parameter, any
binding patterns, or any initializers. It may contain
duplicate identifiers.
Let len be the number of elements in
argumentsList.
Let obj be a newly created arguments
exotic object
with a [[ParameterMap]] internal slot.
Set obj's essential internal methods to the default
ordinary object definitions specified in
9.1.
Set obj.[[GetOwnProperty]] as specified in
9.4.4.1.
Set obj.[[DefineOwnProperty]] as specified in
9.4.4.2.
The abstract operation MakeArgGetter called with String
name and
Environment Recordenv creates a built-in
function object
that when executed returns the value bound for
name in env. It performs the following
steps:
Let steps be the steps of an ArgGetter function
as specified below.
An ArgGetter function is an anonymous built-in function with
[[Name]] and [[Env]] internal slots. When an ArgGetter function
that expects no arguments is called it performs the following
steps:
ArgGetter functions are never directly accessible to
ECMAScript code.
9.4.4.7.2MakeArgSetter (
name, env )
The abstract operation MakeArgSetter called with String
name and
Environment Recordenv creates a built-in
function object
that when executed sets the value bound for name in
env. It performs the following steps:
Let steps be the steps of an ArgSetter function
as specified below.
An ArgSetter function is an anonymous built-in function with
[[Name]] and [[Env]] internal slots. When an ArgSetter function
is called with argument value it performs the
following steps:
ArgSetter functions are never directly accessible to
ECMAScript code.
9.4.5Integer-Indexed Exotic Objects
An
Integer-Indexed exotic object
is an
exotic object
that performs special handling of
integer index
property keys.
Integer-Indexed exotic objects
have the same internal slots as ordinary objects and additionally
[[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]],
[[ContentType]], and [[TypedArrayName]] internal slots.
The abstract operation IntegerIndexedObjectCreate with arguments
prototype and internalSlotsList is used to
specify the creation of new
Integer-Indexed exotic objects. The argument internalSlotsList is a
List
of the names of additional internal slots that must be defined as
part of the object. IntegerIndexedObjectCreate performs the
following steps:
Assert: internalSlotsList contains the names
[[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]],
[[ContentType]], and [[TypedArrayName]].
A module namespace object is an
exotic object
that exposes the bindings exported from an ECMAScript
Module
(See
15.2.3). There is a one-to-one correspondence between the String-keyed
own properties of a module namespace
exotic object
and the binding names exported by the
Module. The exported bindings include any bindings that are indirectly
exported using export * export items. Each
String-valued own property key is the StringValue of the
corresponding exported binding name. These are the only String-keyed
properties of a module namespace
exotic object. Each such property has the attributes { [[Writable]]:
true, [[Enumerable]]: true,
[[Configurable]]: false }. Module namespace
objects are not extensible.
Module namespace objects have the internal slots defined in
Table 31.
Table 31: Internal Slots of Module Namespace Exotic Objects
A
List
containing the String values of the exported names exposed
as own properties of this object. The list is ordered as
if an Array of those String values had been sorted using
Array.prototype.sort using
undefined as comparefn.
[[Prototype]]
Null
This slot always contains the value
null (see
9.4.6.1).
Module namespace exotic objects provide alternative definitions for
all of the internal methods except [[GetPrototypeOf]], which behaves
as defined in
9.1.1.
9.4.6.1[[SetPrototypeOf]] (
V )
When the [[SetPrototypeOf]] internal method of a module namespace
exotic objectO is called with argument V, the following
steps are taken:
When the [[DefineOwnProperty]] internal method of a module
namespace
exotic objectO is called with property key P and
Property DescriptorDesc, the following steps are taken:
When the [[Get]] internal method of a module namespace
exotic objectO is called with property key P and
ECMAScript language valueReceiver, the following steps are taken:
ResolveExport is side-effect free. Each time this operation is
called with a specific exportName,
resolveSet pair as arguments it must return the
same result. An implementation might choose to pre-compute or
cache the ResolveExport results for the [[Exports]] of each
module namespace
exotic object.
9.4.6.8[[Set]] ( P,
V, Receiver )
When the [[Set]] internal method of a module namespace
exotic objectO is called with property key P, value
V, and
ECMAScript language valueReceiver, the following steps are taken:
Return false.
9.4.6.9[[Delete]] ( P )
When the [[Delete]] internal method of a module namespace
exotic objectO is called with property key P, the
following steps are taken:
Append all the entries of symbolKeys to the end of
exports.
Return exports.
9.4.6.11ModuleNamespaceCreate (
module, exports )
The abstract operation ModuleNamespaceCreate with arguments
module, and exports is used to specify the
creation of new module namespace exotic objects. It performs the
following steps:
Let M be a newly created module namespace
exotic object
with the internal slots listed in
Table 31.
Set M's essential internal methods to the
definitions specified in
9.4.6.
Set M.[[Module]] to module.
Let sortedExports be a new
List
containing the same values as the list
exports where the values are ordered as if an Array
of the same values had been sorted using
Array.prototype.sort using
undefined as comparefn.
Set M.[[Exports]] to sortedExports.
Create own properties of M corresponding to the
definitions in
26.3.
Set module.[[Namespace]] to M.
Return M.
9.4.7Immutable Prototype Exotic Objects
An immutable prototype exotic object is an
exotic object
that has a [[Prototype]] internal slot that will not change once it
is initialized.
Immutable prototype exotic objects have the same internal slots as
ordinary objects. They are exotic only in the following internal
methods. All other internal methods of immutable prototype exotic
objects that are not explicitly defined below are instead defined as
in
ordinary objects.
9.4.7.1[[SetPrototypeOf]] (
V )
When the [[SetPrototypeOf]] internal method of an
immutable prototype exotic objectO is called with argument V, the following
steps are taken:
9.5Proxy Object Internal Methods and
Internal Slots
A proxy object is an
exotic object
whose essential internal methods are partially implemented using
ECMAScript code. Every proxy object has an internal slot called
[[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called
the proxy's handler object, or null.
Methods (see
Table 32) of a handler object may be used to augment the implementation for
one or more of the proxy object's internal methods. Every proxy object
also has an internal slot called [[ProxyTarget]] whose value is either
an object or the null value. This object is called
the proxy's target object.
Table 32: Proxy Handler Methods
Internal Method
Handler Method
[[GetPrototypeOf]]
getPrototypeOf
[[SetPrototypeOf]]
setPrototypeOf
[[IsExtensible]]
isExtensible
[[PreventExtensions]]
preventExtensions
[[GetOwnProperty]]
getOwnPropertyDescriptor
[[DefineOwnProperty]]
defineProperty
[[HasProperty]]
has
[[Get]]
get
[[Set]]
set
[[Delete]]
deleteProperty
[[OwnPropertyKeys]]
ownKeys
[[Call]]
apply
[[Construct]]
construct
When a handler method is called to provide the implementation of a
proxy object internal method, the handler method is passed the proxy's
target object as a parameter. A proxy's handler object does not
necessarily have a method corresponding to every essential internal
method. Invoking an internal method on the proxy results in the
invocation of the corresponding internal method on the proxy's target
object if the handler object does not have a method corresponding to
the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy
object are always initialized when the object is created and typically
may not be modified. Some proxy objects are created in a manner that
permits them to be subsequently revoked. When a proxy is
revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are
set to null causing subsequent invocations of
internal methods on that proxy object to throw a
TypeError exception.
Because proxy objects permit the implementation of internal methods to
be provided by arbitrary ECMAScript code, it is possible to define a
proxy object whose handler methods violates the invariants defined in
6.1.7.3. Some of the internal method invariants defined in
6.1.7.3
are essential integrity invariants. These invariants are explicitly
enforced by the proxy object internal methods specified in this
section. An ECMAScript implementation must be robust in the presence
of all possible invariant violations.
In the following algorithm descriptions, assume O is an
ECMAScript proxy object, P is a property key value,
V is any
ECMAScript language value
and Desc is a
Property Descriptor
record.
9.5.1[[GetPrototypeOf]] ( )
When the [[GetPrototypeOf]] internal method of a Proxy
exotic objectO is called, the following steps are taken:
If
SameValue(handlerProto, targetProto) is
false, throw a
TypeError exception.
Return handlerProto.
Note
[[GetPrototypeOf]] for proxy objects enforces the following
invariants:
The result of [[GetPrototypeOf]] must be either an Object or
null.
If the target object is not extensible, [[GetPrototypeOf]]
applied to the proxy object must return the same value as
[[GetPrototypeOf]] applied to the proxy object's target
object.
9.5.2[[SetPrototypeOf]] ( V )
When the [[SetPrototypeOf]] internal method of a Proxy
exotic objectO is called with argument V, the following
steps are taken:
If
SameValue(booleanTrapResult, targetResult) is
false, throw a
TypeError exception.
Return booleanTrapResult.
Note
[[IsExtensible]] for proxy objects enforces the following
invariants:
The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the
same value as [[IsExtensible]] applied to the proxy object's
target object with the same argument.
9.5.4[[PreventExtensions]] ( )
When the [[PreventExtensions]] internal method of a Proxy
exotic objectO is called, the following steps are taken:
If targetDesc is undefined or
targetDesc.[[Configurable]] is
true, then
Throw a TypeError exception.
If resultDesc has a [[Writable]] field and
resultDesc.[[Writable]] is
false, then
If targetDesc.[[Writable]] is
true, throw a
TypeError exception.
Return resultDesc.
Note
[[GetOwnProperty]] for proxy objects enforces the following
invariants:
The result of [[GetOwnProperty]] must be either an Object or
undefined.
A property cannot be reported as non-existent, if it exists as
a non-configurable own property of the target object.
A property cannot be reported as non-existent, if the target
object is not extensible, unless it does not exist as an own
property of the target object.
A property cannot be reported as existent, if the target
object is not extensible, unless it exists as an own property
of the target object.
A property cannot be reported as non-configurable, unless it
exists as a non-configurable own property of the target
object.
A property cannot be reported as both non-configurable and
non-writable, unless it exists as a non-configurable,
non-writable own property of the target object.
9.5.6[[DefineOwnProperty]] (
P, Desc )
When the [[DefineOwnProperty]] internal method of a Proxy
exotic objectO is called with property key P and
Property DescriptorDesc, the following steps are taken:
If settingConfigFalse is
true and
targetDesc.[[Configurable]] is
true, throw a
TypeError exception.
If
IsDataDescriptor(targetDesc) is true,
targetDesc.[[Configurable]] is
false, and
targetDesc.[[Writable]] is
true, then
If Desc has a [[Writable]] field and
Desc.[[Writable]] is
false, throw a
TypeError exception.
Return true.
Note
[[DefineOwnProperty]] for proxy objects enforces the following
invariants:
The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not
extensible.
A property cannot be non-configurable, unless there exists a
corresponding non-configurable own property of the target
object.
A non-configurable property cannot be non-writable, unless
there exists a corresponding non-configurable, non-writable
own property of the target object.
If a property has a corresponding target object property then
applying the
Property Descriptor
of the property to the target object using
[[DefineOwnProperty]] will not throw an exception.
9.5.7[[HasProperty]] ( P )
When the [[HasProperty]] internal method of a Proxy
exotic objectO is called with property key P, the following
steps are taken:
If extensibleTarget is
false, throw a
TypeError exception.
Return booleanTrapResult.
Note
[[HasProperty]] for proxy objects enforces the following
invariants:
The result of [[HasProperty]] is a Boolean value.
A property cannot be reported as non-existent, if it exists as
a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as
an own property of the target object and the target object is
not extensible.
9.5.8[[Get]] ( P,
Receiver )
When the [[Get]] internal method of a Proxy
exotic objectO is called with property key P and
ECMAScript language valueReceiver, the following steps are taken:
Let trapResult be ? Call(trap, handler, « target,
P, Receiver »).
Let targetDesc be ?
target.[[GetOwnProperty]](P).
If targetDesc is not undefined and
targetDesc.[[Configurable]] is
false, then
If
IsDataDescriptor(targetDesc) is true and
targetDesc.[[Writable]] is
false, then
If
SameValue(trapResult,
targetDesc.[[Value]]) is
false, throw a
TypeError exception.
If
IsAccessorDescriptor(targetDesc) is true and
targetDesc.[[Get]] is
undefined, then
If trapResult is not
undefined, throw a
TypeError exception.
Return trapResult.
Note
[[Get]] for proxy objects enforces the following invariants:
The value reported for a property must be the same as the
value of the corresponding target object property if the
target object property is a non-writable, non-configurable own
data property.
The value reported for a property must be
undefined if the corresponding target
object property is a non-configurable own
accessor property
that has undefined as its [[Get]]
attribute.
9.5.9[[Set]] ( P,
V, Receiver )
When the [[Set]] internal method of a Proxy
exotic objectO is called with property key P, value
V, and
ECMAScript language valueReceiver, the following steps are taken:
If targetDesc.[[Set]] is
undefined, throw a
TypeError exception.
Return true.
Note
[[Set]] for proxy objects enforces the following invariants:
The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the
value of the corresponding target object property if the
corresponding target object property is a non-writable,
non-configurable own
data property.
Cannot set the value of a property if the corresponding target
object property is a non-configurable own
accessor property
that has undefined as its [[Set]]
attribute.
9.5.10[[Delete]] ( P )
When the [[Delete]] internal method of a Proxy
exotic objectO is called with property key P, the following
steps are taken:
The Type of each result
List
element is either String or Symbol.
The result
List
must contain the keys of all non-configurable own properties
of the target object.
If the target object is not extensible, then the result
List
must contain all the keys of the own properties of the target
object and no other values.
9.5.12[[Call]] (
thisArgument, argumentsList )
The [[Call]] internal method of a Proxy
exotic objectO is called with parameters thisArgument and
argumentsList, a
List
of ECMAScript language values. The following steps are taken:
A Proxy
exotic object
only has a [[Call]] internal method if the initial value of its
[[ProxyTarget]] internal slot is an object that has a [[Call]]
internal method.
9.5.13[[Construct]] (
argumentsList, newTarget )
The [[Construct]] internal method of a Proxy
exotic objectO is called with parameters
argumentsList which is a possibly empty
List
of ECMAScript language values and newTarget. The
following steps are taken:
Let newObj be ? Call(trap, handler, « target,
argArray, newTarget »).
If
Type(newObj) is not Object, throw a
TypeError exception.
Return newObj.
Note 1
A Proxy
exotic object
only has a [[Construct]] internal method if the initial value of
its [[ProxyTarget]] internal slot is an object that has a
[[Construct]] internal method.
Note 2
[[Construct]] for proxy objects enforces the following
invariants:
The result of [[Construct]] must be an Object.
9.5.14ProxyCreate ( target,
handler )
The abstract operation ProxyCreate with arguments
target and handler is used to specify the
creation of new Proxy exotic objects. It performs the following
steps:
If
Type(target) is not Object, throw a
TypeError exception.
If target is a Proxy
exotic object
and target.[[ProxyHandler]] is
null, throw a
TypeError exception.
If
Type(handler) is not Object, throw a
TypeError exception.
If handler is a Proxy
exotic object
and handler.[[ProxyHandler]] is
null, throw a
TypeError exception.
Let P be a newly created Proxy
exotic object
with internal slots [[ProxyTarget]] and [[ProxyHandler]].
Set P's essential internal methods (except for
[[Call]] and [[Construct]]) to the definitions specified in
9.5.
ECMAScript code is expressed using Unicode. ECMAScript source text is
a sequence of code points. All Unicode code point values from U+0000
to U+10FFFF, including surrogate code points, may occur in source text
where permitted by the ECMAScript grammars. The actual encodings used
to store and interchange ECMAScript source text is not relevant to
this specification. Regardless of the external source text encoding, a
conforming ECMAScript implementation processes the source text as if
it was an equivalent sequence of
SourceCharacter
values, each
SourceCharacter
being a Unicode code point. Conforming ECMAScript implementations are
not required to perform any normalization of source text, or behave as
though they were performing normalization of source text.
The components of a combining character sequence are treated as
individual Unicode code points even though a user might think of the
whole sequence as a single character.
Note
In string literals, regular expression literals, template literals
and identifiers, any Unicode code point may also be expressed
using Unicode escape sequences that explicitly express a code
point's numeric value. Within a comment, such an escape sequence
is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the
behaviour of Unicode escape sequences. In a Java program, if the
Unicode escape sequence \u000A, for example, occurs
within a single-line comment, it is interpreted as a line
terminator (Unicode code point U+000A is LINE FEED (LF)) and
therefore the next code point is not part of the comment.
Similarly, if the Unicode escape sequence
\u000A occurs within a string literal in a Java
program, it is likewise interpreted as a line terminator, which is
not allowed within a string literal—one must write
\n instead of \u000A to cause a LINE
FEED (LF) to be part of the String value of a string literal. In
an ECMAScript program, a Unicode escape sequence occurring within
a comment is never interpreted and therefore cannot contribute to
termination of the comment. Similarly, a Unicode escape sequence
occurring within a string literal in an ECMAScript program always
contributes to the literal and is never interpreted as a line
terminator or as a code point that might terminate the string
literal.
10.1.1Static Semantics: UTF16Encoding (
cp )
The UTF16Encoding of a numeric code point value, cp, is
determined as follows:
Let cp be (lead - 0xD800) × 0x400 + (trail
- 0xDC00) + 0x10000.
Return the code point cp.
10.1.3Static Semantics: CodePointAt (
string, position )
The abstract operation CodePointAt interprets a String
string as a sequence of UTF-16 encoded code points, as
described in
6.1.4, and reads from it a single code point starting with the code unit
at index position. When called, the following steps are
performed:
Eval code is the source text supplied to the built-in
eval function. More precisely, if the parameter to the
built-in eval function is a String, it is treated as an
ECMAScript
Script. The eval code for a particular invocation of eval is
the global code portion of that
Script.
then the source text matching the
BindingIdentifier
(if any) of that declaration or expression is also included in the
function code of the corresponding function.
Function code is generally provided as the bodies of Function
Definitions (14.1), Arrow Function Definitions (14.2), Method Definitions (14.3), Generator Function Definitions (14.4), Async Function Definitions (14.7), Async Generator Function Definitions (14.5), and Async Arrow Functions (14.8). Function code is also derived from the arguments to the
Functionconstructor
(19.2.1.1), the GeneratorFunctionconstructor
(25.2.1.1), and the AsyncFunctionconstructor
(25.7.1.1).
Note 2
The practical effect of including the
BindingIdentifier
in function code is that the Early Errors for
strict mode code
are applied to a
BindingIdentifier
that is the name of a function whose body contains a "use strict"
directive, even if the surrounding code is not
strict mode code.
10.2.1Strict Mode Code
An ECMAScript
Script
syntactic unit may be processed using either unrestricted or strict
mode syntax and semantics. Code is interpreted as
strict mode code in the following situations:
Function code that is supplied as the arguments to the built-in
Function, Generator,
AsyncFunction, and
AsyncGenerator constructors is strict mode code if
the last argument is a String that when processed is a
FunctionBody
that begins with a
Directive Prologue
that contains a
Use Strict Directive.
ECMAScript code that is not strict mode code is called
non-strict code.
10.2.2Non-ECMAScript Functions
An ECMAScript implementation may support the evaluation of function
exotic objects whose evaluative behaviour is expressed in some
implementation-defined form of executable code other than via
ECMAScript code. Whether a
function object
is an ECMAScript code function or a non-ECMAScript function is not
semantically observable from the perspective of an ECMAScript code
function that calls or is called by such a non-ECMAScript function.
11ECMAScript Language: Lexical Grammar
The source text of an ECMAScript
Script or
Module is
first converted into a sequence of input elements, which are tokens,
line terminators, comments, or white space. The source text is scanned
from left to right, repeatedly taking the longest possible sequence of
code points as the next input element.
The use of multiple lexical goals ensures that there are no lexical
ambiguities that would affect automatic semicolon insertion. For
example, there are no syntactic grammar contexts where both a
leading division or division-assignment, and a leading
RegularExpressionLiteral
are permitted. This is not affected by semicolon insertion (see
11.9); in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a
LineTerminator
is U+002F (SOLIDUS) and the syntactic context allows division or
division-assignment, no semicolon is inserted at the
LineTerminator. That is, the above example is interpreted in the same way as:
The Unicode format-control characters (i.e., the characters in
category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT
MARK or RIGHT-TO-LEFT MARK) are control codes used to control the
formatting of a range of text in the absence of higher-level protocols
for this (such as mark-up languages).
It is useful to allow format-control characters in source text to
facilitate editing and display. All format control characters may be
used within comments, and within string literals, template literals,
and regular expression literals.
U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are
format-control characters that are used to make necessary distinctions
when forming words or phrases in certain languages. In ECMAScript
source text these code points may also be used in an
IdentifierName
after the first character.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used
primarily at the start of a text to mark it as Unicode and to allow
detection of the text's encoding and byte order. <ZWNBSP>
characters intended for this purpose can sometimes also appear after
the start of a text, for example as a result of concatenating files.
In ECMAScript source text <ZWNBSP> code points are treated as
white space characters (see
11.2).
The special treatment of certain format-control characters outside of
comments, string literals, and regular expression literals is
summarized in
Table 33.
White space code points are used to improve source text readability
and to separate tokens (indivisible lexical units) from each other,
but are otherwise insignificant. White space code points may occur
between any two tokens and at the start or end of input. White space
code points may occur within a
StringLiteral, a
RegularExpressionLiteral, a
Template, or a
TemplateSubstitutionTail
where they are considered significant code points forming part of a
literal value. They may also occur within a
Comment, but cannot appear within any other kind of token.
The ECMAScript white space code points are listed in
Table 34.
Table 34: White Space Code Points
Code Point
Name
Abbreviation
U+0009
CHARACTER TABULATION
<TAB>
U+000B
LINE TABULATION
<VT>
U+000C
FORM FEED (FF)
<FF>
U+0020
SPACE
<SP>
U+00A0
NO-BREAK SPACE
<NBSP>
U+FEFF
ZERO WIDTH NO-BREAK SPACE
<ZWNBSP>
Other category “Zs”
Any other Unicode “Space_Separator” code point
<USP>
ECMAScript implementations must recognize as
WhiteSpace
code points listed in the “Space_Separator” (“Zs”) category.
Note
Other than for the code points listed in
Table 34, ECMAScript
WhiteSpace
intentionally excludes all code points that have the Unicode
“White_Space” property but which are not classified in category
“Space_Separator” (“Zs”).
Like white space code points, line terminator code points are used to
improve source text readability and to separate tokens (indivisible
lexical units) from each other. However, unlike white space code
points, line terminators have some influence over the behaviour of the
syntactic grammar. In general, line terminators may occur between any
two tokens, but there are a few places where they are forbidden by the
syntactic grammar. Line terminators also affect the process of
automatic semicolon insertion (11.9). A line terminator cannot occur within any token except a
StringLiteral,
Template, or
TemplateSubstitutionTail. <LF> and <CR> line terminators cannot occur within a
StringLiteral
token except as part of a
LineContinuation.
Line terminators are included in the set of white space code points
that are matched by the \s class in regular expressions.
The ECMAScript line terminator code points are listed in
Table 35.
Table 35: Line Terminator Code Points
Code Point
Unicode Name
Abbreviation
U+000A
LINE FEED (LF)
<LF>
U+000D
CARRIAGE RETURN (CR)
<CR>
U+2028
LINE SEPARATOR
<LS>
U+2029
PARAGRAPH SEPARATOR
<PS>
Only the Unicode code points in
Table 35
are treated as line terminators. Other new line or line breaking
Unicode code points are not treated as line terminators but are
treated as white space if they meet the requirements listed in
Table 34. The sequence <CR><LF> is commonly used as a line
terminator. It should be considered a single
SourceCharacter
for the purpose of reporting line numbers.
Comments can be either single or multi-line. Multi-line comments
cannot nest.
Because a single-line comment can contain any Unicode code point
except a
LineTerminator
code point, and because of the general rule that a token is always as
long as possible, a single-line comment always consists of all code
points from the // marker to the end of the line.
However, the
LineTerminator
at the end of the line is not considered to be part of the single-line
comment; it is recognized separately by the lexical grammar and
becomes part of the stream of input elements for the syntactic
grammar. This point is very important, because it implies that the
presence or absence of single-line comments does not affect the
process of automatic semicolon insertion (see
11.9).
Comments behave like white space and are discarded except that, if a
MultiLineComment
contains a line terminator code point, then the entire comment is
considered to be a
LineTerminator
for purposes of parsing by the syntactic grammar.
IdentifierName
and
ReservedWord
are tokens that are interpreted according to the Default Identifier
Syntax given in Unicode Standard Annex #31, Identifier and Pattern
Syntax, with some small modifications.
ReservedWord
is an enumerated subset of
IdentifierName. The syntactic grammar defines
Identifier
as an
IdentifierName
that is not a
ReservedWord. The Unicode identifier grammar is based on character properties
specified by the Unicode Standard. The Unicode code points in the
specified categories in the latest version of the Unicode standard
must be treated as in those categories by all conforming ECMAScript
implementations. ECMAScript implementations may recognize identifier
code points defined in later editions of the Unicode Standard.
Note 1
This standard specifies specific code point additions: U+0024
(DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an
IdentifierName, and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D
(ZERO WIDTH JOINER) are permitted anywhere after the first code
point of an
IdentifierName.
Unicode escape sequences are permitted in an
IdentifierName, where they contribute a single Unicode code point to the
IdentifierName. The code point is expressed by the
CodePoint
of the
UnicodeEscapeSequence
(see
11.8.4). The \ preceding the
UnicodeEscapeSequence
and the u and { } code units, if they
appear, do not contribute code points to the
IdentifierName. A
UnicodeEscapeSequence
cannot be used to put a code point into an
IdentifierName
that would otherwise be illegal. In other words, if a \UnicodeEscapeSequence
sequence were replaced by the
SourceCharacter
it contributes, the result must still be a valid
IdentifierName
that has the exact same sequence of
SourceCharacter
elements as the original
IdentifierName. All interpretations of
IdentifierName
within this specification are based upon their actual code points
regardless of whether or not an escape sequence was used to contribute
any particular code point.
Two
IdentifierNames that are canonically equivalent according to the Unicode standard
are not equal unless, after replacement of each
UnicodeEscapeSequence, they are represented by the exact same sequence of code points.
The sets of code points with Unicode properties “ID_Start” and
“ID_Continue” include, respectively, the code points with Unicode
properties “Other_ID_Start” and “Other_ID_Continue”.
Return the String value consisting of the sequence of code
units corresponding to
IdentifierName. In determining the sequence any occurrences of
\UnicodeEscapeSequence
are first replaced with the code point represented by the
UnicodeEscapeSequence
and then the code points of the entire
IdentifierName
are converted to code units by
UTF16Encoding
each code point.
11.6.2Keywords and Reserved Words
A keyword is a token that matches
IdentifierName, but also has a syntactic use; that is, it appears literally, in a
fixed width font, in some syntactic production. The
keywords of ECMAScript include if, while,
async, await, and many others.
A reserved word is an
IdentifierName
that cannot be used as an identifier. Many keywords are reserved
words, but some are not, and some are reserved only in certain
contexts. if and while are reserved words.
await is reserved only inside async functions and
modules. async is not reserved; it can be used as a
variable name or statement label without restriction.
This specification uses a combination of grammatical productions and
early error
rules to specify which names are valid identifiers and which are
reserved words. All tokens in the
ReservedWord
list below, except for await and yield,
are unconditionally reserved. Exceptions for await and
yield are specified in
12.1, using parameterized syntactic productions. Lastly, several
early error
rules restrict the set of valid identifiers. See
12.1.1,
13.3.1.1,
13.7.5.1, and
14.6.1. In summary, there are five categories of identifier names:
Those that are always allowed as identifiers, and are not
keywords, such as Math, window,
toString, and _;
Those that are never allowed as identifiers, namely the
ReservedWords listed below except await and
yield;
Those that are contextually allowed as identifiers, namely
await and yield;
Those that are contextually disallowed as identifiers, in
strict mode code: let, static,
implements, interface,
package, private,
protected, and public;
Those that are always allowed as identifiers, but also appear as
keywords within certain syntactic productions, at places where
Identifier
is not allowed: as, async,
from, get, of,
set, and target.
The term conditional keyword, or
contextual keyword, is sometimes used to refer to the
keywords that fall in the last three categories, and thus can be
used as identifiers in some contexts and as keywords in others.
Per
5.1.5, keywords in the grammar match literal sequences of specific
SourceCharacter
elements. A code point in a keyword cannot be expressed by a
\UnicodeEscapeSequence.
An
IdentifierName
can contain \UnicodeEscapeSequences, but it is not possible to declare a variable named "else" by
spelling it els\u{65}. The
early error
rules in
12.1.1
rule out identifiers with the same StringValue as a reserved
word.
Note 2
enum is not currently used as a keyword in this
specification. It is a future reserved word, set aside
for use as a keyword in future language extensions.
Similarly, implements, interface,
package, private,
protected, and public are future
reserved words in
strict mode code.
Once the exact MV for a numeric literal has been determined, it is
then rounded to a value of the Number type. If the MV is 0ℝ, then the rounded value is +0; otherwise, the
rounded value must be the
Number value
for the MV (as specified in
6.1.6.1), unless the literal is a
DecimalLiteral
and the literal has more than 20 significant digits, in which case
the
Number value
may be either the
Number value
for the MV of a literal produced by replacing each significant
digit after the 20th with a 0 digit or the
Number value
for the MV of a literal produced by replacing each significant
digit after the 20th with a 0 digit and then
incrementing the literal at the 20th significant digit position. A
digit is significant if it is not part of an
ExponentPart
and
it is not 0; or
there is a nonzero digit to its left and there is a nonzero
digit, not in the
ExponentPart, to its right.
A string literal is zero or more Unicode code points enclosed in
single or double quotes. Unicode code points may also be
represented by an escape sequence. All code points may appear
literally in a string literal except for the closing quote code
points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and
U+000A (LINE FEED). Any code points may appear in the form of an
escape sequence. String literals evaluate to ECMAScript String
values. When generating these String values Unicode code points
are UTF-16 encoded as defined in
10.1.1. Code points belonging to the Basic Multilingual Plane are
encoded as a single code unit element of the string. All other
code points are encoded as two code unit elements of the string.
<LF> and <CR> cannot appear in a string literal,
except as part of a
LineContinuation
to produce the empty code points sequence. The proper way to
include either in the String value of a string literal is to use
an escape sequence such as \n or
\u000A.
Return the String value whose code units are the SV of this
StringLiteral.
11.8.4.2Static Semantics: SV
A string literal stands for a value of the String type. The String
value (SV) of the literal is described in terms of code unit
values contributed by the various parts of the string literal. As
part of this process, some Unicode code points within the string
literal are interpreted as having a
mathematical value
(MV), as described below or in
11.8.3.
The SV of
StringLiteral::""
is the empty code unit sequence.
The SV of
StringLiteral::''
is the empty code unit sequence.
A regular expression literal is an input element that is
converted to a RegExp object (see
21.2) each time the literal is evaluated. Two regular expression
literals in a program evaluate to regular expression objects
that never compare as === to each other even if the
two literals' contents are identical. A RegExp object may also
be created at runtime by new RegExp or calling the
RegExpconstructor
as a function (see
21.2.3).
The productions below describe the syntax for a regular expression
literal and are used by the input element scanner to find the end of
the regular expression literal. The source text comprising the
RegularExpressionBody
and the
RegularExpressionFlags
are subsequently parsed again using the more stringent ECMAScript
Regular Expression grammar (21.2.1).
An implementation may extend the ECMAScript Regular Expression
grammar defined in
21.2.1, but it must not extend the
RegularExpressionBody
and
RegularExpressionFlags
productions defined below or the productions used by these
productions.
Regular expression literals may not be empty; instead of
representing an empty regular expression literal, the code unit
sequence // starts a single-line comment. To
specify an empty regular expression, use: /(?:)/.
A template literal component is interpreted as a sequence of
Unicode code points. The Template Value (TV) of a literal
component is described in terms of code unit values (SV,
11.8.4) contributed by the various parts of the template literal
component. As part of this process, some Unicode code points
within the template component are interpreted as having a
mathematical value
(MV,
11.8.3). In determining a TV, escape sequences are replaced by the
UTF-16 code unit(s) of the Unicode code point represented by the
escape sequence. The Template Raw Value (TRV) is similar to a
Template Value with the difference that in TRVs escape sequences
are interpreted literally.
The TRV of
NotEscapeSequence::x[lookahead ∉
HexDigit]
is the code unit 0x0078 (LATIN SMALL LETTER X).
The TRV of
NotEscapeSequence::xHexDigit[lookahead ∉
HexDigit]
is the sequence consisting of the code unit 0x0078 (LATIN SMALL
LETTER X) followed by the code units of the TRV of
HexDigit.
The TRV of
NotEscapeSequence::u[lookahead ∉
HexDigit][lookahead ≠ {]
is the code unit 0x0075 (LATIN SMALL LETTER U).
The TRV of
NotEscapeSequence::uHexDigit[lookahead ∉
HexDigit]
is the sequence consisting of the code unit 0x0075 (LATIN SMALL
LETTER U) followed by the code units of the TRV of
HexDigit.
The TRV of
NotEscapeSequence::uHexDigitHexDigit[lookahead ∉
HexDigit]
is the sequence consisting of the code unit 0x0075 (LATIN SMALL
LETTER U) followed by the code units of the TRV of the first
HexDigit
followed by the code units of the TRV of the second
HexDigit.
The TRV of
NotEscapeSequence::uHexDigitHexDigitHexDigit[lookahead ∉
HexDigit]
is the sequence consisting of the code unit 0x0075 (LATIN SMALL
LETTER U) followed by the code units of the TRV of the first
HexDigit
followed by the code units of the TRV of the second
HexDigit
followed by the code units of the TRV of the third
HexDigit.
The TRV of
NotEscapeSequence::u{[lookahead ∉
HexDigit]
is the sequence consisting of the code unit 0x0075 (LATIN SMALL
LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET).
The TRV of
NotEscapeSequence::u{NotCodePoint[lookahead ∉
HexDigit]
is the sequence consisting of the code unit 0x0075 (LATIN SMALL
LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET)
followed by the code units of the TRV of
NotCodePoint.
The TRV of
NotEscapeSequence::u{CodePoint[lookahead ∉
HexDigit][lookahead ≠ }]
is the sequence consisting of the code unit 0x0075 (LATIN SMALL
LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET)
followed by the code units of the TRV of
CodePoint.
The TRV of
HexEscapeSequence::xHexDigitHexDigit
is the sequence consisting of the code unit 0x0078 (LATIN SMALL
LETTER X) followed by TRV of the first
HexDigit
followed by the TRV of the second
HexDigit.
The TRV of
UnicodeEscapeSequence::u{CodePoint}
is the sequence consisting of the code unit 0x0075 (LATIN SMALL
LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET)
followed by TRV of
CodePoint
followed by the code unit 0x007D (RIGHT CURLY BRACKET).
The TRV of
LineTerminatorSequence::<CR><LF>
is the sequence consisting of the code unit 0x000A (LINE FEED).
Note
TV excludes the code units of
LineContinuation
while TRV includes them. <CR><LF> and <CR>
LineTerminatorSequences are normalized to <LF> for both TV and TRV. An
explicit
EscapeSequence
is needed to include a <CR> or <CR><LF>
sequence.
11.9Automatic Semicolon Insertion
Most ECMAScript statements and declarations must be terminated with a
semicolon. Such semicolons may always appear explicitly in the source
text. For convenience, however, such semicolons may be omitted from
the source text in certain situations. These situations are described
by saying that semicolons are automatically inserted into the source
code token stream in those situations.
11.9.1Rules of Automatic Semicolon
Insertion
In the following rules, “token” means the actual recognized lexical
token determined using the current lexical
goal symbol
as described in clause
11.
There are three basic rules of semicolon insertion:
When, as the source text is parsed from left to right, a token
(called the offending token) is encountered that is not
allowed by any production of the grammar, then a semicolon is
automatically inserted before the offending token if one or more
of the following conditions is true:
The offending token is separated from the previous token by at
least one
LineTerminator.
The offending token is }.
The previous token is ) and the inserted
semicolon would then be parsed as the terminating semicolon of
a do-while statement (13.7.2).
When, as the source text is parsed from left to right, the end of
the input stream of tokens is encountered and the parser is unable
to parse the input token stream as a single instance of the goal
nonterminal, then a semicolon is automatically inserted at the end
of the input stream.
When, as the source text is parsed from left to right, a token is
encountered that is allowed by some production of the grammar, but
the production is a restricted production and the token
would be the first token for a terminal or nonterminal immediately
following the annotation “[no
LineTerminator
here]” within the restricted production (and therefore such a
token is called a restricted token), and the restricted token is
separated from the previous token by at least one
LineTerminator, then a semicolon is automatically inserted before the
restricted token.
However, there is an additional overriding condition on the
preceding rules: a semicolon is never inserted automatically if the
semicolon would then be parsed as an empty statement or if that
semicolon would become one of the two semicolons in the header of a
for statement (see
13.7.4).
Note
The following are the only restricted productions in the
grammar:
The practical effect of these restricted productions is as
follows:
When a ++ or -- token is encountered
where the parser would treat it as a postfix operator, and at
least one
LineTerminator
occurred between the preceding token and the
++ or -- token, then a semicolon is
automatically inserted before the ++ or
-- token.
When a continue, break,
return, throw, or
yield token is encountered and a
LineTerminator
is encountered before the next token, a semicolon is
automatically inserted after the continue,
break, return, throw,
or yield token.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator should
appear on the same line as its operand.
An
Expression
in a return or throw statement or an
AssignmentExpression
in a yield expression should start on the same
line as the return, throw, or
yield token.
A
LabelIdentifier
in a break or continue statement
should be on the same line as the break or
continue token.
11.9.2Examples of Automatic Semicolon
Insertion
This section is non-normative.
The source
{ 12 } 3
is not a valid sentence in the ECMAScript grammar, even with the
automatic semicolon insertion rules. In contrast, the source
{ 12 } 3
is also not a valid ECMAScript sentence, but is transformed by
automatic semicolon insertion into the following:
{ 1
;2 ;} 3;
which is a valid ECMAScript sentence.
The source
for (a; b
)
is not a valid ECMAScript sentence and is not altered by automatic
semicolon insertion because the semicolon is needed for the header
of a for statement. Automatic semicolon insertion never
inserts one of the two semicolons in the header of a
for statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return;
a + b;
Note 1
The expression a + b is not treated as a value to
be returned by the return statement, because a
LineTerminator
separates it from the token return.
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token ++ is not treated as a postfix operator
applying to the variable b, because a
LineTerminator
occurs between b and ++.
The source
if (a > b)
else c = d
is not a valid ECMAScript sentence and is not altered by automatic
semicolon insertion before the else token, even though
no production of the grammar applies at that point, because an
automatically inserted semicolon would then be parsed as an empty
statement.
The source
a = b + c
(d + e).print()
is not transformed by automatic semicolon insertion,
because the parenthesized expression that begins the second line can
be interpreted as an argument list for a function call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a
left parenthesis, it is a good idea for the programmer to provide an
explicit semicolon at the end of the preceding statement rather than
to rely on automatic semicolon insertion.
11.10Interesting Cases of Automatic
Semicolon Insertion
This section is non-normative.
ECMAScript programs can be written in a style with very few semicolons
by relying on automatic semicolon insertion. As described above,
semicolons are not inserted at every newline, and automatic semicolon
insertion can depend on multiple tokens across line terminators.
As new syntactic features are added to ECMAScript, additional grammar
productions could be added that cause lines relying on automatic
semicolon insertion preceding them to change grammar productions when
parsed.
The interesting cases of automatic semicolon insertion are places
where a semicolon may or may not be inserted depending on preceding
source text according to the rules above. These places are considered
interesting if changes outside of an existing source text grammar
production could change the grammar production of the existing source
text depending on automatic semicolon insertion. The rest of this
section describes a number of interesting cases of automatic semicolon
insertion in this version of ECMAScript.
11.10.1Interesting Cases of Automatic
Semicolon Insertion in Statement Lists
In a
StatementList, many
StatementListItems end in semicolons, which may be omitted using automatic semicolon
insertion. As a consequence of the rules above, at the end of a line
ending an expression, a semicolon is required if the following line
begins with any of the following:
An opening parenthesis ((). Without
a semicolon, the two lines together are treated as a
CallExpression.
An opening square bracket ([).
Without a semicolon, the two lines together are treated as
property access, rather than an
ArrayLiteral
or
ArrayAssignmentPattern.
A template literal (`). Without a
semicolon, the two lines together are interpreted as a tagged
Template (12.3.11), with the previous expression as the
MemberExpression.
Unary + or -. Without a semicolon, the two lines together are interpreted as
a usage of the corresponding binary operator.
A RegExp literal. Without a semicolon, the two
lines together may be parsed instead as the /MultiplicativeOperator, for example if the RegExp has flags.
11.11Cases of Automatic Semicolon
Insertion and “[no
LineTerminator
here]”
This section is non-normative.
ECMAScript contains grammar productions which include “[no
LineTerminator
here]”. These productions are sometimes a means to have optional
operands in the grammar. Introducing a
LineTerminator
in these locations would change the grammar production of a source
text by using the grammar production without the optional operand.
The rest of this section describes a number of productions using “[no
LineTerminator
here]” in this version of ECMAScript.
11.11.1List of Grammar Productions with
Optional Operands and “[no
LineTerminator
here]”
yield and await are permitted as
BindingIdentifier
in the grammar, and prohibited with
static semantics
below, to prohibit automatic semicolon insertion in cases such as
It is a Syntax Error if the code matched by this production is
contained in
strict mode code
and the StringValue of
Identifier
is "arguments" or "eval".
It is a Syntax Error if this phrase is contained in
strict mode code
and the StringValue of
IdentifierName
is: "implements",
"interface", "let",
"package", "private",
"protected", "public",
"static", or "yield".
It is a Syntax Error if the
goal symbol
of the syntactic grammar is
Module
and the StringValue of
IdentifierName
is "await".
It is a Syntax Error if StringValue of
IdentifierName
is the same String value as the StringValue of any
ReservedWord
except for yield or await.
Note
StringValue of
IdentifierName
normalizes any Unicode escape sequences in
IdentifierName
hence such escapes cannot be used to write an
Identifier
whose code point sequence is the same as a
ReservedWord.
undefined is passed for
environment to indicate that a
PutValue
operation should be used to assign the initialization value.
This is the case for var statements and formal
parameter lists of some non-strict functions (See
9.2.10). In those cases a lexical binding is hoisted and
preinitialized prior to evaluation of its initializer.
An
ArrayLiteral
is an expression describing the initialization of an Array
object, using a list, of zero or more expressions each of which
represents an array element, enclosed in square brackets. The
elements need not be literals; they are evaluated each time the
array initializer is evaluated.
Array elements may be elided at the beginning, middle or end of the
element list. Whenever a comma in the element list is not preceded
by an
AssignmentExpression
(i.e., a comma at the beginning or after another comma), the missing
array element contributes to the length of the Array and increases
the index of subsequent elements. Elided array elements are not
defined. If an element is elided at the end of an array, that
element does not contribute to the length of the Array.
CreateDataPropertyOrThrow
is used to ensure that own properties are defined for the
array even if the standard built-in Array prototype object has
been modified in a manner that would preclude the creation of
new own properties using [[Set]].
An object initializer is an expression describing the
initialization of an Object, written in a form resembling a
literal. It is a list of zero or more pairs of property keys and
associated values, enclosed in curly brackets. The values need
not be literals; they are evaluated each time the object
initializer is evaluated.
In certain contexts,
ObjectLiteral
is used as a cover grammar for a more restricted secondary
grammar. The
CoverInitializedName
production is necessary to fully cover these secondary grammars.
However, use of this production results in an early Syntax Error
in normal contexts where an actual
ObjectLiteral
is expected.
Always throw a Syntax Error if code matches this production.
Note
This production exists so that
ObjectLiteral
can serve as a cover grammar for
ObjectAssignmentPattern. It cannot occur in an actual object initializer.
The abstract operation IsValidRegularExpressionLiteral determines
if its argument is a valid regular expression literal. The
following steps are taken:
If FlagText of literal contains any code points
other than g, i, m,
s, u, or y, or if it
contains the same code point more than once, return
false.
Let P be BodyText of literal.
If FlagText of literal contains u,
then
Parse P using the grammars in
21.2.1. The
goal symbol
for the parse is
Pattern[+U, +N]. If P did not conform to the grammar, if any
elements of P were not matched by the parse, or
if any Early Error conditions exist, return
false. Otherwise, return
true.
Parse P using the grammars in
21.2.1. The
goal symbol
for the parse is
Pattern[~U, ~N]. If the result of parsing contains a
GroupName, reparse with the
goal symbolPattern[~U, +N]. If P did not conform to the grammar, if any
elements of P were not matched by the parse, or if
any Early Error conditions exist, return
false. Otherwise, return
true.
Return a
List
whose first element is siteObj, whose second
elements is firstSub, and whose subsequent elements
are the elements of restSub, in order.
restSub may contain no elements.
Append the
Record
{ [[Site]]: templateLiteral, [[Array]]:
template } to templateRegistry.
Return template.
Note 1
The creation of a template object cannot result in an
abrupt completion.
Note 2
Each
TemplateLiteral
in the program code of a
realm
is associated with a unique template object that is used in
the evaluation of tagged Templates (12.2.9.6). The template objects are frozen and the same template
object is used each time a specific tagged Template is
evaluated. Whether template objects are created lazily upon
first evaluation of the
TemplateLiteral
or eagerly prior to first evaluation is an implementation
choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional
non-enumerable properties of template objects.
This algorithm does not apply
GetValue
to the result of evaluating
Expression. The principal motivation for this is so that operators such
as delete and typeof may be applied
to parenthesized expressions.
The abstract operation EvaluatePropertyAccessWithExpressionKey takes
as arguments a value baseValue, a
Parse Nodeexpression, and a Boolean argument strict. It
performs the following steps:
Let propertyNameReference be the result of evaluating
expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
Let propertyKey be ? ToPropertyKey(propertyNameValue).
Return a value of type
Reference
whose base value component is bv, whose referenced
name component is propertyKey, and whose strict
reference flag is strict.
The abstract operation EvaluatePropertyAccessWithIdentifierKey takes
as arguments a value baseValue, a
Parse NodeidentifierName, and a Boolean argument strict.
It performs the following steps:
Let propertyNameString be StringValue of
identifierName.
Return a value of type
Reference
whose base value component is bv, whose referenced
name component is propertyNameString, and whose
strict reference flag is strict.
The abstract operation EvaluateCall takes as arguments a value
func, a value ref, a
Parse Nodearguments, and a Boolean argument
tailPosition. It performs the following steps:
Assert: If tailPosition is true, the
above call will not return here, but instead evaluation will
continue as if the following return has already occurred.
Return a value of type
Reference
that is a
Super Reference
whose base value component is bv, whose referenced
name component is propertyKey, whose thisValue
component is actualThis, and whose strict reference
flag is strict.
12.3.8Argument Lists
Note
The evaluation of an argument list produces a
List
of values.
A tagged template is a function call where the arguments of the
call are derived from a
TemplateLiteral
(12.2.9). The actual arguments include a template object (12.2.9.4) and the values produced by evaluating the expressions
embedded within the
TemplateLiteral.
When a delete operator occurs within
strict mode code, a SyntaxError exception is thrown if its
UnaryExpression
is a direct reference to a variable, function argument, or
function name. In addition, if a delete operator
occurs within
strict mode code
and the property to be deleted has the attribute {
[[Configurable]]: false }, a
TypeError exception is thrown.
No hint is provided in the calls to
ToPrimitive
in steps 5 and 6. All standard objects except Date objects
handle the absence of a hint as if the hint Number were given;
Date objects handle the absence of a hint as if the hint
String were given. Exotic objects may handle the absence of a
hint in some other manner.
Note 2
Step 7 differs from step 3 of the
Abstract Relational Comparison
algorithm, by using the logical-or operation instead of the
logical-and operation.
The result of evaluating a relational operator is always of type
Boolean, reflecting whether the relationship named by the operator
holds between its two operands.
The abstract operation InstanceofOperator(V,
target) implements the generic algorithm for determining
if ECMAScript value V is an instance of object
target either by consulting target's
@@hasinstance method or, if absent, determining whether the value of
target's "prototype" property is
present in V's prototype chain. This abstract operation
performs the following steps:
If
Type(target) is not Object, throw a
TypeError exception.
Let instOfHandler be ? GetMethod(target, @@hasInstance).
If instOfHandler is not undefined,
then
Return ! ToBoolean(?
Call(instOfHandler, target, «
V »)).
If
IsCallable(target) is false, throw a
TypeError exception.
Steps 4 and 5 provide compatibility with previous editions of
ECMAScript that did not use a @@hasInstance method to define the
instanceof operator semantics. If an object does
not define or inherit @@hasInstance it uses the default
instanceof semantics.
12.11Equality Operators
Note
The result of evaluating an equality operator is always of type
Boolean, reflecting whether the relationship named by the operator
holds between its two operands.
If r.[[Value]] is true, return
false. Otherwise, return
true.
Note 1
Given the above definition of equality:
String comparison can be forced by:
`${a}` == `${b}`.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to !(A == B).
A == B is equivalent to B == A,
except in the order of evaluation of A and
B.
Note 3
The equality operator is not always transitive. For example,
there might be two distinct String objects, each representing
the same String value; each String object would be considered
equal to the String value by the == operator, but
the two String objects would not be equal to each other. For
example:
new String("a") == "a" and
"a" == new String("a") are both
true.
new String("a") == new String("a") is
false.
Note 4
Comparison of Strings uses a simple equality test on sequences
of code unit values. There is no attempt to use the more
complex, semantically oriented definitions of character or
string equality and collating order defined in the Unicode
specification. Therefore Strings values that are canonically
equal according to the Unicode standard could test as unequal.
In effect this algorithm assumes that both Strings are already
in normalized form.
The value produced by a && or
|| operator is not necessarily of type Boolean. The
value produced will always be the value of one of the two operand
expressions.
The grammar for a
ConditionalExpression
in ECMAScript is slightly different from that in C and Java, which
each allow the second subexpression to be an
Expression
but restrict the third expression to be a
ConditionalExpression. The motivation for this difference in ECMAScript is to allow an
assignment expression to be governed by either arm of a
conditional and to eliminate the confusing and fairly useless case
of a comma expression as the centre expression.
When an assignment occurs within
strict mode code, it is a runtime error if lref in step 1.f of the
first algorithm or step 7 of the second algorithm it is an
unresolvable reference. If it is, a
ReferenceError exception is thrown. The
LeftHandSideExpression
also may not be a reference to a
data property
with the attribute value { [[Writable]]:
false }, to an
accessor property
with the attribute value { [[Set]]:
undefined }, nor to a non-existent property
of an object for which the
IsExtensible
predicate returns the value false. In these
cases a TypeError exception is thrown.
Left to right evaluation order is maintained by evaluating a
DestructuringAssignmentTarget
that is not a destructuring pattern prior to accessing the
iterator or evaluating the
Initializer.
The value of a
StatementList
is the value of the last value-producing item in the
StatementList. For example, the following calls to the
eval function all return the value 1:
When a
Block
or
CaseBlock
is evaluated a new declarative
Environment Record
is created and bindings for each block scoped variable,
constant, function, or class declared in the block are
instantiated in the
Environment Record.
BlockDeclarationInstantiation is performed as follows using
arguments code and env. code is the
Parse Node
corresponding to the body of the block. env is the
Lexical Environment
in which bindings are to be created.
When undefined is passed for
environment it indicates that a
PutValue
operation should be used to assign the initialization value.
This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are
preinitialized in order to deal with the possibility of
multiple parameters with the same name.
When undefined is passed for
environment it indicates that a
PutValue
operation should be used to assign the initialization value.
This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are
preinitialized in order to deal with the possibility of
multiple parameters with the same name.
With parameters value, environment, and
propertyName.
Note
When undefined is passed for
environment it indicates that a
PutValue
operation should be used to assign the initialization value.
This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are
preinitialized in order to deal with the possibility of
multiple parameters with the same name.
Each else for which the choice of associated
if is ambiguous shall be associated with the nearest
possible if that would otherwise have no corresponding
else.
undefined is passed for
environment to indicate that a
PutValue
operation should be used to assign the initialization value.
This is the case for var statements and the
formal parameter lists of some non-strict functions (see
9.2.10). In those cases a lexical binding is hoisted and
preinitialized prior to evaluation of its initializer.
The abstract operation ForIn/OfHeadEvaluation is called with
arguments TDZnames, expr, and
iterationKind. The value of iterationKind is
either enumerate,
iterate, or
async-iterate.
The abstract operation ForIn/OfBodyEvaluation is called with
arguments lhs, stmt,
iteratorRecord, iterationKind,
lhsKind, labelSet, and optional argument
iteratorKind. The value of lhsKind is either
assignment,
varBinding or
lexicalBinding. The value of
iteratorKind is either sync or
async.
If iteratorKind is not present, set
iteratorKind to sync.
Return an Iterator object (25.1.1.2) whose next method iterates over all the
String-valued keys of enumerable properties of O.
The iterator object is never directly accessible to ECMAScript
code. The mechanics and order of enumerating the properties is
not specified but must conform to the rules specified below.
The iterator's throw and return methods
are null and are never invoked. The iterator's
next method processes object properties to determine
whether the property key should be returned as an iterator value.
Returned property keys do not include keys that are Symbols.
Properties of the target object may be deleted during enumeration.
A property that is deleted before it is processed by the
iterator's next method is ignored. If new properties
are added to the target object during enumeration, the newly added
properties are not guaranteed to be processed in the active
enumeration. A
property name
will be returned by the iterator's next method at
most once in any enumeration.
Enumerating the properties of the target object includes
enumerating properties of its prototype, and the prototype of the
prototype, and so on, recursively; but a property of a prototype
is not processed if it has the same name as a property that has
already been processed by the iterator's next method.
The values of [[Enumerable]] attributes are not considered when
determining if a property of a prototype object has already been
processed. The enumerable property names of prototype objects must
be obtained by invoking EnumerateObjectProperties passing the
prototype object as the argument. EnumerateObjectProperties must
obtain the own property keys of the target object by calling its
[[OwnPropertyKeys]] internal method. Property attributes of the
target object must be obtained by calling its [[GetOwnProperty]]
internal method.
the value of the [[Prototype]] internal slot of O or
an object in its prototype chain changes,
a property is removed from O or an object in its
prototype chain,
a property is added to an object in O's prototype
chain, or
the value of the [[Enumerable]] attribute of a property of
O or an object in its prototype chain changes.
Note 1
Hosts are not required to implement the algorithm in
13.7.5.16.2.1
directly. They may choose any implementation whose behavior
will not deviate from that algorithm unless one of the
constraints in the previous paragraph is violated.
The following is an informative definition of an ECMAScript
generator function that conforms to these rules:
function* EnumerateObjectProperties(obj) {
const visited = newSet();
for (const key ofReflect.ownKeys(obj)) {
if (typeof key === "symbol") continue;
const desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
const proto = Reflect.getPrototypeOf(obj);
if (proto === null) return;
for (const protoKey of EnumerateObjectProperties(proto)) {
if (!visited.has(protoKey)) yield protoKey;
}
}
Note 2
The list of exotic objects for which implementations are not
required to match
CreateForInIterator
was chosen because implementations historically differed in
behavior for those cases, and agreed in all others.
13.7.5.16For-In Iterator Objects
A For-In Iterator is an object that represents a specific
iteration over some specific object. For-In Iterator objects are
never directly accessible to ECMAScript code; they exist soley to
illustrate the behavior of
EnumerateObjectProperties.
13.7.5.16.1CreateForInIterator (
object )
The abstract operation CreateForInIterator with argument
object is used to create a For-In Iterator object
which iterates over the own and inherited enumerable string
properties of object in a specific order. It performs
the following steps:
13.7.5.16.3Properties of For-In
Iterator Instances
For-In Iterator instances are ordinary objects that inherit
properties from the
%ForInIteratorPrototype%
intrinsic object. For-In Iterator instances are initially
created with the internal slots listed in
Table 38.
Table 38: Internal Slots of For-In Iterator Instances
Internal Slot
Description
[[Object]]
The Object value whose properties are being iterated.
[[ObjectWasVisited]]
true if the iterator has invoked
[[OwnPropertyKeys]] on [[Object]],
false otherwise.
[[VisitedKeys]]
A list of String values which have been emitted by
this iterator thus far.
[[RemainingKeys]]
A list of String values remaining to be emitted for
the current object, before iterating the properties of
its prototype (if its prototype is not
null).
It is a Syntax Error if this
ContinueStatement
is not nested, directly or indirectly (but not crossing function
boundaries), within an
IterationStatement.
A return statement causes a function to cease
execution and, in most cases, returns a value to the caller. If
Expression
is omitted, the return value is undefined.
Otherwise, the return value is the value of
Expression. A return statement may not actually return a value
to the caller depending on surrounding context. For example, in a
try block, a return statement's
completion record may be replaced with another completion record
during evaluation of the finally block.
The with statement adds an object
Environment Record
for a computed object to the lexical environment of the
running execution context. It then executes a statement using this augmented lexical
environment. Finally, it restores the original lexical
environment.
No matter how control leaves the embedded
Statement, whether normally or by some form of
abrupt completion
or exception, the LexicalEnvironment is always restored to its
former state.
This operation does not execute C's
StatementList
(if any). The
CaseBlock
algorithm uses its return value to determine which
StatementList
to start executing.
A
Statement
may be prefixed by a label. Labelled statements are only used in
conjunction with labelled break and
continue statements. ECMAScript has no
goto statement. A
Statement
can be part of a
LabelledStatement, which itself can be part of a
LabelledStatement, and so on. The labels introduced this way are collectively
referred to as the “current label set” when describing the
semantics of individual statements.
The try statement encloses a block of code in which
an exceptional condition can occur, such as a runtime error or a
throw statement. The catch clause
provides the exception-handling code. When a catch clause catches
an exception, its
CatchParameter
is bound to that exception.
Evaluating a
DebuggerStatement
may allow an implementation to cause a breakpoint when run under
a debugger. If a debugger is not present or active this
statement has no observable effect.
Various ECMAScript language elements cause the creation of
ECMAScript function objects (9.2). Evaluation of such functions starts with the execution of their
[[Call]] internal method (9.2.1).
The ExpectedArgumentCount of a
FormalParameterList
is the number of
FormalParameters
to the left of either the rest parameter or the first
FormalParameter
with an Initializer. A
FormalParameter
without an initializer is allowed after the first parameter with
an initializer but such parameters are considered to be optional
with undefined as their default value.
The abstract operation IsAnonymousFunctionDefinition determines if
its argument is a function definition that does not bind a name. The
argument expr is the result of parsing an
AssignmentExpression
or
Initializer. The following steps are taken:
If IsFunctionDefinition of expr is
false, return false.
When undefined is passed for
environment it indicates that a
PutValue
operation should be used to assign the initialization value.
This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are
preinitialized in order to deal with the possibility of multiple
parameters with the same name.
A "prototype" property is automatically
created for every function defined using a
FunctionDeclaration
or
FunctionExpression, to allow for the possibility that the function will be used
as a
constructor.
Normally, Contains does not look inside most function forms.
However, Contains is used to detect new.target,
this, and super usage within an
ArrowFunction.
When undefined is passed for
environment it indicates that a
PutValue
operation should be used to assign the initialization value.
This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are
preinitialized in order to deal with the possibility of multiple
parameters with the same name.
Set closure.[[SourceText]] to the source text matched
by
ArrowFunction.
Return closure.
Note
An
ArrowFunction
does not define local bindings for arguments,
super, this, or
new.target. Any reference to
arguments, super, this,
or new.target within an
ArrowFunction
must resolve to a binding in a lexically enclosing environment.
Typically this will be the Function Environment of an
immediately enclosing function. Even though an
ArrowFunction
may contain references to super, the
function object
created in step 4 is not made into a method by performing
MakeMethod. An
ArrowFunction
that references super is always contained within a
non-ArrowFunction
and the necessary state to implement super is
accessible via the scope that is captured by the
function object
of the
ArrowFunction.
YieldExpression
cannot be used within the
FormalParameters
of a generator function because any expressions that are part of
FormalParameters
are evaluated before the resulting generator object is in a
resumable state.
Let innerResult be ? Call(throw, iterator, «
received.[[Value]] »).
If generatorKind is
async, then set
innerResult to ? Await(innerResult).
NOTE: Exceptions from the inner iterator
throw method are propagated. Normal
completions from an inner throw method
are processed similarly to an inner
next.
If
Type(innerResult) is not Object, throw a
TypeError exception.
NOTE: If iterator does not have a
throw method, this throw is going to
terminate the yield* loop. But first we
need to give iterator a chance to clean
up.
Let closeCompletion be
Completion
{ [[Type]]: normal,
[[Value]]: empty, [[Target]]:
empty }.
If generatorKind is
async, perform
? AsyncIteratorClose(iteratorRecord,
closeCompletion).
YieldExpression
and
AwaitExpression
cannot be used within the
FormalParameters
of an async generator function because any expressions that are
part of
FormalParameters
are evaluated before the resulting async generator object is in a
resumable state.
When
Module
is the syntactic
goal symbol
and the
[Await]
parameter is absent, await is parsed as a
keyword
and will be a Syntax error. When
Script
is the syntactic
goal symbol, await may be parsed as an identifier when the
[Await]
parameter is absent. This includes the following contexts:
Normally, Contains does not look inside most function forms.
However, Contains is used to detect new.target,
this, and super usage within an
AsyncArrowFunction.
Return the result of HasCallInTailPosition of
body with argument call.
Note
Tail Position calls are only defined in
strict mode code
because of a common non-standard language extension (see
9.2.4) that enables observation of the chain of caller contexts.
14.9.2Static Semantics:
HasCallInTailPosition
With parameter call.
Note
call is a
Parse Node
that represents a specific range of source text. When the
following algorithms compare call to another
Parse Node, it is a test of whether they represent the same source text.
Return HasCallInTailPosition of
Block
with argument call.
14.9.2.2Expression Rules
Note
A potential tail position call that is immediately followed by
return
GetValue
of the call result is also a possible tail position call.
Function calls cannot return reference values, so such a
GetValue
operation will always return the same value as the actual
function call result.
A tail position call must either release any transient internal
resources associated with the currently executing function
execution context
before invoking the target function or reuse those resources in
support of the target function.
Note
For example, a tail position call should only grow an
implementation's activation record stack by the amount that the
size of the target function's activation record exceeds the size
of the calling function's activation record. If the target
function's activation record is smaller, then the total size of
the stack should decrease.
It is a Syntax Error if
StatementList
Contains super unless the source code containing
super is eval code that is being processed by a
direct eval. Additional
early error
rules for super within
direct eval
are defined in
18.2.1.1.
The abstract operation ParseScript with arguments
sourceText, realm, and
hostDefined creates a
Script Record
based upon the result of parsing sourceText as a
Script. ParseScript performs the following steps:
Assert: sourceText is an ECMAScript source text (see
clause
10).
Parse sourceText using
Script
as the
goal symbol
and analyse the parse result for any Early Error conditions. If
the parse was successful and no early errors were found, let
body be the resulting parse tree. Otherwise, let
body be a
List
of one or more SyntaxError objects
representing the parsing errors and/or early errors. Parsing and
early error
detection may be interweaved in an implementation-dependent
manner. If more than one parsing error or
early error
is present, the number and ordering of error objects in the list
is implementation-dependent, but at least one must be present.
An implementation may parse script source text and analyse it
for Early Error conditions prior to evaluation of ParseScript
for that script source text. However, the reporting of any
errors must be deferred until the point where this specification
actually performs ParseScript upon that source text.
15.1.10ScriptEvaluation (
scriptRecord )
Let globalEnv be
scriptRecord.[[Realm]].[[GlobalEnv]].
When an
execution context
is established for evaluating scripts, declarations are
instantiated in the current
global environment. Each global binding declared in the code is instantiated.
GlobalDeclarationInstantiation is performed as follows using
arguments script and env. script is
the
ScriptBody
for which the
execution context
is being established. env is the global lexical
environment in which bindings are to be created.
If vn is not an element of
declaredFunctionNames, then
Let vnDefinable be ?
envRec.CanDeclareGlobalVar(vn).
If vnDefinable is
false, throw a
TypeError exception.
If vn is not an element of
declaredVarNames, then
Append vn to
declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step
if the
global object
is an ordinary object. However, if the
global object
is a Proxy
exotic object
it may exhibit behaviours that cause abnormal terminations in
some of the following steps.
NOTE: Annex
B.3.3.2
adds additional steps at this point.
Let lexDeclarations be the
LexicallyScopedDeclarations of script.
For each element d in lexDeclarations, do
NOTE: Lexically declared names are only instantiated here
but not initialized.
Early errors specified in
15.1.1
prevent name conflicts between function/var declarations and
let/const/class declarations as well as redeclaration of
let/const/class bindings for declaration contained within a
single
Script. However, such conflicts and redeclarations that span more
than one
Script
are detected as runtime errors during
GlobalDeclarationInstantiation. If any such errors are detected,
no bindings are instantiated for the script. However, if the
global object
is defined using Proxy exotic objects then the runtime tests for
conflicting declarations may be unreliable resulting in an
abrupt completion
and some global declarations not being instantiated. If this
occurs, the code for the
Script
is not evaluated.
Unlike explicit var or function declarations, properties that
are directly created on the
global object
result in global bindings that may be shadowed by
let/const/class declarations.
It is a Syntax Error if the LexicallyDeclaredNames of
ModuleItemList
contains any duplicate entries.
It is a Syntax Error if any element of the
LexicallyDeclaredNames of
ModuleItemList
also occurs in the VarDeclaredNames of
ModuleItemList.
It is a Syntax Error if the ExportedNames of
ModuleItemList
contains any duplicate entries.
It is a Syntax Error if any element of the ExportedBindings of
ModuleItemList
does not also occur in either the VarDeclaredNames of
ModuleItemList, or the LexicallyDeclaredNames of
ModuleItemList.
It is a Syntax Error if ContainsDuplicateLabels of
ModuleItemList
with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of
ModuleItemList
with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of
ModuleItemList
with arguments « » and « » is true.
Note
The duplicate ExportedNames rule implies that multiple
export defaultExportDeclaration
items within a
ModuleBody
is a Syntax Error. Additional error conditions relating to
conflicting or duplicate declarations are checked during
module linking prior to evaluation of a
Module. If any such errors are detected the
Module
is not evaluated.
The abstract operation ImportedLocalNames with argument
importEntries creates a
List
of all of the local name bindings defined by a
List
of ImportEntry Records (see
Table 45). ImportedLocalNames performs the following steps:
A Module Record encapsulates structural information
about the imports and exports of a single module. This information
is used to link the imports and exports of sets of connected
modules. A Module Record includes four fields that are only used
when evaluating a module.
For specification purposes Module Record values are values of the
Record
specification type and can be thought of as existing in a simple
object-oriented hierarchy where Module Record is an abstract class
with both abstract and concrete subclasses. This specification
defines the abstract subclass named
Cyclic Module Record
and its concrete subclass named
Source Text Module Record. Other specifications and implementations may define additional
Module Record subclasses corresponding to alternative module
definition facilities that they defined.
Module Record defines the fields listed in
Table 40. All Module Definition subclasses include at least those fields.
Module Record also defines the abstract method list in
Table 41. All Module definition subclasses must provide concrete
implementations of these abstract methods.
The
Lexical Environment
containing the top level bindings for this module. This
field is set when the module is linked.
[[Namespace]]
Object | undefined
The Module Namespace Object (26.3) if one has been created for this module. Otherwise
undefined.
[[HostDefined]]
Any, default value is undefined.
Field reserved for use by host environments that need to
associate additional information with a module.
Table 41: Abstract Methods of Module Records
Method
Purpose
GetExportedNames([exportStarSet])
Return a list of all names that are either directly or
indirectly exported from this module.
ResolveExport(exportName [,
resolveSet])
Return the binding of a name exported by this module.
Bindings are represented by a
ResolvedBinding Record, of the form { [[Module]]:
Module Record, [[BindingName]]: String }. If the export is a
Module Namespace Object without a direct binding in
any module, [[BindingName]] will be set to
"*namespace*". Return
null if the name cannot be
resolved, or "ambiguous" if
multiple bindings were found.
Each time this operation is called with a specific
exportName, resolveSet pair as
arguments it must return the same result if it
completes normally.
Link()
Prepare the module for evaluation by transitively
resolving all module dependencies and creating a
module
Environment Record.
Evaluate()
If this module has already been evaluated
successfully, return undefined; if
it has already been evaluated unsuccessfully, throw
the exception that was produced. Otherwise,
transitively evaluate all module dependencies of this
module and then evaluate this module.
Link must have completed successfully prior to
invoking this method.
15.2.1.16Cyclic Module Records
A Cyclic Module Record is
used to represent information about a module that can participate
in dependency cycles with other modules that are subclasses of the
Cyclic Module Record
type. Module Records that are not subclasses of the
Cyclic Module Record
type must not participate in dependency cycles with Source Text
Module Records.
In addition to the fields defined in
Table 40
Cyclic Module Records have the additional fields listed in
Table 42
Table 42: Additional Fields of Cyclic Module Records
A completion of type
throw representing the exception
that occurred during evaluation.
undefined if no exception occurred or
if [[Status]] is not evaluated.
Auxiliary field used during Link and Evaluate only. If
[[Status]] is linking or
evaluating, this nonnegative
number records the point at which the module was first
visited during the ongoing depth-first traversal of the
dependency graph.
Auxiliary field used during Link and Evaluate only. If
[[Status]] is linking or
evaluating, this is either the
module's own [[DFSIndex]] or that of an "earlier" module
in the same strongly connected component.
A
List
of all the
ModuleSpecifier
strings used by the module represented by this record to
request the importation of a module. The
List
is source code occurrence ordered.
In addition to the methods defined in
Table 41
Cyclic Module Records have the additional methods listed in
Table 43
Table 43: Additional Abstract Methods of Cyclic Module Records
On success, Link transitions this module's [[Status]] from
unlinked to
linked. On failure, an exception is
thrown and this module's [[Status]] remains
unlinked.
This abstract method performs the following steps (most of the
work is done by the auxiliary function
InnerModuleLinking):
15.2.1.16.1.1InnerModuleLinking (
module, stack, index )
The InnerModuleLinking abstract operation is used by Link to
perform the actual linking process for the
Cyclic Module Recordmodule, as well as recursively on all other modules
in the dependency graph. The stack and
index parameters, as well as a module's
[[DFSIndex]] and [[DFSAncestorIndex]] fields, keep track of
the depth-first search (DFS) traversal. In particular,
[[DFSAncestorIndex]] is used to discover strongly connected
components (SCCs), such that all modules in an SCC transition
to linked together.
This abstract operation performs the following steps:
15.2.1.16.2.1InnerModuleEvaluation
( module, stack, index )
The InnerModuleEvaluation abstract operation is used by
Evaluate to perform the actual evaluation process for the
Source Text Module Recordmodule, as well as recursively on all other modules
in the dependency graph. The stack and
index parameters, as well as module's
[[DFSIndex]] and [[DFSAncestorIndex]] fields, are used the
same way as in
InnerModuleLinking.
This abstract operation performs the following steps:
If requiredModule and
module are the same
Module Record, set done to true.
Return index.
15.2.1.16.3Example Cyclic Module
Record Graphs
This non-normative section gives a series of examples of the
linking and evaluation of a few common module graphs, with a
specific focus on how errors can occur.
First consider the following simple module graph:
Let's first assume that there are no error conditions. When a
host first calls A.Link(), this will complete
successfully by assumption, and recursively link modules
B and C as well, such that
A.[[Status]] = B.[[Status]] =
C.[[Status]] = linked. This
preparatory step can be performed at any time. Later, when the
host is ready to incur any possible side effects of the modules,
it can call A.Evaluate(), which will complete
successfully (again by assumption), recursively having evaluated
first C and then B. Each module's
[[Status]] at this point will be
evaluated.
Consider then cases involving linking errors. If
InnerModuleLinking
of C succeeds but, thereafter, fails for
B, for example because it imports something that
C does not provide, then the original
A.Link() will fail, and both A and
B's [[Status]] remain
unlinked. C's [[Status]] has
become linked, though.
Finally, consider a case involving evaluation errors. If
InnerModuleEvaluation
of C succeeds but, thereafter, fails for
B, for example because B contains code
that throws an exception, then the original
A.Evaluate() will fail. The resulting exception will
be recorded in both A and B's
[[EvaluationError]] fields, and their [[Status]] will become
evaluated. C will also become
evaluated but, in contrast to
A and B, will remain without an
[[EvaluationError]], as it successfully completed evaluation.
Storing the exception ensures that any time a host tries to
reuse A or B by calling their Evaluate()
method, it will encounter the same exception. (Hosts are not
required to reuse Cyclic Module Records; similarly, hosts are
not required to expose the exception objects thrown by these
methods. However, the specification enables such uses.)
The difference here between linking and evaluation errors is due
to how evaluation must be only performed once, as it can cause
side effects; it is thus important to remember whether
evaluation has already been performed, even if unsuccessfully.
(In the error case, it makes sense to also remember the
exception because otherwise subsequent Evaluate() calls would
have to synthesize a new one.) Linking, on the other hand, is
side-effect-free, and thus even if it fails, it can be retried
at a later time with no issues.
Now consider a different type of error condition:
In this scenario, module A declares a dependency on
some other module, but no
Module Record
exists for that module, i.e.
HostResolveImportedModule
throws an exception when asked for it. This could occur for a
variety of reasons, such as the corresponding resource not
existing, or the resource existing but
ParseModule
throwing an exception when trying to parse the resulting source
text. Hosts can choose to expose the cause of failure via the
exception they throw from
HostResolveImportedModule. In any case, this exception causes a linking failure, which
as before results in A's [[Status]] remaining
unlinked.
Lastly, consider a module graph with a cycle:
Here we assume that the entry point is module A, so
that the host proceeds by calling A.Link(), which
performs
InnerModuleLinking
on A. This in turn calls
InnerModuleLinking
on B. Because of the cycle, this again triggers
InnerModuleLinking
on A, but at this point it is a no-op since
A.[[Status]] is already
linking. B.[[Status]] itself
remains linking when control gets back to
A and
InnerModuleLinking
is triggered on C. After this returns with
C.[[Status]] being linked ,
both A and B transition from
linking to
linked together; this is by design, since
they form a strongly connected component.
An analogous story occurs for the evaluation phase of a cyclic
module graph, in the success case.
Now consider a case where A has an linking error; for
example, it tries to import a binding from C that
does not exist. In that case, the above steps still occur,
including the early return from the second call to
InnerModuleLinking
on A. However, once we unwind back to the original
InnerModuleLinking
on A, it fails during
InitializeEnvironment, namely right after C.ResolveExport(). The thrown
SyntaxError exception propagates up to
A.Link, which resets all modules that are currently
on its stack (these are always exactly the modules
that are still linking). Hence both
A and B become
unlinked. Note that C is left
as linked.
Finally, consider a case where A has an evaluation
error; for example, its source code throws an exception. In that
case, the evaluation-time analog of the above steps still
occurs, including the early return from the second call to
InnerModuleEvaluation
on A. However, once we unwind back to the original
InnerModuleEvaluation
on A, it fails by assumption. The exception thrown
propagates up to A.Evaluate(), which records the
error in all modules that are currently on its
stack (i.e., the modules that are still
evaluating). Hence both A and
B become evaluated and the
exception is recorded in both A and B's
[[EvaluationError]] fields, while C is left as
evaluated with no [[EvaluationError]].
15.2.1.17Source Text Module Records
A
Source Text Module Record
is used to represent information about a module that was defined
from ECMAScript source text (10) that was parsed using the
goal symbolModule. Its fields contain digested information about the names that
are imported by the module and its concrete methods use this
digest to link, link, and evaluate the module.
In addition to the fields defined in
Table 42, Source Text Module Records have the additional fields listed in
Table 44. Each of these fields is initially set in
ParseModule.
Table 44: Additional Fields of Source Text Module Records
A
List
of ExportEntry records derived from the code of this
module that correspond to reexported imports that occur
within the module or exports from
export * as namespace declarations.
A
List
of ExportEntry records derived from the code of this
module that correspond to
export * declarations that occur within the
module, not including
export * as namespace declarations.
An ImportEntry Record is a
Record
that digests information about a single declarative import. Each
ImportEntry Record
has the fields defined in
Table 45:
The name under which the desired binding is exported by
the module identified by [[ModuleRequest]]. The value
"*" indicates that the import request
is for the target module's namespace object.
[[LocalName]]
String
The name that is used to locally access the imported
value from within the importing module.
Note 1
Table 46
gives examples of ImportEntry records fields used to represent
the syntactic import forms:
Table 46 (Informative): Import Forms Mappings to
ImportEntry Records
An ExportEntry Record is a
Record
that digests information about a single declarative export. Each
ExportEntry Record
has the fields defined in
Table 47:
The name under which the desired binding is exported by
the module identified by [[ModuleRequest]].
null if the
ExportDeclaration
does not have a
ModuleSpecifier. "*" indicates that the export
request is for all exported bindings.
[[LocalName]]
String | null
The name that is used to locally access the exported
value from within the importing module.
null if the exported value is not
locally accessible from within the module.
Note 2
Table 48
gives examples of the ExportEntry record fields used to
represent the syntactic export forms:
Table 48 (Informative): Export Forms Mappings to
ExportEntry Records
Export Statement Form
[[ExportName]]
[[ModuleRequest]]
[[ImportName]]
[[LocalName]]
export var v;
"v"
null
null
"v"
export default function f() {}
"default"
null
null
"f"
export default function () {}
"default"
null
null
"*default*"
export default 42;
"default"
null
null
"*default*"
export {x};
"x"
null
null
"x"
export {v as x};
"x"
null
null
"v"
export {x} from "mod";
"x"
"mod"
"x"
null
export {v as x} from "mod";
"x"
"mod"
"v"
null
export * from "mod";
null
"mod"
"*"
null
export * as ns from "mod";
"ns"
"mod"
"*"
null
The following definitions specify the required concrete methods
and other
abstract operations
for Source Text Module Records
The abstract operation ParseModule with arguments
sourceText, realm, and
hostDefined creates a
Source Text Module Record
based upon the result of parsing sourceText as a
Module. ParseModule performs the following steps:
Assert: sourceText is an ECMAScript source text (see
clause
10).
Parse sourceText using
Module
as the
goal symbol
and analyse the parse result for any Early Error conditions.
If the parse was successful and no early errors were found,
let body be the resulting parse tree. Otherwise,
let body be a
List
of one or more SyntaxError objects
representing the parsing errors and/or early errors. Parsing
and
early error
detection may be interweaved in an implementation-dependent
manner. If more than one parsing error or
early error
is present, the number and ordering of error objects in the
list is implementation-dependent, but at least one must be
present.
An implementation may parse module source text and analyse
it for Early Error conditions prior to the evaluation of
ParseModule for that module source text. However, the
reporting of any errors must be deferred until the point
where this specification actually performs ParseModule upon
that source text.
ResolveExport attempts to resolve an imported binding to the
actual defining module and local binding name. The defining
module may be the module represented by the
Module Record
this method was invoked on or some other module that is imported
by that module. The parameter resolveSet is used to
detect unresolved circular import/export paths. If a pair
consisting of specific
Module Record
and exportName is reached that is already in
resolveSet, an import circularity has been
encountered. Before recursively calling ResolveExport, a pair
consisting of module and exportName is
added to resolveSet.
If a defining module is found, a
ResolvedBinding Record
{ [[Module]], [[BindingName]] } is returned. This record
identifies the resolved binding of the originally requested
export, unless this is the export of a namespace with no local
binding. In this case, [[BindingName]] will be set to
"*namespace*". If no definition was found or
the request is found to be circular, null is
returned. If the request is found to be ambiguous, the string
"ambiguous" is returned.
This abstract method performs the following steps:
If resolveSet is not present, set
resolveSet to a new empty
List.
Assert: resolveSet is a
List
of
Record
{ [[Module]], [[ExportName]] }.
If starResolution is
null, set
starResolution to resolution.
Else,
Assert: There is more than one * import
that includes the requested name.
If resolution.[[Module]] and
starResolution.[[Module]] are not the
same
Module Record
or
SameValue(resolution.[[BindingName]],
starResolution.[[BindingName]]) is
false, return
"ambiguous".
NOTE: The above call cannot fail because imported module
requests are a subset of
module.[[RequestedModules]], and these have
been resolved earlier in this algorithm.
HostResolveImportedModule is an implementation-defined abstract
operation that provides the concrete
Module Record
subclass instance that corresponds to the
ModuleSpecifier
String, specifier, occurring within the context of the
script or module represented by the
Script Record
or
Module RecordreferencingScriptOrModule.
referencingScriptOrModule may also be
null, if the resolution is being performed in
the context of an
import()
expression, and there is no active script or module at that time.
Note
An example of when referencingScriptOrModule can be
null is in a web browser host. There, if a
user clicks on a control given by
there will be no active script or module at the time the
import()
expression runs. More generally, this can happen in any
situation where the host pushes execution contexts with
null ScriptOrModule components onto the
execution context stack.
The implementation of HostResolveImportedModule must conform to
the following requirements:
The normal return value must be an instance of a concrete
subclass of
Module Record.
If a
Module Record
corresponding to the pair referencingScriptOrModule,
specifier does not exist or cannot be created, an
exception must be thrown.
Each time this operation is called with a specific
referencingScriptOrModule, specifier pair
as arguments it must return the same
Module Record
instance if it completes normally.
Multiple different referencingScriptOrModule,
specifier pairs may map to the same
Module Record
instance. The actual mapping semantic is implementation-defined
but typically a normalization process is applied to
specifier as part of the mapping process. A typical
normalization process would include actions such as alphabetic
case folding and expansion of relative and abbreviated path
specifiers.
HostImportModuleDynamically is an implementation-defined abstract
operation that performs any necessary setup work in order to make
available the module corresponding to the
ModuleSpecifier
String, specifier, occurring within the context of the
script or module represented by the
Script Record
or
Module RecordreferencingScriptOrModule. (referencingScriptOrModule
may also be null, if there is no active script
or module when the
import()
expression occurs.) It then performs
FinishDynamicImport
to finish the dynamic import process.
The implementation of HostImportModuleDynamically must conform to
the following requirements:
The abstract operation must always complete normally with
undefined. Success or failure must instead be
signaled as discussed below.
The host environment must conform to one of the two following
sets of requirements:
Success path
At some future time, the host environment must perform
FinishDynamicImport(referencingScriptOrModule,
specifier, promiseCapability,
NormalCompletion(undefined)).
The completion value of any subsequent call to
HostResolveImportedModule
after
FinishDynamicImport
has completed, given the arguments
referencingScriptOrModule and
specifier, must be a module which has already
been evaluated, i.e. whose Evaluate concrete method has
already been called and returned a normal completion.
Failure path
At some future time, the host environment must perform
FinishDynamicImport(referencingScriptOrModule,
specifier, promiseCapability, an
abrupt completion), with the
abrupt completion
representing the cause of failure.
If the host environment takes the success path once for a given
referencingScriptOrModule, specifier pair,
it must always do so for subsequent calls.
The operation must not call
promiseCapability.[[Resolve]] or
promiseCapability.[[Reject]], but instead must treat
promiseCapability as an opaque identifying value to
be passed through to
FinishDynamicImport.
The actual process performed is implementation-defined, but
typically consists of performing whatever I/O operations are
necessary to allow
HostResolveImportedModule
to synchronously retrieve the appropriate
Module Record, and then calling its Evaluate concrete method. This might
require performing similar normalization as
HostResolveImportedModule
does.
FinishDynamicImport completes the process of a dynamic import
originally started by an
import()
call, resolving or rejecting the promise returned by that call as
appropriate according to completion. It is performed by
host environments as part of
HostImportModuleDynamically.
If completion is an
abrupt completion, then perform ! Call(promiseCapability.[[Reject]],
undefined, «
completion.[[Value]] »).
Else,
Assert: completion is a normal completion and
completion.[[Value]] is
undefined.
The GetModuleNamespace abstract operation retrieves the Module
Namespace
Exotic object
representing module's exports, lazily creating it the
first time it was requested, and storing it in
module.[[Namespace]] for future retrieval.
This abstract operation performs the following steps:
The only way GetModuleNamespace can throw is via one of the
triggered
HostResolveImportedModule
calls. Unresolvable names are simply excluded from the
namespace at this point. They will lead to a real linking
error later unless they are all ambiguous star exports that
are not explicitly requested anywhere.
Assert: All dependencies of m have been transitively
resolved and m is ready for evaluation.
Return ? m.Evaluate().
Note
An implementation may parse a sourceText as a
Module, analyse it for Early Error conditions, and link it prior to
the execution of the TopLevelModuleEvaluationJob for that
sourceText. An implementation may also resolve,
pre-parse and pre-analyse, and pre-link module dependencies of
sourceText. However, the reporting of any errors
detected by these actions must be deferred until the
TopLevelModuleEvaluationJob is actually executed.
For each
IdentifierNamen in ReferencedBindings of
NamedExports: It is a Syntax Error if StringValue of n is a
ReservedWord
or if the StringValue of n is one of:
"implements", "interface",
"let", "package",
"private", "protected",
"public", or "static".
Return a new
List
containing the
ExportEntry Record
{ [[ModuleRequest]]: null, [[ImportName]]:
null, [[LocalName]]: localName,
[[ExportName]]: "default" }.
Return a new
List
containing the
ExportEntry Record
{ [[ModuleRequest]]: null, [[ImportName]]:
null, [[LocalName]]: localName,
[[ExportName]]: "default" }.
Return a new
List
containing the
ExportEntry Record
{ [[ModuleRequest]]: module, [[ImportName]]:
importName, [[LocalName]]: localName,
[[ExportName]]: sourceName }.
Let sourceName be the StringValue of the first
IdentifierName.
Let exportName be the StringValue of the second
IdentifierName.
If module is null, then
Let localName be sourceName.
Let importName be null.
Else,
Let localName be null.
Let importName be sourceName.
Return a new
List
containing the
ExportEntry Record
{ [[ModuleRequest]]: module, [[ImportName]]:
importName, [[LocalName]]: localName,
[[ExportName]]: exportName }.
It is not necessary to treat export defaultAssignmentExpression
as a constant declaration because there is no syntax that
permits assignment to the internal bound name used to
reference a module's default object.
An implementation must report most errors at the time the relevant
ECMAScript language construct is evaluated. An
early error is an error that can be detected
and reported prior to the evaluation of any construct in the
Script
containing the error. The presence of an
early error
prevents the evaluation of the construct. An implementation must report
early errors in a
Script as
part of parsing that
Script in
ParseScript. Early errors in a
Module are
reported at the point when the
Module would
be evaluated and the
Module is
never initialized. Early errors in eval code are reported at the
time eval is called and prevent evaluation of the
eval code. All errors that are not early errors are runtime
errors.
An implementation must report as an
early error
any occurrence of a condition that is listed in a “Static Semantics:
Early Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early errors
even if the compiler can prove that a construct cannot execute without
error under any circumstances. An implementation may issue an early
warning in such a case, but it should not report the error until the
relevant construct is actually executed.
An implementation shall report all errors as specified, except for the
following:
Except as restricted in
16.2, an implementation may extend
Script
syntax,
Module
syntax, and regular expression pattern or flag syntax. To permit this,
all operations (such as calling eval, using a regular
expression literal, or using the Function or
RegExpconstructor) that are allowed to throw SyntaxError are
permitted to exhibit implementation-defined behaviour instead of
throwing SyntaxError when they encounter an
implementation-defined extension to the script syntax or regular
expression pattern or flag syntax.
Except as restricted in
16.2, an implementation may provide additional types, values, objects,
properties, and functions beyond those described in this
specification. This may cause constructs (such as looking up a
variable in the global scope) to have implementation-defined behaviour
instead of throwing an error (such as
ReferenceError).
16.1HostReportErrors (
errorList )
HostReportErrors is an implementation-defined abstract operation that
allows host environments to report parsing errors, early errors, and
runtime errors.
An implementation of HostReportErrors must complete normally in all
cases. The default implementation of HostReportErrors is to
unconditionally return an empty normal completion.
Note
errorList will be a
List
of ECMAScript language values. If the errors are parsing errors or
early errors, these will always be
SyntaxError objects. Runtime errors, however,
can be any ECMAScript value.
16.2Forbidden Extensions
An implementation must not extend this specification in the following
ways:
If an implementation extends any
function object
with an own property named "caller" the value of
that property, as observed using [[Get]] or [[GetOwnProperty]], must
not be a
strict function
object. If it is an
accessor property, the function that is the value of the property's [[Get]]
attribute must never return a
strict function
when called.
Neither mapped nor unmapped arguments objects may be created with an
own property named "caller".
The behaviour of the following methods must not be extended except
as specified in ECMA-402:
Object.prototype.toLocaleString,
Array.prototype.toLocaleString,
Number.prototype.toLocaleString,
Date.prototype.toLocaleDateString,
Date.prototype.toLocaleString,
Date.prototype.toLocaleTimeString,
String.prototype.localeCompare,
%TypedArray%.prototype.toLocaleString.
The RegExp pattern grammars in
21.2.1
and
B.1.4
must not be extended to recognize any of the source characters A-Z
or a-z as
IdentityEscape[+U]
when the [U] grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows
the token : to immediately follow source text that
matches the
BindingIdentifier
nonterminal symbol.
There are certain built-in objects available whenever an ECMAScript
Script or
Module
begins execution. One, the
global object, is part of the lexical environment of the executing program. Others
are accessible as initial properties of the
global object
or indirectly as properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a
function is a built-in
function object
with the characteristics described in
9.3. Unless specified otherwise, the [[Extensible]] internal slot of a
built-in object initially has the value true. Every
built-in
function object
has a [[Realm]] internal slot whose value is the
Realm Record
of the
realm
for which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments.
Some of them furthermore are constructors: they are functions intended
for use with the new operator. For each built-in function,
this specification describes the arguments required by that function and
the properties of that
function object. For each built-in
constructor, this specification furthermore describes properties of the prototype
object of that
constructor
and properties of specific object instances returned by a
new expression that invokes that
constructor.
Unless otherwise specified in the description of a particular function,
if a built-in function or
constructor
is given fewer arguments than the function is specified to require, the
function or
constructor
shall behave exactly as if it had been given sufficient additional
arguments, each such argument being the
undefined value. Such missing arguments are
considered to be “not present” and may be identified in that manner by
specification algorithms. In the description of a particular function,
the terms “this value” and “NewTarget” have the
meanings given in
9.3.
Unless otherwise specified in the description of a particular function,
if a built-in function or
constructor
described is given more arguments than the function is specified to
allow, the extra arguments are evaluated by the call and then ignored by
the function. However, an implementation may define implementation
specific behaviour relating to such arguments as long as the behaviour
is not the throwing of a TypeError exception that is
predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of
built-in functions are encouraged to do so by adding new functions
rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in
constructor
has the Function prototype object, which is the initial value of the
expression Function.prototype (19.2.3), as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the
Object prototype object, which is the initial value of the expression
Object.prototype (19.1.3), as the value of its [[Prototype]] internal slot, except the Object
prototype object itself.
Built-in function objects that are not identified as constructors do not
implement the [[Construct]] internal method unless otherwise specified
in the description of a particular function.
Each built-in function defined in this specification is created by
calling the
CreateBuiltinFunction
abstract operation (9.3.3).
Every built-in
function object, including constructors, has a "length" property
whose value is an
integer. Unless otherwise specified, this value is equal to the largest number
of named arguments shown in the subclause headings for the function
description. Optional parameters (which are indicated with brackets:
[]) or rest parameters (which are shown using
the form «...name») are not included in the default argument count.
Note 2
For example, the
function object
that is the initial value of the "map" property
of the Array prototype object is described under the subclause
heading «Array.prototype.map (callbackFn [ , thisArg])» which shows
the two named arguments callbackFn and thisArg, the latter being
optional; therefore the value of the
"length" property of that
function object
is 1.
Unless otherwise specified, the "length" property of
a built-in
function object
has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]:
true }.
Every built-in
function object, including constructors, that is not identified as an anonymous
function has a "name" property whose value is a
String. Unless otherwise specified, this value is the name that is given
to the function in this specification. For functions that are specified
as properties of objects, the name value is the
property name
string used to access the function. Functions that are specified as get
or set accessor functions of built-in properties have
"get " or "set " prepended to the
property name
string. The value of the "name" property is
explicitly specified for each built-in functions whose property key is a
Symbol value.
Unless otherwise specified, the "name" property of a
built-in
function object, if it exists, has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
Every other
data property
described in clauses 18 through 26 and in Annex
B.2
has the attributes { [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]:
true } unless otherwise specified.
Every
accessor property
described in clauses 18 through 26 and in Annex
B.2
has the attributes { [[Enumerable]]: false,
[[Configurable]]: true } unless otherwise specified.
If only a get accessor function is described, the set accessor function
is the default value, undefined. If only a set
accessor is described the get accessor is the default value,
undefined.
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a
function.
has a [[Prototype]] internal slot whose value is
implementation-dependent.
may have host defined properties in addition to the properties defined
in this specification. This may include a property whose value is the
global object itself.
This property has the attributes { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: true }.
18.1.2Infinity
The value of Infinity is +∞ (see
6.1.6.1). This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
18.1.3NaN
The value of NaN is NaN (see
6.1.6.1). This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
18.1.4undefined
The value of undefined is
undefined (see
6.1.1). This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
18.2Function Properties of the Global
Object
18.2.1eval ( x )
The eval function is the %eval% intrinsic
object. When the eval function is called with one
argument x, the following steps are taken:
If F.[[ConstructorKind]] is
derived, let
inDerivedConstructor be
true; otherwise, let
inDerivedConstructor be
false.
Else,
Let inFunction be false.
Let inMethod be false.
Let inDerivedConstructor be
false.
Perform the following substeps in an implementation-dependent
order, possibly interleaving parsing and error detection:
Let script be the ECMAScript code that is the
result of parsing x, interpreted as UTF-16
encoded Unicode text as described in
6.1.4, for the
goal symbolScript. If the parse fails, throw a
SyntaxError exception. If any early
errors are detected, throw a
SyntaxError exception (but see also
clause
16).
If script Contains
ScriptBody
is false, return
undefined.
NOTE: If direct is true,
runningContext will be the
execution context
that performed the
direct eval. If direct is false,
runningContext will be the
execution context
for the invocation of the eval function.
The eval code cannot instantiate variable or function bindings
in the variable environment of the calling context that
invoked the eval if the calling context is evaluating formal
parameter initializers or if either the code of the calling
context or the eval code is
strict mode code. Instead such bindings are instantiated in a new
VariableEnvironment that is only accessible to the eval code.
Bindings introduced by let, const,
or class declarations are always instantiated in
a new LexicalEnvironment.
HostEnsureCanCompileStrings is an implementation-defined abstract
operation that allows host environments to block certain
ECMAScript functions which allow developers to compile strings
into ECMAScript code.
An implementation of HostEnsureCanCompileStrings may complete
normally or abruptly. Any abrupt completions will be propagated to
its callers. The default implementation of
HostEnsureCanCompileStrings is to unconditionally return an empty
normal completion.
An alternative version of this algorithm is described in
B.3.5.
18.2.2isFinite ( number )
The isFinite function is the
%isFinite% intrinsic object. When the
isFinite function is called with one argument
number, the following steps are taken:
A reliable way for ECMAScript code to test if a value
X is a NaN is an expression of
the form X !== X. The result will be
true if and only if X is a
NaN.
18.2.4parseFloat ( string )
The parseFloat function produces a
Number value
dictated by interpretation of the contents of the
string argument as a decimal literal.
The parseFloat function is the
%parseFloat% intrinsic object. When the
parseFloat function is called with one argument
string, the following steps are taken:
parseFloat may interpret only a leading portion of
string as a
Number value; it ignores any code units that cannot be interpreted as part
of the notation of a decimal literal, and no indication is given
that any such code units were ignored.
18.2.5parseInt ( string,
radix )
The parseInt function produces an
integer
value dictated by interpretation of the contents of the
string argument according to the specified
radix. Leading white space in string is
ignored. If radix is undefined or 0,
it is assumed to be 10 except when the number begins with the code
unit pairs 0x or 0X, in which case a radix
of 16 is assumed. If radix is 16, the number may also
optionally begin with the code unit pairs 0x or
0X.
The parseInt function is the
%parseInt% intrinsic object. When the
parseInt function is called, the following steps are
taken:
If S is not empty and the first code unit of
S is the code unit 0x002D (HYPHEN-MINUS), set
sign to -1.
If S is not empty and the first code unit of
S is the code unit 0x002B (PLUS SIGN) or the code
unit 0x002D (HYPHEN-MINUS), remove the first code unit from
S.
If the length of S is at least 2 and the first
two code units of S are either
"0x" or "0X", then
Remove the first two code units from S.
Set R to 16.
If S contains a code unit that is not a radix-R
digit, let Z be the substring of
S consisting of all code units before the first such
code unit; otherwise, let Z be S.
If Z is empty, return NaN.
Let mathInt be the
mathematical integer
value that is represented by Z in radix-R
notation, using the letters A-Z and a-z
for digits with values 10 through 35. (However, if
R is 10 and Z contains more than 20
significant digits, every significant digit after the 20th may
be replaced by a 0 digit, at the option of the implementation;
and if R is not 2, 4, 8, 10, 16, or 32, then
mathInt may be an implementation-dependent
approximation to the
mathematical integer
value that is represented by Z in radix-R
notation.)
parseInt may interpret only a leading portion of
string as an
integer
value; it ignores any code units that cannot be interpreted as
part of the notation of an
integer, and no indication is given that any such code units were
ignored.
18.2.6URI Handling Functions
Uniform Resource Identifiers, or URIs, are Strings that identify
resources (e.g. web pages or files) and transport protocols by which
to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript
language itself does not provide any support for using URIs except
for functions that encode and decode URIs as described in
18.2.6.2,
18.2.6.3,
18.2.6.4
and
18.2.6.5
Note
Many implementations of ECMAScript provide additional functions
and methods that manipulate web pages; these functions are
beyond the scope of this standard.
18.2.6.1URI Syntax and Semantics
A URI is composed of a sequence of components separated by
component separators. The general form is:
Scheme:First/Second;Third?Fourth
where the italicized names represent components and
“:”, “/”, “;” and
“?” are reserved for use as separators. The
encodeURI and decodeURI functions are
intended to work with complete URIs; they assume that any reserved
code units in the URI are intended to have special meaning and so
are not encoded. The encodeURIComponent and
decodeURIComponent functions are intended to work
with the individual component parts of a URI; they assume that any
reserved code units represent text and so must be encoded so that
they are not interpreted as reserved code units when the component
is part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
The above syntax is based upon RFC 2396 and does not reflect
changes introduced by the more recent RFC 3986.
Runtime Semantics
When a code unit to be included in a URI is not listed above or is
not intended to have the special meaning sometimes given to the
reserved code units, that code unit must be encoded. The code unit
is transformed into its UTF-8 encoding, with
surrogate pairs
first converted from UTF-16 to the corresponding code point value.
(Note that for code units in the range [0, 127] this results in a
single octet with the same value.) The resulting sequence of
octets is then transformed into a String with each octet
represented by an escape sequence of the form
"%xx".
This syntax of Uniform Resource Identifiers is based upon
RFC 2396 and does not reflect the more recent RFC 3986 which
replaces RFC 2396. A formal description and implementation
of UTF-8 is given in RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6
octets. The only octet of a sequence of one has the
higher-order bit set to 0, the remaining 7 bits being used
to encode the character value. In a sequence of n octets, n
> 1, the initial octet has the n higher-order bits set to
1, followed by a bit set to 0. The remaining bits of that
octet contain bits from the value of the character to be
encoded. The following octets all have the higher-order bit
set to 1 and the following bit set to 0, leaving 6 bits in
each to contain bits from the character to be encoded. The
possible UTF-8 encodings of ECMAScript characters are
specified in
Table 49.
Table 49 (Informative): UTF-8 Encodings
Code Unit Value
Representation
1st Octet
2nd Octet
3rd Octet
4th Octet
0x0000 - 0x007F
00000000 0zzzzzzz
0zzzzzzz
0x0080 - 0x07FF
00000yyy yyzzzzzz
110yyyyy
10zzzzzz
0x0800 - 0xD7FF
xxxxyyyy yyzzzzzz
1110xxxx
10yyyyyy
10zzzzzz
0xD800 - 0xDBFF
followed by
0xDC00 - 0xDFFF
110110vv vvwwwwxx
followed by
110111yy yyzzzzzz
11110uuu
10uuwwww
10xxyyyy
10zzzzzz
0xD800 - 0xDBFF
not followed by
0xDC00 - 0xDFFF
causes URIError
0xDC00 - 0xDFFF
causes URIError
0xE000 - 0xFFFF
xxxxyyyy yyzzzzzz
1110xxxx
10yyyyyy
10zzzzzz
Where
uuuuu = vvvv + 1
to account for the addition of 0x10000 as in section 3.8 of
the Unicode Standard (Surrogates).
The above transformation combines each
surrogate pair
(for which code unit values in the inclusive range 0xD800 to
0xDFFF are reserved) into a UTF-32 representation and
encodes the resulting 21-bit value into UTF-8. Decoding
reconstructs the
surrogate pair.
RFC 3629 prohibits the decoding of invalid UTF-8 octet
sequences. For example, the invalid sequence C0 80 must not
decode into the code unit 0x0000. Implementations of the
Decode algorithm are required to throw a
URIError when encountering such invalid
sequences.
18.2.6.2decodeURI (
encodedURI )
The decodeURI function computes a new version of a
URI in which each escape sequence and UTF-8 encoding of the sort
that might be introduced by the encodeURI function is
replaced with the UTF-16 encoding of the code points that it
represents. Escape sequences that could not have been introduced
by encodeURI are not replaced.
The decodeURI function is the
%decodeURI% intrinsic object. When the
decodeURI function is called with one argument
encodedURI, the following steps are taken:
The decodeURIComponent function computes a new
version of a URI in which each escape sequence and UTF-8 encoding
of the sort that might be introduced by the
encodeURIComponent function is replaced with the
UTF-16 encoding of the code points that it represents.
The decodeURIComponent function is the
%decodeURIComponent% intrinsic object. When the
decodeURIComponent function is called with one
argument encodedURIComponent, the following steps are
taken:
Let componentString be ? ToString(encodedURIComponent).
The encodeURI function computes a new version of a
UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced
by one, two, three, or four escape sequences representing the
UTF-8 encoding of the code points.
The encodeURI function is the
%encodeURI% intrinsic object. When the
encodeURI function is called with one argument
uri, the following steps are taken:
The code point # is not encoded to an escape
sequence even though it is not a reserved or unescaped URI
code point.
18.2.6.5encodeURIComponent (
uriComponent )
The encodeURIComponent function computes a new
version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced
by one, two, three, or four escape sequences representing the
UTF-8 encoding of the code point.
The encodeURIComponent function is the
%encodeURIComponent% intrinsic object. When the
encodeURIComponent function is called with one
argument uriComponent, the following steps are taken:
The assign function is used to copy the values of all
of the enumerable own properties from one or more source objects
to a target object. When the
assign function is called, the following steps are
taken:
The defineProperties function is used to add own
properties and/or update the attributes of existing own properties
of an object. When the defineProperties function is
called, the following steps are taken:
19.1.2.4Object.defineProperty (
O, P, Attributes )
The defineProperty function is used to add an own
property and/or update the attributes of an existing own property
of an object. When the defineProperty function is
called, the following steps are taken:
If
Type(O) is not Object, throw a
TypeError exception.
The function created for adder is never directly
accessible to ECMAScript code.
19.1.2.7.1CreateDataPropertyOnObject
Functions
A CreateDataPropertyOnObject function is an anonymous built-in
function. When a CreateDataPropertyOnObject function is called
with arguments key and value, the
following steps are taken:
19.1.2.11.1Runtime Semantics:
GetOwnPropertyKeys ( O, type )
The abstract operation GetOwnPropertyKeys is called with
arguments O and type where O is
an Object and type is one of the ECMAScript
specification types String or Symbol. The following steps are
taken:
The ordering of steps 1 and 2 is chosen to ensure that any
exception that would have been thrown by step 1 in previous
editions of this specification will continue to be thrown even
if the this value is
undefined or null.
19.1.3.3Object.prototype.isPrototypeOf
( V )
When the isPrototypeOf method is called with argument
V, the following steps are taken:
The ordering of steps 1 and 2 preserves the behaviour
specified by previous editions of this specification for the
case where V is not an object and the
this value is
undefined or null.
19.1.3.4Object.prototype.propertyIsEnumerable ( V )
When the propertyIsEnumerable method is called with
argument V, the following steps are taken:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and 2 is chosen to ensure that any
exception that would have been thrown by step 1 in previous
editions of this specification will continue to be thrown even
if the this value is
undefined or null.
The optional parameters to this function are not used but are
intended to correspond to the parameter pattern used by ECMA-402
toLocaleString functions. Implementations that do not
include ECMA-402 support must not use those parameter positions
for other purposes.
Note 1
This function provides a generic
toLocaleString implementation for objects that
have no locale-specific toString behaviour.
Array, Number, Date,
and
%TypedArray%
provide their own locale-sensitive
toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this
default implementation.
19.1.3.6Object.prototype.toString ( )
When the toString method is called, the following
steps are taken:
If the this value is
undefined, return
"[object Undefined]".
If the this value is
null, return
"[object Null]".
This function is the %ObjProto_toString% intrinsic
object.
Note
Historically, this function was occasionally used to access
the String value of the [[Class]] internal slot that was used
in previous editions of this specification as a nominal type
tag for various built-in objects. The above definition of
toString preserves compatibility for legacy code
that uses toString as a test for those specific
kinds of built-in objects. It does not provide a reliable type
testing mechanism for other kinds of built-in or program
defined objects. In addition, programs can use @@toStringTag
in ways that will invalidate the reliability of such legacy
type tests.
19.1.3.7Object.prototype.valueOf ( )
When the valueOf method is called, the following
steps are taken:
is the initial value of the "Function" property
of the
global object.
creates and initializes a new
function object
when called as a function rather than as a
constructor. Thus the function call Function(…) is equivalent
to the object creation expression
new Function(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Function behaviour must include a
super call to the Functionconstructor
to create and initialize a subclass instance with the internal
slots necessary for built-in function behaviour. All ECMAScript
syntactic forms for defining function objects create instances of
Function. There is no syntactic means to create
instances of Function subclasses except for the
built-in GeneratorFunction,
AsyncFunction, and
AsyncGeneratorFunction subclasses.
19.2.1.1Function ( p1,
p2, … , pn, body )
The last argument specifies the body (executable code) of a
function; any preceding arguments specify formal parameters.
When the Function function is called with some
arguments p1, p2, … , pn,
body (where n might be 0, that is, there are
no “ p ” arguments, and where body might
also not be provided), the following steps are taken:
It is permissible but not necessary to have one argument for
each formal parameter to be specified. For example, all three
of the following expressions produce the same result:
The abstract operation CreateDynamicFunction is called with
arguments constructor, newTarget,
kind, and args. constructor is
the
constructor
function that is performing this action, newTarget is
the
constructor
that new was initially applied to,
kind is either normal,
generator, async,
or asyncGenerator, and args is
a
List
containing the actual argument values that were passed to
constructor. The following steps are taken:
Perform the following substeps in an
implementation-dependent order, possibly interleaving
parsing and error detection:
Let parameters be the result of parsing
P, interpreted as UTF-16 encoded Unicode text
as described in
6.1.4, using parameterGoal as the
goal symbol. Throw a SyntaxError exception if
the parse fails.
Let body be the result of parsing
bodyText, interpreted as UTF-16 encoded
Unicode text as described in
6.1.4, using goal as the
goal symbol. Throw a SyntaxError exception if
the parse fails.
Let prefix be the prefix associated with
kind in
Table 50.
Let sourceText be the
string-concatenation
of prefix, " anonymous(",
P, 0x000A (LINE FEED), ") {",
bodyText, and "}".
Set F.[[SourceText]] to sourceText.
Return F.
Note
A "prototype" property is created for
every non-async function created using CreateDynamicFunction
to provide for the possibility that the function will be
used as a
constructor.
accepts any arguments and returns
undefined when invoked.
does not have a [[Construct]] internal method; it cannot be used
as a
constructor
with the new operator.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
does not have a "prototype" property.
has a "length" property whose value is 0.
has a "name" property whose value is the empty
String.
Note
The Function prototype object is specified to be a
function object
to ensure compatibility with ECMAScript code that was created
prior to the ECMAScript 2015 specification.
The thisArg value is passed without modification as
the this value. This is a change from
Edition 3, where an undefined or
nullthisArg is replaced with
the
global object
and
ToObject
is applied to all other values and that result is passed as
the this value. Even though the
thisArg is passed without modification, non-strict
functions still perform these transformations upon entry to
the function.
Note 2
If func is an arrow function or a
bound function
then the thisArg will be ignored by the function
[[Call]] in step 5.
If this method was called with more than one argument, then in
left to right order, starting with the second argument, append
each argument as the last element of argList.
The thisArg value is passed without modification as
the this value. This is a change from
Edition 3, where an undefined or
nullthisArg is replaced with
the
global object
and
ToObject
is applied to all other values and that result is passed as
the this value. Even though the
thisArg is passed without modification, non-strict
functions still perform these transformations upon entry to
the function.
Note 2
If func is an arrow function or a
bound function
then the thisArg will be ignored by the function
[[Call]] in step 5.
19.2.3.4Function.prototype.constructor
The initial value of
Function.prototype.constructor is
%Function%.
19.2.3.5Function.prototype.toString (
)
When the toString method is called, the following
steps are taken:
Let func be the this value.
If func is a
Bound Function exotic object
or a
built-in function object, then return an implementation-dependent String source code
representation of func. The representation must
have the syntax of a
NativeFunction. Additionally, if func is a
Well-known Intrinsic Object
and is not identified as an anonymous function, the portion of
the returned String that would be matched by
PropertyName
must be the initial value of the
"name" property of func.
If
Type(func) is Object and func has a
[[SourceText]] internal slot and
Type(func.[[SourceText]]) is String and
! HostHasSourceTextAvailable(func) is
true, then return
func.[[SourceText]].
If
Type(func) is Object and
IsCallable(func) is true, then return an
implementation-dependent String source code representation of
func. The representation must have the syntax of a
NativeFunction.
The value of the "name" property of this
function is "[Symbol.hasInstance]".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
Note
This is the default implementation of
@@hasInstance that most functions inherit.
@@hasInstance is called by the
instanceof operator to determine whether a value
is an instance of a specific
constructor. An expression such as
v instanceof F
evaluates as
F[@@hasInstance](v)
A
constructor
function can control which objects are recognized as its
instances by instanceof by exposing a different
@@hasInstance method on the function.
This property is non-writable and non-configurable to prevent
tampering that could be used to globally expose the target
function of a
bound function.
19.2.4Function Instances
Every Function instance is an ECMAScript
function object
and has the internal slots listed in
Table 29. Function objects created using the
Function.prototype.bind method (19.2.3.2) have the internal slots listed in
Table 30.
Function instances have the following properties:
19.2.4.1length
The value of the "length" property is an
integer
that indicates the typical number of arguments expected by the
function. However, the language permits the function to be invoked
with some other number of arguments. The behaviour of a function
when invoked on a number of arguments other than the number
specified by its "length" property depends on
the function. This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
19.2.4.2name
The value of the "name" property is a String
that is descriptive of the function. The name has no semantic
significance but is typically a variable or
property name
that is used to refer to the function at its point of definition
in ECMAScript code. This property has the attributes {
[[Writable]]: false, [[Enumerable]]:
false, [[Configurable]]:
true }.
Anonymous functions objects that do not have a contextual name
associated with them by this specification do not have a
"name" own property but inherit the
"name" property of
%Function.prototype%.
19.2.4.3prototype
Function instances that can be used as a
constructor
have a "prototype" property. Whenever such a
Function instance is created another ordinary object is also
created and is the initial value of the function's
"prototype" property. Unless otherwise
specified, the value of the
"prototype" property is used to initialize the
[[Prototype]] internal slot of the object created when that
function is invoked as a
constructor.
This property has the attributes { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: false }.
HostHasSourceTextAvailable is an implementation-defined abstract
operation that allows host environments to prevent the source text
from being provided for a given function.
An implementation of HostHasSourceTextAvailable must complete
normally in all cases. This operation must be deterministic with
respect to its parameters. Each time it is called with a specific
func as its argument, it must return the same completion
record. The default implementation of HostHasSourceTextAvailable is
to unconditionally return a normal completion with a value of
true.
is the initial value of the "Boolean" property
of the
global object.
creates and initializes a new Boolean object when called as a
constructor.
performs a type conversion when called as a function rather than
as a
constructor.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Boolean behaviour must include a
super call to the Booleanconstructor
to create and initialize the subclass instance with a
[[BooleanData]] internal slot.
19.3.1.1Boolean ( value )
When Boolean is called with argument
value, the following steps are taken:
Boolean instances are ordinary objects that inherit properties from
the Boolean prototype object. Boolean instances have a
[[BooleanData]] internal slot. The [[BooleanData]] internal slot is
the Boolean value represented by this Boolean object.
For each element e of the GlobalSymbolRegistry
List, do
If
SameValue(e.[[Key]], stringKey) is
true, return e.[[Symbol]].
Assert: GlobalSymbolRegistry does not currently contain an entry
for stringKey.
Let newSymbol be a new unique Symbol value whose
[[Description]] value is stringKey.
Append the
Record
{ [[Key]]: stringKey, [[Symbol]]:
newSymbol } to the GlobalSymbolRegistry
List.
Return newSymbol.
The GlobalSymbolRegistry is a
List
that is globally available. It is shared by all realms. Prior to
the evaluation of any ECMAScript code it is initialized as a new
empty
List. Elements of the GlobalSymbolRegistry are Records with the
structure defined in
Table 51.
19.4.3.5Symbol.prototype [
@@toPrimitive ] ( hint )
This function is called by ECMAScript language operators to
convert a Symbol object to a primitive value. The allowed values
for hint are "default",
"number", and "string".
When the @@toPrimitive method is called with argument
hint, the following steps are taken:
The value of the "name" property of this
function is "[Symbol.toPrimitive]".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
19.4.3.6Symbol.prototype [
@@toStringTag ]
The initial value of the @@toStringTag property is the String
value "Symbol".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
19.4.4Properties of Symbol Instances
Symbol instances are ordinary objects that inherit properties from
the Symbol prototype object. Symbol instances have a [[SymbolData]]
internal slot. The [[SymbolData]] internal slot is the Symbol value
represented by this Symbol object.
19.5Error Objects
Instances of Error objects are thrown as exceptions when runtime
errors occur. The Error objects may also serve as base objects for
user-defined exception classes.
is the initial value of the "Error" property of
the
global object.
creates and initializes a new Error object when called as a
function rather than as a
constructor. Thus the function call Error(…) is equivalent to
the object creation expression new Error(…) with the
same arguments.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Error behaviour must include a
super call to the Errorconstructor
to create and initialize subclass instances with an [[ErrorData]]
internal slot.
19.5.1.1Error ( message )
When the Error function is called with argument
message, the following steps are taken:
If NewTarget is undefined, let
newTarget be the
active function object; else let newTarget be NewTarget.
If msg is undefined, set
msg to the empty String; otherwise set
msg to ? ToString(msg).
If name is the empty String, return msg.
If msg is the empty String, return name.
Return the
string-concatenation
of name, the code unit 0x003A (COLON), the code
unit 0x0020 (SPACE), and msg.
19.5.4Properties of Error Instances
Error instances are ordinary objects that inherit properties from
the Error prototype object and have an [[ErrorData]] internal slot
whose value is undefined. The only specified uses
of [[ErrorData]] is to identify Error and
NativeError instances as Error objects within
Object.prototype.toString.
19.5.5Native Error Types Used in This
Standard
A new instance of one of the NativeError objects below is
thrown when a runtime error is detected. All of these objects share
the same structure, as described in
19.5.6.
19.5.5.1EvalError
This exception is not currently used within this specification.
This object remains for compatibility with previous editions of
this specification.
19.5.5.2RangeError
Indicates a value that is not in the set or range of allowable
values.
19.5.5.3ReferenceError
Indicate that an invalid reference value has been detected.
19.5.5.4SyntaxError
Indicates that a parsing error has occurred.
19.5.5.5TypeError
TypeError is used to indicate an unsuccessful operation when none
of the other NativeError objects are an appropriate
indication of the failure cause.
19.5.5.6URIError
Indicates that one of the global URI handling functions was used
in a way that is incompatible with its definition.
19.5.6NativeError Object
Structure
When an ECMAScript implementation detects a runtime error, it throws
a new instance of one of the NativeError objects defined
in
19.5.5. Each of these objects has the structure described below,
differing only in the name used as the
constructor
name instead of NativeError, in the
"name" property of the prototype object, and in
the implementation-defined "message" property of
the prototype object.
For each error object, references to NativeError in the
definition should be replaced with the appropriate error object name
from
19.5.5.
creates and initializes a new NativeError object when
called as a function rather than as a
constructor. A call of the object as a function is equivalent to calling
it as a
constructor
with the same arguments. Thus the function call
NativeError(…) is equivalent to the
object creation expression
new NativeError(…) with the same
arguments.
is designed to be subclassable. It may be used as the value of
an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
NativeError behaviour must include a
super call to the NativeErrorconstructor
to create and initialize subclass instances with an
[[ErrorData]] internal slot.
19.5.6.1.1NativeError (
message )
When a NativeError function is called with argument
message, the following steps are taken:
If NewTarget is undefined, let
newTarget be the
active function object; else let newTarget be NewTarget.
The actual value of the string passed in step 2 is either
"%EvalError.prototype%",
"%RangeError.prototype%",
"%ReferenceError.prototype%",
"%SyntaxError.prototype%",
"%TypeError.prototype%", or
"%URIError.prototype%" corresponding to which
NativeErrorconstructor
is being defined.
19.5.6.2Properties of the
NativeError Constructors
has a [[Prototype]] internal slot whose value is
%Error%.
has a "name" property whose value is the
String value "NativeError".
has the following properties:
19.5.6.2.1NativeError.prototype
The initial value of
NativeError.prototype is a
NativeError prototype object (19.5.6.3). Each NativeErrorconstructor
has a distinct prototype object.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
19.5.6.3Properties of the
NativeError Prototype Objects
Each NativeError prototype object:
is an ordinary object.
is not an Error instance and does not have an [[ErrorData]]
internal slot.
has a [[Prototype]] internal slot whose value is
%Error.prototype%.
19.5.6.3.1NativeError.prototype.constructor
The initial value of the
"constructor" property of the prototype for a
given NativeErrorconstructor
is the corresponding intrinsic object %NativeError%
(19.5.6.1).
19.5.6.3.2NativeError.prototype.message
The initial value of the "message" property
of the prototype for a given NativeErrorconstructor
is the empty String.
19.5.6.3.3NativeError.prototype.name
The initial value of the "name" property of
the prototype for a given NativeErrorconstructor
is the String value consisting of the name of the
constructor
(the name used instead of NativeError).
19.5.6.4Properties of
NativeError Instances
NativeError instances are ordinary objects that inherit
properties from their NativeError prototype object and
have an [[ErrorData]] internal slot whose value is
undefined. The only specified use of
[[ErrorData]] is by
Object.prototype.toString (19.1.3.6) to identify Error or NativeError instances.
is the initial value of the "Number" property
of the
global object.
creates and initializes a new Number object when called as a
constructor.
performs a type conversion when called as a function rather than
as a
constructor.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Number behaviour must include a
super call to the Numberconstructor
to create and initialize the subclass instance with a
[[NumberData]] internal slot.
20.1.1.1Number ( value )
When Number is called with argument value,
the following steps are taken:
The value of Number.EPSILON is the difference between
1 and the smallest value greater than 1 that is representable as a
Number value, which is approximately 2.2204460492503130808472633361816 x
10 - 16.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.1.2.2Number.isFinite (
number )
When Number.isFinite is called with one argument
number, the following steps are taken:
This function differs from the global isNaN function
(18.2.3) in that it does not convert its argument to a Number before
determining whether it is NaN.
20.1.2.5Number.isSafeInteger (
number )
When Number.isSafeInteger is called with one argument
number, the following steps are taken:
The value of Number.MAX_SAFE_INTEGER is the
largest
integer
n such that n and n + 1 are both exactly representable as a
Number value.
The value of Number.MAX_SAFE_INTEGER is
9007199254740991 (253 - 1).
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.1.2.7Number.MAX_VALUE
The value of Number.MAX_VALUE is the largest positive
finite value of the Number type, which is approximately
1.7976931348623157 × 10308.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.1.2.8Number.MIN_SAFE_INTEGER
Note
The value of Number.MIN_SAFE_INTEGER is the
smallest
integer
n such that n and n - 1 are both exactly representable as a
Number value.
The value of Number.MIN_SAFE_INTEGER is
-9007199254740991 (-(253 - 1)).
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.1.2.9Number.MIN_VALUE
The value of Number.MIN_VALUE is the smallest
positive value of the Number type, which is approximately
5 × 10-324.
In the IEEE 754-2008 double precision binary representation, the
smallest possible value is a denormalized number. If an
implementation does not support denormalized values, the value of
Number.MIN_VALUE must be the smallest non-zero
positive value that can actually be represented by the
implementation.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.1.2.10Number.NaN
The value of Number.NaN is NaN.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.1.2.11Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is
-∞.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.1.2.15Number.prototype
The initial value of Number.prototype is
%Number.prototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.1.3Properties of the Number Prototype
Object
The Number prototype object:
is the intrinsic object %NumberPrototype%.
is an ordinary object.
is itself a Number object; it has a [[NumberData]] internal slot
with the value +0.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
Unless explicitly stated otherwise, the methods of the Number
prototype object defined below are not generic and the
this value passed to them must be either a
Number value
or an object that has a [[NumberData]] internal slot that has been
initialized to a
Number value.
The abstract operation
thisNumberValue(value) performs the following steps:
The phrase “this
Number value” within the specification of a method refers to the result
returned by calling the abstract operation
thisNumberValue
with the this value of the method invocation
passed as the argument.
20.1.3.1Number.prototype.constructor
The initial value of Number.prototype.constructor is
%Number%.
Return a String containing this
Number value
represented in decimal exponential notation with one digit before
the significand's decimal point and
fractionDigits digits after the significand's decimal
point. If fractionDigits is
undefined, include as many significand digits
as necessary to uniquely specify the Number (just like in
ToString
except that in this case the Number is always output in
exponential notation). Specifically, perform the following steps:
If f < 0 or f > 100, throw a
RangeError exception.
If x = 0, then
Let m be the String value consisting of
f + 1 occurrences of the code unit 0x0030
(DIGIT ZERO).
Let e be 0.
Else,
If fractionDigits is not
undefined, then
Let e and n be integers such
that 10f ≤ n <
10f + 1 and for which
ℝ(n) × 10ℝℝ(e) -
ℝ(n)
-
ℝ(x) is as close to zero as possible. If
there are two such sets of e and
n, pick the e and
n for which
ℝ(n) × 10ℝℝ(e) -
ℝ(f)
is larger.
Else,
Let e, n, and f be
integers such that f ≥ 0, 10f
≤ n < 10f + 1, the
Number value
for
ℝ(n) × 10ℝℝ(e) -
ℝ(f)
is x, and f is as small as
possible. Note that the decimal representation of
n has f + 1ℝ
digits, n is not divisible by 10, and the
least significant digit of n is not
necessarily uniquely determined by these criteria.
Let m be the String value consisting of the
digits of the decimal representation of n (in
order, with no leading zeroes).
If f ≠ 0, then
Let a be the first code unit of m,
and let b be the remaining f code
units of m.
For implementations that provide more accurate conversions
than required by the rules above, it is recommended that the
following alternative version of step 10.b.i be used as a
guideline:
Let e, n, and f be
integers such that f ≥ 0, 10f
≤ n < 10f + 1, the
Number value
for
ℝ(n) × 10ℝℝ(e) -
ℝ(f)
is x, and f is as small as possible.
If there are multiple possibilities for n,
choose the value of n for which
ℝ(n) × 10ℝℝ(e) -
ℝ(f)
is closest in value to x. If there are two such
possible values of n, choose the one that is
even.
toFixed returns a String containing this
Number value
represented in decimal fixed-point notation with
fractionDigits digits after the decimal point. If
fractionDigits is undefined, 0
is assumed.
Let n be an
integer
for which
ℝ(n) ÷ 10ℝℝ(f)
-
ℝ(x) is as close to zero as possible. If there
are two such n, pick the larger n.
If n = 0, let m be the String
"0". Otherwise, let m be the
String value consisting of the digits of the decimal
representation of n (in order, with no leading
zeroes).
If f ≠ 0, then
Let k be the length of m.
If k ≤ f, then
Let z be the String value consisting of
f + 1 - k occurrences of the
code unit 0x0030 (DIGIT ZERO).
The output of toFixed may be more precise than
toString for some values because toString only
prints enough significant digits to distinguish the number
from adjacent number values. For example,
(1000000000000000128).toString() returns
"1000000000000000100", while
(1000000000000000128).toFixed(0) returns
"1000000000000000128".
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
Number.prototype.toLocaleString method as specified
in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of
the toLocaleString method is used.
Produces a String value that represents this
Number value
formatted according to the conventions of the host environment's
current locale. This function is implementation-dependent, and it
is permissible, but not encouraged, for it to return the same
thing as toString.
The meanings of the optional parameters to this method are defined
in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not use those parameter positions for
anything else.
Return a String containing this
Number value
represented either in decimal exponential notation with one digit
before the significand's decimal point and
precision - 1 digits
after the significand's decimal point or in decimal fixed notation
with precision significant digits. If
precision is undefined, call
ToString
instead. Specifically, perform the following steps:
If p < 1 or p > 100, throw a
RangeError exception.
If x = 0, then
Let m be the String value consisting of
p occurrences of the code unit 0x0030 (DIGIT
ZERO).
Let e be 0.
Else,
Let e and n be integers such that
10p - 1 ≤ n < 10p
and for which
ℝ(n) × 10ℝℝ(e) -
ℝ(p) + 1ℝ
-
ℝ(x) is as close to zero as possible. If there
are two such sets of e and n, pick
the e and n for which
ℝ(n) × 10ℝℝ(e) -
ℝ(p) + 1ℝ
is larger.
Let m be the String value consisting of the
digits of the decimal representation of n (in
order, with no leading zeroes).
Set m to the
string-concatenation
of the first e + 1 code units of m,
the code unit 0x002E (FULL STOP), and the remaining
p - (e + 1) code units of
m.
Else,
Set m to the
string-concatenation
of the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E
(FULL STOP), -(e + 1) occurrences of the code
unit 0x0030 (DIGIT ZERO), and the String m.
Return the String representation of this
Number value
using the radix specified by radixNumber. Letters
a-z are used for digits with values
10 through 35. The precise algorithm is
implementation-dependent, however the algorithm should be a
generalization of that specified in
6.1.6.1.20.
The toString function is not generic; it throws a
TypeError exception if its
this value is not a Number or a Number object.
Therefore, it cannot be transferred to other kinds of objects for
use as a method.
The "length" property of the
toString method is 1.
Number instances are ordinary objects that inherit properties from
the Number prototype object. Number instances also have a
[[NumberData]] internal slot. The [[NumberData]] internal slot is
the
Number value
represented by this Number object.
is the initial value of the "BigInt" property
of the
global object.
performs a type conversion when called as a function rather than
as a
constructor.
is not intended to be used with the new operator or
to be subclassed. It may be used as the value of an
extends clause of a class definition but a
super call to the BigIntconstructor
will cause an exception.
20.2.1.1BigInt ( value )
When BigInt is called with argument value,
the following steps are taken:
If NewTarget is not undefined, throw a
TypeError exception.
The phrase “this BigInt value” within the specification of a method
refers to the result returned by calling the abstract operation
thisBigIntValue
with the this value of the method invocation
passed as the argument.
20.2.3.1BigInt.prototype.constructor
The initial value of BigInt.prototype.constructor is
the intrinsic object
%BigInt%.
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
BigInt.prototype.toLocaleString method as specified
in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of
the toLocaleString method is used.
Produces a String value that represents this BigInt value
formatted according to the conventions of the host environment's
current locale. This function is implementation-dependent, and it
is permissible, but not encouraged, for it to return the same
thing as toString.
The meanings of the optional parameters to this method are defined
in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not use those parameter positions for
anything else.
20.2.3.3BigInt.prototype.toString ( [
radix ] )
Note
The optional radix should be an
integer
value in the inclusive range 2 to 36. If radix not
present or is undefined the Number 10 is
used as the value of radix.
Return the String representation of this
Number value
using the radix specified by radixNumber. Letters
a-z are used for digits with values
10 through 35. The precise algorithm is
implementation-dependent, however the algorithm should be a
generalization of that specified in
6.1.6.2.23.
The toString function is not generic; it throws a
TypeError exception if its
this value is not a BigInt or a BigInt object.
Therefore, it cannot be transferred to other kinds of objects for
use as a method.
does not have a [[Construct]] internal method; it cannot be used as
a
constructor
with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a
function.
Note
In this specification, the phrase “the
Number value
for x” has a technical meaning defined in
6.1.6.1.
20.3.1Value Properties of the Math
Object
20.3.1.1Math.E
The
Number value
for eℝ, the base of the natural logarithms, which is approximately
2.7182818284590452354.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.3.1.2Math.LN10
The
Number value
for the natural logarithm of 10ℝ, which is approximately 2.302585092994046.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.3.1.3Math.LN2
The
Number value
for the natural logarithm of 2ℝ, which is approximately 0.6931471805599453.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.3.1.4Math.LOG10E
The
Number value
for the base-10 logarithm of eℝ, the base of the natural logarithms; this value is approximately
0.4342944819032518.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
Note
The value of Math.LOG10E is approximately the
reciprocal of the value of Math.LN10.
20.3.1.5Math.LOG2E
The
Number value
for the base-2 logarithm of eℝ, the base of the natural logarithms; this value is approximately
1.4426950408889634.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
Note
The value of Math.LOG2E is approximately the
reciprocal of the value of Math.LN2.
20.3.1.6Math.PI
The
Number value
for πℝ, the ratio of the circumference of a circle to its diameter,
which is approximately 3.1415926535897932.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.3.1.7Math.SQRT1_2
The
Number value
for the square root of ½ℝ, which is approximately 0.7071067811865476.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
Note
The value of Math.SQRT1_2 is approximately the
reciprocal of the value of Math.SQRT2.
20.3.1.8Math.SQRT2
The
Number value
for the square root of 2ℝ, which is approximately 1.4142135623730951.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.3.1.9Math [ @@toStringTag ]
The initial value of the @@toStringTag property is the String
value "Math".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
20.3.2Function Properties of the Math
Object
Each of the following Math object functions applies the
ToNumber
abstract operation to each of its arguments (in left-to-right order
if there is more than one). If
ToNumber
returns an
abrupt completion, that
Completion Record
is immediately returned. Otherwise, the function performs a
computation on the resulting
Number value(s). The value returned by each function is a Number.
In the function descriptions below, the symbols
NaN, -0,
+0, -∞ and
+∞ refer to the Number values described in
6.1.6.1.
Note
The behaviour of the functions acos,
acosh, asin, asinh,
atan, atanh, atan2,
cbrt, cos, cosh,
exp, expm1, hypot,
log,log1p, log2,
log10, pow, random,
sin, sinh, sqrt,
tan, and tanh is not precisely
specified here except to require specific results for certain
argument values that represent boundary cases of interest. For
other argument values, these functions are intended to compute
approximations to the results of familiar mathematical
functions, but some latitude is allowed in the choice of
approximation algorithms. The general intent is that an
implementer should be able to use the same mathematical library
for ECMAScript on a given hardware platform that is available to
C programmers on that platform.
Although the choice of algorithms is left to the implementation,
it is recommended (but not specified by this standard) that
implementations use the approximation algorithms for IEEE
754-2008 arithmetic contained in fdlibm, the freely
distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
20.3.2.1Math.abs ( x )
Returns the absolute value of x; the result has the
same magnitude as x but has positive sign.
If x is NaN, the result is
NaN.
If x is -0, the result is
+0.
If x is -∞, the result is
+∞.
20.3.2.2Math.acos ( x )
Returns an implementation-dependent approximation to the arc
cosine of x. The result is expressed in radians and
ranges from +0 to +π.
If x is NaN, the result is
NaN.
If x is greater than 1, the result is
NaN.
If x is less than -1, the result is
NaN.
If x is exactly 1, the result is
+0.
20.3.2.3Math.acosh ( x )
Returns an implementation-dependent approximation to the inverse
hyperbolic cosine of x.
If x is NaN, the result is
NaN.
If x is less than 1, the result is NaN.
If x is 1, the result is +0.
If x is +∞, the result is
+∞.
20.3.2.4Math.asin ( x )
Returns an implementation-dependent approximation to the arc sine
of x. The result is expressed in radians and ranges
from -π / 2 to +π / 2.
If x is NaN, the result is
NaN.
If x is greater than 1, the result is
NaN.
If x is less than -1, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
20.3.2.5Math.asinh ( x )
Returns an implementation-dependent approximation to the inverse
hyperbolic sine of x.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
If x is -∞, the result is
-∞.
20.3.2.6Math.atan ( x )
Returns an implementation-dependent approximation to the arc
tangent of x. The result is expressed in radians and
ranges from -π / 2 to +π / 2.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is an
implementation-dependent approximation to +π / 2.
If x is -∞, the result is an
implementation-dependent approximation to -π / 2.
20.3.2.7Math.atanh ( x )
Returns an implementation-dependent approximation to the inverse
hyperbolic tangent of x.
If x is NaN, the result is
NaN.
If x is less than -1, the result is
NaN.
If x is greater than 1, the result is
NaN.
If x is -1, the result is -∞.
If x is +1, the result is +∞.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
20.3.2.8Math.atan2 ( y,
x )
Returns an implementation-dependent approximation to the arc
tangent of the quotient
y / x of
the arguments y and x, where the signs of
y and x are used to determine the quadrant
of the result. Note that it is intentional and traditional for the
two-argument arc tangent function that the argument named
y be first and the argument named x be
second. The result is expressed in radians and ranges from -π to
+π.
If either x or y is
NaN, the result is NaN.
If y > 0 and x is
+0, the result is an implementation-dependent
approximation to +π / 2.
If y > 0 and x is
-0, the result is an implementation-dependent
approximation to +π / 2.
If y is +0 and x >
0, the result is +0.
If y is +0 and x is
+0, the result is +0.
If y is +0 and x is
-0, the result is an implementation-dependent
approximation to +π.
If y is +0 and x <
0, the result is an implementation-dependent approximation to
+π.
If y is -0 and x >
0, the result is -0.
If y is -0 and x is
+0, the result is -0.
If y is -0 and x is
-0, the result is an implementation-dependent
approximation to -π.
If y is -0 and x <
0, the result is an implementation-dependent approximation to
-π.
If y < 0 and x is
+0, the result is an implementation-dependent
approximation to -π / 2.
If y < 0 and x is
-0, the result is an implementation-dependent
approximation to -π / 2.
If y > 0 and y is finite and
x is +∞, the result is
+0.
If y > 0 and y is finite and
x is -∞, the result is an
implementation-dependent approximation to +π.
If y < 0 and y is finite and
x is +∞, the result is
-0.
If y < 0 and y is finite and
x is -∞, the result is an
implementation-dependent approximation to -π.
If y is +∞ and x is
finite, the result is an implementation-dependent approximation
to +π / 2.
If y is -∞ and x is
finite, the result is an implementation-dependent approximation
to -π / 2.
If y is +∞ and x is
+∞, the result is an implementation-dependent
approximation to +π / 4.
If y is +∞ and x is
-∞, the result is an implementation-dependent
approximation to +3π / 4.
If y is -∞ and x is
+∞, the result is an implementation-dependent
approximation to -π / 4.
If y is -∞ and x is
-∞, the result is an implementation-dependent
approximation to -3π / 4.
20.3.2.9Math.cbrt ( x )
Returns an implementation-dependent approximation to the cube root
of x.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
If x is -∞, the result is
-∞.
20.3.2.10Math.ceil ( x )
Returns the smallest (closest to -∞)
Number value
that is not less than x and is an
integer. If x is already an
integer, the result is x.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
If x is -∞, the result is
-∞.
If x is less than 0 but greater than -1, the result
is -0.
The value of Math.ceil(x) is the same as the value of
-Math.floor(-x).
20.3.2.11Math.clz32 ( x )
When Math.clz32 is called with one argument
x, the following steps are taken:
Let p be the number of leading zero bits in the
32-bit binary representation of n.
Return p.
Note
If n is 0, p will be 32. If the most
significant bit of the 32-bit binary encoding of
n is 1, p will be 0.
20.3.2.12Math.cos ( x )
Returns an implementation-dependent approximation to the cosine of
x. The argument is expressed in radians.
If x is NaN, the result is
NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is
NaN.
If x is -∞, the result is
NaN.
20.3.2.13Math.cosh ( x )
Returns an implementation-dependent approximation to the
hyperbolic cosine of x.
If x is NaN, the result is
NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is
+∞.
If x is -∞, the result is
+∞.
Note
The value of Math.cosh(x) is the same as the
value of (Math.exp(x) + Math.exp(-x)) / 2.
20.3.2.14Math.exp ( x )
Returns an implementation-dependent approximation to the
exponential function of x (e raised to the
power of x, where e is the base of the
natural logarithms).
If x is NaN, the result is
NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is
+∞.
If x is -∞, the result is
+0.
20.3.2.15Math.expm1 ( x )
Returns an implementation-dependent approximation to subtracting 1
from the exponential function of x (e raised
to the power of x, where e is the base of
the natural logarithms). The result is computed in a way that is
accurate even when the value of x is close 0.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
If x is -∞, the result is -1.
20.3.2.16Math.floor ( x )
Returns the greatest (closest to +∞)
Number value
that is not greater than x and is an
integer. If x is already an
integer, the result is x.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
If x is -∞, the result is
-∞.
If x is greater than 0 but less than 1, the result is
+0.
Note
The value of Math.floor(x) is the same as the
value of -Math.ceil(-x).
20.3.2.17Math.fround ( x )
When Math.fround is called with argument
x, the following steps are taken:
If x is NaN, return
NaN.
If x is one of +0,
-0, +∞,
-∞, return x.
Let x32 be the result of converting x to
a value in IEEE 754-2008 binary32 format using roundTiesToEven
mode.
Let x64 be the result of converting
x32 to a value in IEEE 754-2008 binary64 format.
Return the ECMAScript
Number value
corresponding to x64.
20.3.2.18Math.hypot (
value1, value2, ...values )
Math.hypot returns an implementation-dependent
approximation of the square root of the sum of squares of its
arguments.
If no arguments are passed, the result is +0.
If any argument is +∞, the result is
+∞.
If any argument is -∞, the result is
+∞.
If no argument is +∞ or
-∞, and any argument is
NaN, the result is NaN.
If all arguments are either +0 or
-0, the result is +0.
Note
Implementations should take care to avoid the loss of
precision from overflows and underflows that are prone to
occur in naive implementations when this function is called
with two or more arguments.
20.3.2.19Math.imul ( x,
y )
When Math.imul is called with arguments
x and y, the following steps are taken:
Returns an implementation-dependent approximation to the natural
logarithm of x.
If x is NaN, the result is
NaN.
If x is less than 0, the result is
NaN.
If x is +0 or
-0, the result is -∞.
If x is 1, the result is +0.
If x is +∞, the result is
+∞.
20.3.2.21Math.log1p ( x )
Returns an implementation-dependent approximation to the natural
logarithm of 1 + x. The result is computed in a way
that is accurate even when the value of x is close to zero.
If x is NaN, the result is
NaN.
If x is less than -1, the result is
NaN.
If x is -1, the result is -∞.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
20.3.2.22Math.log10 ( x )
Returns an implementation-dependent approximation to the base 10
logarithm of x.
If x is NaN, the result is
NaN.
If x is less than 0, the result is
NaN.
If x is +0, the result is
-∞.
If x is -0, the result is
-∞.
If x is 1, the result is +0.
If x is +∞, the result is
+∞.
20.3.2.23Math.log2 ( x )
Returns an implementation-dependent approximation to the base 2
logarithm of x.
If x is NaN, the result is
NaN.
If x is less than 0, the result is
NaN.
If x is +0, the result is
-∞.
If x is -0, the result is
-∞.
If x is 1, the result is +0.
If x is +∞, the result is
+∞.
20.3.2.24Math.max ( value1,
value2, ...values )
Given zero or more arguments, calls
ToNumber
on each of the arguments and returns the largest of the resulting
values.
If no arguments are given, the result is -∞.
If any value is NaN, the result is
NaN.
The comparison of values to determine the largest value is done
using the
Abstract Relational Comparison
algorithm except that +0 is considered to be
larger than -0.
20.3.2.25Math.min ( value1,
value2, ...values )
Given zero or more arguments, calls
ToNumber
on each of the arguments and returns the smallest of the resulting
values.
If no arguments are given, the result is +∞.
If any value is NaN, the result is
NaN.
The comparison of values to determine the smallest value is done
using the
Abstract Relational Comparison
algorithm except that +0 is considered to be
larger than -0.
Returns a
Number value
with positive sign, greater than or equal to 0 but less than 1,
chosen randomly or pseudo randomly with approximately uniform
distribution over that range, using an implementation-dependent
algorithm or strategy. This function takes no arguments.
Each Math.random function created for distinct realms
must produce a distinct sequence of values from successive calls.
20.3.2.28Math.round ( x )
Returns the
Number value
that is closest to x and is an
integer. If two integers are equally close to x, then the
result is the
Number value
that is closer to +∞. If x is
already an
integer, the result is x.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
If x is -∞, the result is
-∞.
If x is greater than 0 but less than 0.5, the result
is +0.
If x is less than 0 but greater than or equal to
-0.5, the result is -0.
Note 1
Math.round(3.5) returns 4, but
Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same
as the value of Math.floor(x + 0.5). When
x is -0 or is less than 0 but
greater than or equal to -0.5,
Math.round(x) returns -0, but
Math.floor(x + 0.5) returns
+0. Math.round(x) may also
differ from the value of
Math.floor(x + 0.5)because of internal rounding
when computing x + 0.5.
20.3.2.29Math.sign ( x )
Returns the sign of x, indicating whether
x is positive, negative, or zero.
If x is NaN, the result is
NaN.
If x is -0, the result is
-0.
If x is +0, the result is
+0.
If x is negative and not -0, the
result is -1.
If x is positive and not +0, the
result is +1.
20.3.2.30Math.sin ( x )
Returns an implementation-dependent approximation to the sine of
x. The argument is expressed in radians.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞ or
-∞, the result is NaN.
20.3.2.31Math.sinh ( x )
Returns an implementation-dependent approximation to the
hyperbolic sine of x.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
If x is -∞, the result is
-∞.
Note
The value of Math.sinh(x) is the same as the
value of (Math.exp(x) - Math.exp(-x)) / 2.
20.3.2.32Math.sqrt ( x )
Returns an implementation-dependent approximation to the square
root of x.
If x is NaN, the result is
NaN.
If x is less than 0, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is
+∞.
20.3.2.33Math.tan ( x )
Returns an implementation-dependent approximation to the tangent
of x. The argument is expressed in radians.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞ or
-∞, the result is NaN.
20.3.2.34Math.tanh ( x )
Returns an implementation-dependent approximation to the
hyperbolic tangent of x.
If x is NaN, the result is
NaN.
If x is +0, the result is
+0.
If x is -0, the result is
-0.
If x is +∞, the result is +1.
If x is -∞, the result is -1.
Note
The value of Math.tanh(x) is the same as the
value of
(Math.exp(x) - Math.exp(-x)) / (Math.exp(x) +
Math.exp(-x)).
20.3.2.35Math.trunc ( x )
Returns the integral part of the number x, removing any
fractional digits. If x is already an
integer, the result is x.
If x is NaN, the result is
NaN.
If x is -0, the result is
-0.
If x is +0, the result is
+0.
If x is +∞, the result is
+∞.
If x is -∞, the result is
-∞.
If x is greater than 0 but less than 1, the result is
+0.
If x is less than 0 but greater than -1, the result
is -0.
20.4Date Objects
20.4.1Overview of Date Objects and
Definitions of Abstract Operations
The following functions are
abstract operations
that operate on time values (defined in
20.4.1.1). Note that, in every case, if any argument to one of these
functions is NaN, the result will be
NaN.
20.4.1.1Time Values and Time Range
Time measurement in ECMAScript is analogous to time measurement in
POSIX, in particular sharing definition in terms of the proleptic
Gregorian calendar, an epoch of midnight at the beginning of 01
January, 1970 UTC, and an accounting of every day as comprising
exactly 86,400 seconds (each of which is 1000 milliseconds long).
An ECMAScript time value is a Number, either a finite
integer
representing an instant in time to millisecond precision or
NaN representing no specific instant. A time
value that is a multiple of
24 × 60 × 60 × 1000 = 86,400,000
(i.e., is equal to 86,400,000 × d for some
integerd) represents the instant at the start of the UTC day
that follows the epoch by d whole UTC days (preceding
the epoch for negative d). Every other finite time
value t is defined relative to the greatest preceding
time value s that is such a multiple, and represents
the instant that occurs within the same UTC day as
s but follows it by t −
s milliseconds.
Time values do not account for UTC leap seconds—there are no time
values representing instants within positive leap seconds, and
there are time values representing instants removed from the UTC
timeline by negative leap seconds. However, the definition of time
values nonetheless yields piecewise alignment with UTC, with
discontinuities only at leap second boundaries and zero difference
outside of leap seconds.
A Number can exactly represent all integers from
-9,007,199,254,740,992 to 9,007,199,254,740,992 (20.1.2.8
and
20.1.2.6). A time value supports a slightly smaller range of
-8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds. This
yields a supported time value range of exactly -100,000,000 days
to 100,000,000 days relative to midnight at the beginning of 01
January, 1970 UTC.
The exact moment of midnight at the beginning of 01 January, 1970
UTC is represented by the time value +0.
Note
The 400 year cycle of the proleptic Gregorian calendar
contains 97 leap years. This yields an average of 365.2425
days per year, which is 31,556,952,000 milliseconds.
Therefore, the maximum range a Number could represent exactly
with millisecond precision is approximately -285,426 to
285,426 years relative to 1970. The smaller range supported by
a time value as specified in this section is approximately
-273,790 to 273,790 years relative to 1970.
ECMAScript uses a proleptic Gregorian calendar to map a day number
to a year number and to determine the month and date within that
year. In this calendar, leap years are precisely those which are
(divisible by 4) and ((not divisible by 100) or (divisible by
400)). The number of days in year number y is therefore
defined by
All non-leap years have 365 days with the usual number of days per
month and leap years have an extra day in February. The day number
of the first day of year y is given by:
A month value of 0 specifies January; 1 specifies February; 2
specifies March; 3 specifies April; 4 specifies May; 5 specifies
June; 6 specifies July; 7 specifies August; 8 specifies September;
9 specifies October; 10 specifies November; and 11 specifies
December. Note that
MonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.
20.4.1.5Date Number
A date number is identified by an
integer
in the range 1 through 31, inclusive. The mapping
DateFromTime(t) from a
time valuet to a date number is defined by:
A weekday value of 0 specifies Sunday; 1 specifies Monday; 2
specifies Tuesday; 3 specifies Wednesday; 4 specifies Thursday; 5
specifies Friday; and 6 specifies Saturday. Note that
WeekDay(0) = 4, corresponding to
Thursday, 01 January, 1970.
20.4.1.7LocalTZA ( t,
isUTC )
LocalTZA( t, isUTC ) is an
implementation-defined algorithm that returns the local time zone
adjustment, or offset, in milliseconds. The local political rules
for standard time and daylight saving time in effect at
t should be used to determine the result in the way
specified in this section.
When isUTC is true,
LocalTZA( tUTC, true )
should return the offset of the local time zone from UTC measured
in milliseconds at time represented by
time valuetUTC. When the result is added to
tUTC, it should yield the corresponding Number
tlocal.
When isUTC is false,
LocalTZA( tlocal, false )
should return the offset of the local time zone from UTC measured
in milliseconds at local time represented by Number
tlocal. When the result is subtracted from
tlocal, it should yield the corresponding
time valuetUTC.
Input t is nominally a
time value
but may be any
Number value. This can occur when isUTC is false and tlocal represents a
time value
that is already offset outside of the
time value
range at the range boundaries. The algorithm must not limit
tlocal to the
time value
range, so that such inputs are supported.
When
tlocal
represents local time repeating multiple times at a negative time
zone transition (e.g. when the daylight saving time ends or the
time zone offset is decreased due to a time zone rule change) or
skipped local time at a positive time zone transitions (e.g. when
the daylight saving time starts or the time zone offset is
increased due to a time zone rule change),
tlocal
must be interpreted using the time zone offset before the
transition.
If an implementation does not support a conversion described above
or if political rules for time t are not available
within the implementation, the result must be 0.
Note
It is recommended that implementations use the time zone
information of the IANA Time Zone Database
https://www.iana.org/time-zones/.
1:30 AM on November 5, 2017 in America/New_York is repeated
twice (fall backward), but it must be interpreted as 1:30 AM
UTC-04 instead of 1:30 AM UTC-05. LocalTZA(TimeClip(MakeDate(MakeDay(2017, 10, 5),
MakeTime(1, 30, 0, 0))), false) is
-4 ×
msPerHour.
2:30 AM on March 12, 2017 in America/New_York does not exist,
but it must be interpreted as 2:30 AM UTC-05 (equivalent to
3:30 AM UTC-04). LocalTZA(TimeClip(MakeDate(MakeDay(2017, 2, 12),
MakeTime(2, 30, 0, 0))), false) is
-5 ×
msPerHour.
Local time zone offset values may be positive
or negative.
20.4.1.8LocalTime ( t )
The abstract operation LocalTime with argument
t converts t from UTC to local time by
performing the following steps:
Two different input time values
tUTC
are converted to the same local time
tlocal at a
negative time zone transition when there are repeated times
(e.g. the daylight saving time ends or the time zone
adjustment is decreased.).
LocalTime(UTC(tlocal))
is not necessarily always equal to
tlocal. Correspondingly,
UTC(LocalTime(tUTC))
is not necessarily always equal to
tUTC.
20.4.1.9UTC ( t )
The abstract operation UTC with argument t converts
t from local time to UTC by performing the following
steps:
The abstract operation MakeTime calculates a number of
milliseconds from its four arguments, which must be ECMAScript
Number values. This operator functions as follows:
If hour is not finite or min is not
finite or sec is not finite or ms is not
finite, return NaN.
Let t be h*msPerHour+m*msPerMinute+s*msPerSecond+milli, performing the arithmetic
according to IEEE 754-2008 rules (that is, as if using the
ECMAScript operators * and +).
Return t.
20.4.1.12MakeDay ( year,
month, date )
The abstract operation MakeDay calculates a number of days from
its three arguments, which must be ECMAScript Number values. This
operator functions as follows:
If year is not finite or month is not
finite or date is not finite, return
NaN.
Find a value t such that
YearFromTime(t) is ym and
MonthFromTime(t) is mn and
DateFromTime(t) is 1; but if this is not possible (because
some argument is out of range), return NaN.
The abstract operation MakeDate calculates a number of
milliseconds from its two arguments, which must be ECMAScript
Number values. This operator functions as follows:
If day is not finite or time is not
finite, return NaN.
The abstract operation TimeClip calculates a number of
milliseconds from its argument, which must be an ECMAScript
Number value. This operator functions as follows:
The point of step 4 is that an implementation is permitted a
choice of internal representations of time values, for example
as a 64-bit signed
integer
or as a 64-bit floating-point value. Depending on the
implementation, this internal representation may or may not
distinguish -0 and +0.
20.4.1.15Date Time String Format
ECMAScript defines a string interchange format for date-times
based upon a simplification of the ISO 8601 calendar date extended
format. The format is as follows:
YYYY-MM-DDTHH:mm:ss.sssZ
Where the elements are as follows:
YYYY
is the year in the proleptic Gregorian calendar as four
decimal digits from 0000 to 9999, or as an
expanded year
of "+" or
"-" followed by six decimal digits.
-
"-" (hyphen) appears literally twice in
the string.
MM
is the month of the year as two decimal digits from 01
(January) to 12 (December).
DD
is the day of the month as two decimal digits from 01 to
31.
T
"T" appears literally in the string, to
indicate the beginning of the time element.
HH
is the number of complete hours that have passed since
midnight as two decimal digits from 00 to 24.
:
":" (colon) appears literally twice in
the string.
mm
is the number of complete minutes since the start of the
hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the
minute as two decimal digits from 00 to 59.
.
"." (dot) appears literally in the
string.
sss
is the number of complete milliseconds since the start of
the second as three decimal digits.
Z
is the UTC offset representation specified as
"Z" (for UTC with no offset) or an
offset of either "+" or
"-" followed by a time expression
HH:mm (indicating local time ahead of or
behind UTC, respectively)
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the
above date-only forms immediately followed by one of the following
time forms with an optional UTC offset representation appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
A string containing out-of-bounds or nonconforming elements is not
a valid instance of this format.
Note 1
As every day both starts and ends with midnight, the two
notations 00:00 and 24:00 are
available to distinguish the two midnights that can be
associated with one date. This means that the following two
notations refer to exactly the same point in time:
1995-02-04T24:00 and
1995-02-05T00:00. This interpretation of the
latter form as "end of a calendar day" is consistent with ISO
8601, even though that specification reserves it for
describing time intervals and does not permit it within
representations of single points in time.
Note 2
There exists no international standard that specifies
abbreviations for civil time zones like CET, EST, etc. and
sometimes the same abbreviation is even used for two very
different time zones. For this reason, both ISO 8601 and this
format specify numeric representations of time zone offsets.
20.4.1.15.1Expanded Years
Covering
the full
time value
range of approximately 273,790 years forward or backward from 01
January, 1970 (20.4.1.1) requires representing years before 0 or after 9999. ISO 8601
permits expansion of the year representation, but only by mutual
agreement of the partners in information interchange. In the
simplified ECMAScript format, such an expanded year
representation shall have 6 digits and is always prefixed with a
+ or - sign. The year 0 is considered positive and hence
prefixed with a + sign. Strings matching the
Date Time String Format
with expanded years representing instants in time outside the
range of a
time value
are treated as unrecognizable by
Date.parse
and cause that function to return NaN without
falling back to implementation-specific behavior or heuristics.
is the initial value of the "Date" property of
the
global object.
creates and initializes a new Date object when called as a
constructor.
returns a String representing the current time (UTC) when called
as a function rather than as a
constructor.
is a single function whose behaviour is overloaded based upon the
number and types of its arguments.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Date behaviour must include a super call
to the Dateconstructor
to create and initialize the subclass instance with a
[[DateValue]] internal slot.
The now function returns a
Number value
that is the
time value
designating the UTC date and time of the occurrence of the call to
now.
20.4.3.2Date.parse (
string )
The parse function applies the
ToString
operator to its argument. If
ToString
results in an
abrupt completion
the
Completion Record
is immediately returned. Otherwise, parse interprets
the resulting String as a date and time; it returns a Number, the
UTC
time value
corresponding to the date and time. The String may be interpreted
as a local time, a UTC time, or a time in some other time zone,
depending on the contents of the String. The function first
attempts to parse the String according to the format described in
Date Time String Format (20.4.1.15), including expanded years. If the String does not conform to
that format the function may fall back to any
implementation-specific heuristics or implementation-specific date
formats. Strings that are unrecognizable or contain out-of-bounds
format element values shall cause Date.parse to
return NaN.
If the String conforms to the
Date Time String Format, substitute values take the place of absent format elements.
When the MM or DD elements are absent,
"01" is used. When the HH,
mm, or ss elements are absent,
"00" is used. When the sss element
is absent, "000" is used. When the UTC offset
representation is absent, date-only forms are interpreted as a UTC
time and date-time forms are interpreted as a local time.
If x is any Date object whose milliseconds amount is
zero within a particular implementation of ECMAScript, then all of
the following expressions should produce the same numeric value in
that implementation, if all the properties referenced have their
initial values:
is not required to produce the same
Number value
as the preceding three expressions and, in general, the value
produced by Date.parse is implementation-dependent
when given any String value that does not conform to the Date Time
String Format (20.4.1.15) and that could not be produced in that implementation by the
toString or toUTCString method.
20.4.3.3Date.prototype
The initial value of Date.prototype is
%Date.prototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
20.4.3.4Date.UTC ( year [ ,
month [ , date [ , hours [ ,
minutes [ , seconds [ , ms ] ] ]
] ] ] )
When the UTC function is called, the following steps
are taken:
The UTC function differs from the
Dateconstructor
in two ways: it returns a
time value
as a Number, rather than creating a Date object, and it
interprets the arguments in UTC rather than as local time.
20.4.4Properties of the Date Prototype
Object
The Date prototype object:
is the intrinsic object %DatePrototype%.
is itself an ordinary object.
is not a Date instance and does not have a [[DateValue]] internal
slot.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
Unless explicitly defined otherwise, the methods of the Date
prototype object defined below are not generic and the
this value passed to them must be an object that
has a [[DateValue]] internal slot that has been initialized to a
time value.
The abstract operation
thisTimeValue(value) performs the following steps:
If
Type(value) is Object and value has a
[[DateValue]] internal slot, then
Return value.[[DateValue]].
Throw a TypeError exception.
In following descriptions of functions that are properties of the
Date prototype object, the phrase “this Date object” refers to the object that is the this value
for the invocation of the function. If the Type of the
this value is not Object, a
TypeError exception is thrown. The phrase “this time value” within the specification of a method refers to the result
returned by calling the abstract operation thisTimeValue with the
this value of the method invocation passed as the
argument.
20.4.4.1Date.prototype.constructor
The initial value of Date.prototype.constructor is
%Date%.
The "length" property of the
setFullYear method is 3.
Note
If month is not present, this method behaves as if
month was present with the value
getMonth(). If date is not present, it
behaves as if date was present with the value
getDate().
20.4.4.22Date.prototype.setHours (
hour [ , min [ , sec [ ,
ms ] ] ] )
The "length" property of the
setHours method is 4.
Note
If min is not present, this method behaves as if
min was present with the value
getMinutes(). If sec is not present,
it behaves as if sec was present with the value
getSeconds(). If ms is not present, it
behaves as if ms was present with the value
getMilliseconds().
The "length" property of the
setMinutes method is 3.
Note
If sec is not present, this method behaves as if
sec was present with the value
getSeconds(). If ms is not present,
this behaves as if ms was present with the value
getMilliseconds().
20.4.4.25Date.prototype.setMonth (
month [ , date ] )
The "length" property of the
setUTCFullYear method is 3.
Note
If month is not present, this method behaves as if
month was present with the value
getUTCMonth(). If date is not present,
it behaves as if date was present with the value
getUTCDate().
20.4.4.30Date.prototype.setUTCHours (
hour [ , min [ , sec [ ,
ms ] ] ] )
The "length" property of the
setUTCHours method is 4.
Note
If min is not present, this method behaves as if
min was present with the value
getUTCMinutes(). If sec is not
present, it behaves as if sec was present with the
value getUTCSeconds(). If ms is not
present, it behaves as if ms was present with the
value getUTCMilliseconds().
The "length" property of the
setUTCMinutes method is 3.
Note
If sec is not present, this method behaves as if
sec was present with the value
getUTCSeconds(). If ms is not present,
it function behaves as if ms was present with the
value return by getUTCMilliseconds().
20.4.4.33Date.prototype.setUTCMonth (
month [ , date ] )
If
this time value
is not a finite Number or if it corresponds with a year that
cannot be represented in the
Date Time String Format, this function throws a RangeError exception.
Otherwise, it returns a String representation of
this time value
in that format on the UTC time scale, including all format
elements and the UTC offset representation "Z".
20.4.4.37Date.prototype.toJSON (
key )
This function provides a String representation of a Date object
for use by JSON.stringify (24.5.2).
When the toJSON method is called with argument
key, the following steps are taken:
The toJSON function is intentionally generic; it
does not require that its this value be a
Date object. Therefore, it can be transferred to other kinds
of objects for use as a method. However, it does require that
any such object have a toISOString method.
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
Date.prototype.toLocaleDateString method as specified
in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of
the toLocaleDateString method is used.
This function returns a String value. The contents of the String
are implementation-dependent, but are intended to represent the
“date” portion of the Date in the current time zone in a
convenient, human-readable form that corresponds to the
conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined
in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not use those parameter positions for
anything else.
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
Date.prototype.toLocaleString method as specified in
the ECMA-402 specification. If an ECMAScript implementation does
not include the ECMA-402 API the following specification of the
toLocaleString method is used.
This function returns a String value. The contents of the String
are implementation-dependent, but are intended to represent the
Date in the current time zone in a convenient, human-readable form
that corresponds to the conventions of the host environment's
current locale.
The meaning of the optional parameters to this method are defined
in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not use those parameter positions for
anything else.
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
Date.prototype.toLocaleTimeString method as specified
in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of
the toLocaleTimeString method is used.
This function returns a String value. The contents of the String
are implementation-dependent, but are intended to represent the
“time” portion of the Date in the current time zone in a
convenient, human-readable form that corresponds to the
conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined
in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not use those parameter positions for
anything else.
For any Date object d whose milliseconds amount
is zero, the result of
Date.parse(d.toString()) is equal to
d.valueOf(). See
20.4.3.2.
Note 2
The toString function is not generic; it throws a
TypeError exception if its
this value is not a Date object. Therefore,
it cannot be transferred to other kinds of objects for use as
a method.
If yv ≥ 0, let yearSign be the empty
string; otherwise, let yearSign be
"-".
Let year be the String representation of
abs(yv), formatted as a decimal number.
Let paddedYear be ! StringPad(year, 4, "0",
start).
Return the
string-concatenation
of weekday, the code unit 0x0020 (SPACE),
month, the code unit 0x0020 (SPACE),
day, the code unit 0x0020 (SPACE),
yearSign, and paddedYear.
Table 52: Names of days of the week
Number
Name
0
"Sun"
1
"Mon"
2
"Tue"
3
"Wed"
4
"Thu"
5
"Fri"
6
"Sat"
Table 53: Names of months of the year
Number
Name
0
"Jan"
1
"Feb"
2
"Mar"
3
"Apr"
4
"May"
5
"Jun"
6
"Jul"
7
"Aug"
8
"Sep"
9
"Oct"
10
"Nov"
11
"Dec"
20.4.4.41.3Runtime Semantics:
TimeZoneString ( tv )
If offset ≥ 0, let offsetSign be
"+"; otherwise, let
offsetSign be "-".
Let offsetMin be the String representation of
MinFromTime(abs(offset)), formatted as a two-digit decimal
number, padded to the left with a zero if necessary.
Let offsetHour be the String representation of
HourFromTime(abs(offset)), formatted as a two-digit decimal
number, padded to the left with a zero if necessary.
Let tzName be an implementation-defined string
that is either the empty string or the
string-concatenation
of the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT
PARENTHESIS), an implementation-dependent timezone name, and
the code unit 0x0029 (RIGHT PARENTHESIS).
Return the
string-concatenation
of offsetSign, offsetHour,
offsetMin, and tzName.
The toUTCString method returns a String value
representing the instance in time corresponding to
this time value. The format of the String is based upon "HTTP-date" from RFC
7231, generalized to support the full range of times supported by
ECMAScript Date objects. It performs the following steps:
If yv ≥ 0, let yearSign be the empty
string; otherwise, let yearSign be
"-".
Let year be the String representation of
abs(yv), formatted as a decimal number.
Let paddedYear be ! StringPad(year, 4, "0",
start).
Return the
string-concatenation
of weekday, ",", the code unit
0x0020 (SPACE), day, the code unit 0x0020 (SPACE),
month, the code unit 0x0020 (SPACE),
yearSign, paddedYear, the code unit
0x0020 (SPACE), and
TimeString(tv).
20.4.4.45Date.prototype [
@@toPrimitive ] ( hint )
This function is called by ECMAScript language operators to
convert a Date object to a primitive value. The allowed values for
hint are "default",
"number", and "string". Date
objects, are unique among built-in ECMAScript object in that they
treat "default" as being equivalent to
"string", All other built-in ECMAScript objects
treat "default" as being equivalent to
"number".
When the @@toPrimitive method is called with argument
hint, the following steps are taken:
Let O be the this value.
If
Type(O) is not Object, throw a
TypeError exception.
If hint is the String value
"string" or the String value
"default", then
The value of the "name" property of this
function is "[Symbol.toPrimitive]".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
20.4.5Properties of Date Instances
Date instances are ordinary objects that inherit properties from the
Date prototype object. Date instances also have a [[DateValue]]
internal slot. The [[DateValue]] internal slot is the
time value
represented by
this Date object.
is the initial value of the "String" property
of the
global object.
creates and initializes a new String object when called as a
constructor.
performs a type conversion when called as a function rather than
as a
constructor.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
String behaviour must include a
super call to the Stringconstructor
to create and initialize the subclass instance with a
[[StringData]] internal slot.
21.1.1.1String ( value )
When String is called with argument value,
the following steps are taken:
If value is not present, let s be the
empty String.
If ! IsInteger(nextCP) is false, throw a
RangeError exception.
If nextCP < 0 or nextCP >
0x10FFFF, throw a RangeError exception.
Append the elements of the
UTF16Encoding
of nextCP to the end of elements.
Set nextIndex to nextIndex + 1.
Return the String value whose code units are, in order, the
elements in the
Listelements. If length is 0, the empty
string is returned.
The "length" property of the
fromCodePoint function is 1.
21.1.2.3String.prototype
The initial value of String.prototype is
%String.prototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
21.1.2.4String.raw (
template, ...substitutions )
The String.raw function may be called with a variable
number of arguments. The first argument is template and
the remainder of the arguments form the
Listsubstitutions. The following steps are taken:
Let substitutions be a
List
consisting of all of the arguments passed to this function,
starting with the second argument. If fewer than two arguments
were passed, the
List
is empty.
Let numberOfSubstitutions be the number of elements
in substitutions.
Append in order the code unit elements of
nextSeg to the end of
stringElements.
If nextIndex + 1 = literalSegments,
then
Return the String value whose code units are, in
order, the elements in the
ListstringElements. If
stringElements has no elements, the empty
string is returned.
If nextIndex <
numberOfSubstitutions, let next be
substitutions[nextIndex].
Append in order the code unit elements of
nextSub to the end of
stringElements.
Set nextIndex to nextIndex + 1.
Note
String.raw is intended for use as a tag function of a Tagged
Template (12.3.11). When called as such, the first argument will be a well
formed template object and the rest parameter will contain the
substitution values.
21.1.3Properties of the String Prototype
Object
The String prototype object:
is the intrinsic object %StringPrototype%.
is a String
exotic object
and has the internal methods specified for such objects.
has a [[StringData]] internal slot whose value is the empty
String.
has a "length" property whose initial value is
0 and whose attributes are { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
Unless explicitly stated otherwise, the methods of the String
prototype object defined below are not generic and the
this value passed to them must be either a String
value or an object that has a [[StringData]] internal slot that has
been initialized to a String value.
The abstract operation
thisStringValue(value) performs the following steps:
Returns a single element String containing the code unit at
index pos within the String value resulting from
converting this object to a String. If there is no element at
that index, the result is the empty String. The result is a
String value, not a String object.
If pos is a value of Number type that is an
integer, then the result of x.charAt(pos) is equal to
the result of x.substring(pos, pos + 1).
When the charAt method is called with one argument
pos, the following steps are taken:
If position < 0 or position ≥
size, return the empty String.
Return the String value of length 1, containing one code unit
from S, namely the code unit at index
position.
Note 2
The charAt function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
21.1.3.2String.prototype.charCodeAt (
pos )
Note 1
Returns a Number (a nonnegative
integer
less than 216) that is the numeric value of the
code unit at index pos within the String resulting
from converting this object to a String. If there is no
element at that index, the result is NaN.
When the charCodeAt method is called with one
argument pos, the following steps are taken:
Return a value of Number type, whose value is the numeric
value of the code unit at index position within the
String S.
Note 2
The charCodeAt function is intentionally generic;
it does not require that its this value be
a String object. Therefore it can be transferred to other
kinds of objects for use as a method.
21.1.3.3String.prototype.codePointAt (
pos )
Note 1
Returns a nonnegative
integer
Number less than or equal to 0x10FFFF that is the code point
value of the UTF-16 encoded code point (6.1.4) starting at the string element at index
pos within the String resulting from converting
this object to a String. If there is no element at that index,
the result is undefined. If a valid UTF-16
surrogate pair
does not begin at pos, the result is the code unit
at pos.
When the codePointAt method is called with one
argument pos, the following steps are taken:
The codePointAt function is intentionally
generic; it does not require that its
this value be a String object. Therefore it
can be transferred to other kinds of objects for use as a
method.
21.1.3.4String.prototype.concat (
...args )
Note 1
When the concat method is called it returns the
String value consisting of the code units of the
this object (converted to a String)
followed by the code units of each of the arguments converted
to a String. The result is a String value, not a String
object.
When the concat method is called with zero or more
arguments, the following steps are taken:
The concat function is intentionally generic; it
does not require that its this value be a
String object. Therefore it can be transferred to other kinds
of objects for use as a method.
21.1.3.5String.prototype.constructor
The initial value of String.prototype.constructor is
%String%.
If the sequence of code units of S starting at
start of length searchLength is the same
as the full code unit sequence of searchStr, return
true.
Otherwise, return false.
Note 1
Returns true if the sequence of code units
of searchString converted to a String is the same
as the corresponding code units of this object (converted to a
String) starting at endPosition - length(this).
Otherwise returns false.
Note 2
Throwing an exception if the first argument is a RegExp is
specified in order to allow future editions to define
extensions that allow such argument values.
Note 3
The endsWith function is intentionally generic;
it does not require that its this value be
a String object. Therefore, it can be transferred to other
kinds of objects for use as a method.
21.1.3.7String.prototype.includes (
searchString [ , position ] )
The includes method takes two arguments,
searchString and position, and performs the
following steps:
If there exists any
integerk not smaller than start such that
k + searchLen is not greater than
len, and for all nonnegative integers
j less than searchLen, the code unit at
index k + j within S is the
same as the code unit at index j within
searchStr, return true; but if
there is no such
integerk, return false.
Note 1
If searchString appears as a substring of the
result of converting this object to a String, at one or more
indices that are greater than or equal to position,
return true; otherwise, returns
false. If position is
undefined, 0 is assumed, so as to search
all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is
specified in order to allow future editions to define
extensions that allow such argument values.
Note 3
The includes function is intentionally generic;
it does not require that its this value be
a String object. Therefore, it can be transferred to other
kinds of objects for use as a method.
21.1.3.8String.prototype.indexOf (
searchString [ , position ] )
Note 1
If searchString appears as a substring of the
result of converting this object to a String, at one or more
indices that are greater than or equal to position,
then the smallest such index is returned; otherwise, -1 is
returned. If position is
undefined, 0 is assumed, so as to search
all of the String.
The indexOf method takes two arguments,
searchString and position, and performs the
following steps:
Return the smallest possible
integerk not smaller than start such that
k + searchLen is not greater than
len, and for all nonnegative integers
j less than searchLen, the code unit at
index k + j within S is the
same as the code unit at index j within
searchStr; but if there is no such
integerk, return the value -1.
Note 2
The indexOf function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
21.1.3.9String.prototype.lastIndexOf (
searchString [ , position ] )
Note 1
If searchString appears as a substring of the
result of converting this object to a String at one or more
indices that are smaller than or equal to position,
then the greatest such index is returned; otherwise, -1 is
returned. If position is
undefined, the length of the String value
is assumed, so as to search all of the String.
The lastIndexOf method takes two arguments,
searchString and position, and performs the
following steps:
Return the largest possible nonnegative
integerk not larger than start such that
k + searchLen is not greater than
len, and for all nonnegative integers
j less than searchLen, the code unit at
index k + j within S is the
same as the code unit at index j within
searchStr; but if there is no such
integerk, return the value -1.
Note 2
The lastIndexOf function is intentionally
generic; it does not require that its
this value be a String object. Therefore,
it can be transferred to other kinds of objects for use as a
method.
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
localeCompare method as specified in the ECMA-402
specification. If an ECMAScript implementation does not include
the ECMA-402 API the following specification of the
localeCompare method is used.
When the localeCompare method is called with argument
that, it returns a Number other than
NaN that represents the result of a
locale-sensitive String comparison of the
this value (converted to a String) with
that (converted to a String). The two Strings are
S and That. The two Strings are compared in
an implementation-defined fashion. The result is intended to order
String values in the sort order specified by a host default
locale, and will be negative, zero, or positive, depending on
whether S comes before That in the sort
order, the Strings are equal, or S comes after
That in the sort order, respectively.
Before performing the comparisons, the following steps are
performed to prepare the Strings:
The meaning of the optional second and third parameters to this
method are defined in the ECMA-402 specification; implementations
that do not include ECMA-402 support must not assign any other
interpretation to those parameter positions.
The localeCompare method, if considered as a function
of two arguments this and that, is a
consistent comparison function (as defined in
22.1.3.27) on the set of all Strings.
The actual return values are implementation-defined to permit
implementers to encode additional information in the value, but
the function is required to define a total ordering on all
Strings. This function must treat Strings that are canonically
equivalent according to the Unicode standard as identical and must
return 0 when comparing Strings that are considered
canonically equivalent.
Note 1
The localeCompare method itself is not directly
suitable as an argument to
Array.prototype.sort because the latter requires
a function of two arguments.
Note 2
This function is intended to rely on whatever
language-sensitive comparison functionality is available to
the ECMAScript environment from the host environment, and to
compare according to the rules of the host environment's
current locale. However, regardless of the host provided
comparison capabilities, this function must treat Strings that
are canonically equivalent according to the Unicode standard
as identical. It is recommended that this function should not
honour Unicode compatibility equivalences or decompositions.
For a definition and discussion of canonical equivalence see
the Unicode Standard, chapters 2 and 3, as well as Unicode
Standard Annex #15, Unicode Normalization Forms (https://unicode.org/reports/tr15/) and Unicode Technical Note #5, Canonical Equivalence in
Applications (https://www.unicode.org/notes/tn5/). Also see Unicode Technical Standard #10, Unicode Collation
Algorithm (https://unicode.org/reports/tr10/).
Note 3
The localeCompare function is intentionally
generic; it does not require that its
this value be a String object. Therefore,
it can be transferred to other kinds of objects for use as a
method.
21.1.3.11String.prototype.match (
regexp )
When the match method is called with argument
regexp, the following steps are taken:
The match function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
21.1.3.12String.prototype.matchAll (
regexp )
Performs a regular expression match of the String representing the
this value against regexp and
returns an iterator. Each iteration result's value is an Array
object containing the results of the match, or
null if the String did not match.
When the matchAll method is called, the following
steps are taken:
The matchAll function is intentionally generic, it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
Note 2
Similarly to String.prototype.split,
String.prototype.matchAll is designed to typically
act without mutating its inputs.
21.1.3.13String.prototype.normalize (
[ form ] )
When the normalize method is called with one argument
form, the following steps are taken:
If f is not one of "NFC",
"NFD", "NFKC", or
"NFKD", throw a
RangeError exception.
Let ns be the String value that is the result of
normalizing S into the normalization form named by
f as specified in
https://unicode.org/reports/tr15/.
Return ns.
Note
The normalize function is intentionally generic;
it does not require that its this value be
a String object. Therefore it can be transferred to other
kinds of objects for use as a method.
Return the String value that is made from n copies
of S appended together.
Note 1
This method creates the String value consisting of the code
units of the this object (converted to
String) repeated count times.
Note 2
The repeat function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
Search string for the first occurrence of
searchString and let pos be the index
within string of the first code unit of the matched
substring and let matched be
searchString. If no occurrences of
searchString were found, return string.
If functionalReplace is true,
then
Let replValue be ? Call(replaceValue, undefined, «
matched, pos, string »).
Let replStr be ! GetSubstitution(matched, string, pos,
captures, undefined,
replaceValue).
Let tailPos be pos + the number of code
units in matched.
Let newString be the
string-concatenation
of the first pos code units of string,
replStr, and the trailing substring of
string starting at index tailPos. If
pos is 0, the first element of the concatenation
will be the empty String.
Return newString.
Note
The replace function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
Let result be the String value derived from
replacement by copying code unit elements from
replacement to result while performing
replacements as specified in
Table 54. These $ replacements are done left-to-right,
and, once such a replacement is performed, the new
replacement text is not subject to further replacements.
Return result.
Table 54: Replacement Text Symbol Substitutions
Code units
Unicode Characters
Replacement text
0x0024, 0x0024
$$
$
0x0024, 0x0026
$&
matched
0x0024, 0x0060
$`
If position is 0, the replacement is the
empty String. Otherwise the replacement is the
substring of str that starts at index 0 and
whose last code unit is at index position -
1.
0x0024, 0x0027
$'
If tailPos ≥ stringLength, the
replacement is the empty String. Otherwise the
replacement is the substring of str that
starts at index tailPos and continues to
the end of str.
0x0024, N
Where
0x0031 ≤ N ≤ 0x0039
$n where
n is one of
1 2 3 4 5 6 7 8 9 and $n is
not followed by a decimal digit
The nth element of
captures, where n is a single
digit in the range 1 to 9. If n ≤
m and the nth element
of captures is
undefined, use the empty String
instead. If n > m, no
replacement is done.
0x0024, N, N
Where
0x0030 ≤ N ≤ 0x0039
$nn where
n is one of
0 1 2 3 4 5 6 7 8 9
The nnth element of
captures, where nn is a
two-digit decimal number in the range 01 to 99. If
nn ≤ m and the nnth element of captures is
undefined, use the empty String
instead. If nn is 00 or nn >
m, no replacement is done.
0x0024, 0x003C
$<
If namedCaptures is
undefined, the replacement
text is the String "$<".
The search function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
21.1.3.19String.prototype.slice (
start, end )
The slice method takes two arguments,
start and end, and returns a substring of
the result of converting this object to a String, starting from
index start and running to, but not including, index
end (or through the end of the String if
end is undefined). If
start is negative, it is treated as
sourceLength + start
where sourceLength is the length of the String. If
end is negative, it is treated as
sourceLength + end
where sourceLength is the length of the String. The
result is a String value, not a String object. The following steps
are taken:
Return the String value containing span consecutive
code units from S beginning with the code unit at
index from.
Note
The slice function is intentionally generic; it
does not require that its this value be a
String object. Therefore it can be transferred to other kinds
of objects for use as a method.
Returns an Array object into which substrings of the result of
converting this object to a String have been stored. The
substrings are determined by searching from left to right for
occurrences of separator; these occurrences are not
part of any substring in the returned array, but serve to divide
up the String value. The value of separator may be a
String of any length or it may be an object, such as a RegExp,
that has a @@split method.
When the split method is called, the following steps
are taken:
The value of separator may be an empty String. In
this case, separator does not match the empty
substring at the beginning or end of the input String, nor
does it match the empty substring at the end of the previous
separator match. If separator is the empty String,
the String is split up into individual code unit elements; the
length of the result array equals the length of the String,
and each substring contains one code unit.
If the this object is (or converts to) the
empty String, the result depends on whether
separator can match the empty String. If it can,
the result array contains no elements. Otherwise, the result
array contains one element, which is the empty String.
If separator is undefined, then
the result array contains just one String, which is the
this value (converted to a String). If
limit is not undefined, then the
output array is truncated so that it contains no more than
limit elements.
Note 2
The split function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
21.1.3.20.1Runtime Semantics:
SplitMatch ( S, q, R )
The abstract operation SplitMatch takes three parameters, a
String S, an
integerq, and a String R, and performs the
following steps in order to return either
false or the end index of a match:
If there exists an
integeri between 0 (inclusive) and
r (exclusive) such that the code unit at index
q + i within S is different
from the code unit at index i within
R, return false.
Return q + r.
21.1.3.21String.prototype.startsWith (
searchString [ , position ] )
If searchLength + start is greater than
len, return false.
If the sequence of code units of S starting at
start of length searchLength is the same
as the full code unit sequence of searchStr, return
true.
Otherwise, return false.
Note 1
This method returns true if the sequence of
code units of searchString converted to a String is
the same as the corresponding code units of this object
(converted to a String) starting at index position.
Otherwise returns false.
Note 2
Throwing an exception if the first argument is a RegExp is
specified in order to allow future editions to define
extensions that allow such argument values.
Note 3
The startsWith function is intentionally generic;
it does not require that its this value be
a String object. Therefore, it can be transferred to other
kinds of objects for use as a method.
21.1.3.22String.prototype.substring (
start, end )
The substring method takes two arguments,
start and end, and returns a substring of
the result of converting this object to a String, starting from
index start and running to, but not including, index
end of the String (or through the end of the String if
end is undefined). The result is a
String value, not a String object.
If either argument is NaN or negative, it is
replaced with zero; if either argument is larger than the length
of the String, it is replaced with the length of the String.
Return the String value whose length is to -
from, containing code units from S,
namely the code units with indices from through
to - 1, in ascending order.
Note
The substring function is intentionally generic;
it does not require that its this value be
a String object. Therefore, it can be transferred to other
kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
toLocaleLowerCase method as specified in the ECMA-402
specification. If an ECMAScript implementation does not include
the ECMA-402 API the following specification of the
toLocaleLowerCase method is used.
This function interprets a String value as a sequence of UTF-16
encoded code points, as described in
6.1.4.
This function works exactly the same as
toLowerCase except that its result is intended to
yield the correct result for the host environment's current
locale, rather than a locale-independent result. There will only
be a difference in the few cases (such as Turkish) where the rules
for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined
in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not use those parameter positions for
anything else.
Note
The toLocaleLowerCase function is intentionally
generic; it does not require that its
this value be a String object. Therefore,
it can be transferred to other kinds of objects for use as a
method.
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
toLocaleUpperCase method as specified in the ECMA-402
specification. If an ECMAScript implementation does not include
the ECMA-402 API the following specification of the
toLocaleUpperCase method is used.
This function interprets a String value as a sequence of UTF-16
encoded code points, as described in
6.1.4.
This function works exactly the same as
toUpperCase except that its result is intended to
yield the correct result for the host environment's current
locale, rather than a locale-independent result. There will only
be a difference in the few cases (such as Turkish) where the rules
for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined
in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not use those parameter positions for
anything else.
Note
The toLocaleUpperCase function is intentionally
generic; it does not require that its
this value be a String object. Therefore,
it can be transferred to other kinds of objects for use as a
method.
21.1.3.25String.prototype.toLowerCase
( )
This function interprets a String value as a sequence of UTF-16
encoded code points, as described in
6.1.4. The following steps are taken:
Let cpList be a
List
containing in order the code points as defined in
6.1.4
of S, starting at the first element of
S.
Let cuList be a
List
where the elements are the result of
toLowercase(cpList), according to the Unicode
Default Case Conversion algorithm.
Let L be the String value whose code units are the
UTF16Encoding
of the code points of cuList.
Return L.
The result must be derived according to the locale-insensitive
case mappings in the Unicode Character Database (this explicitly
includes not only the UnicodeData.txt file, but also all
locale-insensitive mappings in the SpecialCasings.txt file that
accompanies it).
Note 1
The case mapping of some code points may produce multiple code
points. In this case the result String may not be the same
length as the source String. Because both
toUpperCase and toLowerCase have
context-sensitive behaviour, the functions are not
symmetrical. In other words,
s.toUpperCase().toLowerCase() is not necessarily
equal to s.toLowerCase().
Note 2
The toLowerCase function is intentionally
generic; it does not require that its
this value be a String object. Therefore,
it can be transferred to other kinds of objects for use as a
method.
21.1.3.26String.prototype.toString ( )
When the toString method is called, the following
steps are taken:
For a String object, the toString method happens
to return the same thing as the valueOf method.
21.1.3.27String.prototype.toUpperCase
( )
This function interprets a String value as a sequence of UTF-16
encoded code points, as described in
6.1.4.
This function behaves in exactly the same way as
String.prototype.toLowerCase, except that the String
is mapped using the toUppercase algorithm of the Unicode Default
Case Conversion.
Note
The toUpperCase function is intentionally
generic; it does not require that its
this value be a String object. Therefore,
it can be transferred to other kinds of objects for use as a
method.
21.1.3.28String.prototype.trim ( )
This function interprets a String value as a sequence of UTF-16
encoded code points, as described in
6.1.4.
The trim function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
21.1.3.28.1Runtime Semantics:
TrimString ( string, where )
The abstract operation TrimString is called with arguments
string and where, and interprets the
String value string as a sequence of UTF-16 encoded
code points, as described in
6.1.4. It performs the following steps:
Let T be the String value that is a copy of
S with both leading and trailing white space
removed.
Return T.
The definition of white space is the union of
WhiteSpace
and
LineTerminator. When determining whether a Unicode code point is in Unicode
general category “Space_Separator” (“Zs”), code unit sequences
are interpreted as UTF-16 encoded code point sequences as
specified in
6.1.4.
21.1.3.29String.prototype.trimEnd ( )
This function interprets a String value as a sequence of UTF-16
encoded code points, as described in
6.1.4.
The trimEnd function is intentionally generic; it
does not require that its this value be a
String object. Therefore, it can be transferred to other kinds
of objects for use as a method.
21.1.3.30String.prototype.trimStart (
)
This function interprets a String value as a sequence of UTF-16
encoded code points, as described in
6.1.4.
The trimStart function is intentionally generic;
it does not require that its this value be
a String object. Therefore, it can be transferred to other
kinds of objects for use as a method.
21.1.3.31String.prototype.valueOf ( )
When the valueOf method is called, the following
steps are taken:
When the @@iterator method is called it returns an
Iterator object (25.1.1.2) that iterates over the code points of a String value, returning
each code point as a String value. The following steps are taken:
The value of the "name" property of this
function is "[Symbol.iterator]".
21.1.4Properties of String Instances
String instances are String exotic objects and have the internal
methods specified for such objects. String instances inherit
properties from the String prototype object. String instances also
have a [[StringData]] internal slot.
String instances have a "length" property, and a
set of enumerable properties with
integer-indexed names.
21.1.4.1length
The number of elements in the String value represented by this
String object.
Once a String object is initialized, this property is unchanging.
It has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]:
false }.
21.1.5String Iterator Objects
A String Iterator is an object, that represents a specific iteration
over some specific String instance object. There is not a named
constructor
for String Iterator objects. Instead, String iterator objects are
created by calling certain methods of String instance objects.
21.1.5.1CreateStringIterator (
string )
Several methods of String objects return Iterator objects. The
abstract operation CreateStringIterator with argument
string is used to create such iterator objects. It
performs the following steps:
The initial value of the @@toStringTag property is the String
value "String Iterator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
21.1.5.3Properties of String Iterator
Instances
String Iterator instances are ordinary objects that inherit
properties from the
%StringIteratorPrototype%
intrinsic object. String Iterator instances are initially created
with the internal slots listed in
Table 55.
Table 55: Internal Slots of String Iterator Instances
Internal Slot
Description
[[IteratedString]]
The String value whose code units are being iterated.
[[StringNextIndex]]
The
integer index
of the next string element (code unit) to be examined by
this iterator.
21.2RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated
flags.
Note
The form and functionality of regular expressions is modelled
after the regular expression facility in the Perl 5 programming
language.
21.2.1Patterns
The RegExpconstructor
applies the following grammar to the input pattern String. An error
occurs if the grammar cannot interpret the String as an expansion of
Pattern.
It is a Syntax Error if IsCharacterClass of the first
ClassAtom
is true or IsCharacterClass of the second
ClassAtom
is true.
It is a Syntax Error if IsCharacterClass of the first
ClassAtom
is false and IsCharacterClass of the second
ClassAtom
is false and the CharacterValue of the first
ClassAtom
is larger than the CharacterValue of the second
ClassAtom.
It is a Syntax Error if IsCharacterClass of
ClassAtomNoDash
is true or IsCharacterClass of
ClassAtom
is true.
It is a Syntax Error if IsCharacterClass of
ClassAtomNoDash
is false and IsCharacterClass of
ClassAtom
is false and the CharacterValue of
ClassAtomNoDash
is larger than the CharacterValue of
ClassAtom.
It is a Syntax Error if the
List
of Unicode code points that is SourceText of
UnicodePropertyName
is not identical to a
List
of Unicode code points that is a Unicode
property name
or property alias listed in the “Property name
and aliases” column of
Table 57.
It is a Syntax Error if the
List
of Unicode code points that is SourceText of
UnicodePropertyValue
is not identical to a
List
of Unicode code points that is a value or value alias for the
Unicode property or property alias given by SourceText of
UnicodePropertyName
listed in the “Property value and aliases” column of the
corresponding tables
Table 59
or
Table 60.
It is a Syntax Error if the
List
of Unicode code points that is SourceText of
LoneUnicodePropertyNameOrValue
is not identical to a
List
of Unicode code points that is a Unicode general category or
general category alias listed in the “Property value and
aliases” column of
Table 59, nor a binary property or binary property alias listed in the
“Property name
and aliases” column of
Table 58.
A regular expression pattern is converted into an internal procedure
using the process described below. An implementation is encouraged
to use more efficient algorithms than the ones listed below, as long
as the results are the same. The internal procedure is used as the
value of a RegExp object's [[RegExpMatcher]] internal slot.
A
Pattern
is either a BMP pattern or a Unicode pattern depending upon whether
or not its associated flags contain a u. A BMP pattern
matches against a String interpreted as consisting of a sequence of
16-bit values that are Unicode code points in the range of the Basic
Multilingual Plane. A Unicode pattern matches against a String
interpreted as consisting of Unicode code points encoded using
UTF-16. In the context of describing the behaviour of a BMP pattern
“character” means a single 16-bit Unicode BMP code point. In the
context of describing the behaviour of a Unicode pattern “character”
means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of
the corresponding non-encoded code point.
The syntax and semantics of
Pattern
is defined as if the source code for the
Pattern
was a
List
of
SourceCharacter
values where each
SourceCharacter
corresponds to a Unicode code point. If a BMP pattern contains a
non-BMP
SourceCharacter
the entire pattern is encoded using UTF-16 and the individual code
units of that encoding are used as the elements of the
List.
Note
For example, consider a pattern expressed in source text as the
single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF).
Interpreted as a Unicode pattern, it would be a single element
(character)
List
consisting of the single code point 0x1D11E. However,
interpreted as a BMP pattern, it is first UTF-16 encoded to
produce a two element
List
consisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp
constructor
as ECMAScript String values in which non-BMP characters are
UTF-16 encoded. For example, the single character MUSICAL SYMBOL
G CLEF pattern, expressed as a String value, is a String of
length 2 whose elements were the code units 0xD834 and 0xDD1E.
So no further translation of the string would be necessary to
process it as a BMP pattern consisting of two pattern
characters. However, to process it as a Unicode pattern
UTF16Decode
must be used in producing a
List
consisting of a single pattern character, the code point
U+1D11E.
An implementation may not actually perform such translations to
or from UTF-16, but the semantics of this specification requires
that the result of pattern matching be as if such translations
were performed.
21.2.2.1Notation
The descriptions below use the following variables:
Input is a
List
consisting of all of the characters, in order, of the String
being matched by the regular expression pattern. Each character
is either a code unit or a code point, depending upon the kind
of pattern involved. The notation Input[n]
means the nth character of
Input, where n can range between 0
(inclusive) and InputLength (exclusive).
InputLength is the number of characters in
Input.
NcapturingParens is the total number of
left-capturing parentheses (i.e. the total number of
Atom::(GroupSpecifierDisjunction)
Parse Nodes) in the pattern. A left-capturing parenthesis is any
( pattern character that is matched by the
( terminal of the
Atom::(GroupSpecifierDisjunction)
production.
DotAll is true if the RegExp
object's [[OriginalFlags]] internal slot contains
"s" and otherwise is
false.
IgnoreCase is true if the RegExp
object's [[OriginalFlags]] internal slot contains
"i" and otherwise is
false.
Multiline is true if the RegExp
object's [[OriginalFlags]] internal slot contains
"m" and otherwise is
false.
Unicode is true if the RegExp
object's [[OriginalFlags]] internal slot contains
"u" and otherwise is
false.
Furthermore, the descriptions below use the following internal
data structures:
A CharSet is a mathematical set of characters, either
code units or code points depending up the state of the
Unicode flag. “All characters” means either all code
unit values or all code point values also depending upon the
state of Unicode.
A State is an ordered pair (endIndex,
captures) where endIndex is an
integer
and captures is a
List
of NcapturingParens values. States are used to
represent partial match states in the regular expression
matching algorithms. The endIndex is one plus the
index of the last input character matched so far by the pattern,
while captures holds the results of capturing
parentheses. The nth element of
captures is either a
List
that represents the value obtained by the nth set of capturing parentheses or
undefined if the nth set of capturing parentheses hasn't been reached
yet. Due to backtracking, many States may be in use at any time
during the matching process.
A MatchResult is either a State or the special token
failure that indicates that the match
failed.
A Continuation procedure is an internal closure (i.e.
an internal procedure with some arguments already bound to
values) that takes one State argument and returns a MatchResult
result. If an internal closure references variables which are
bound in the function that creates the closure, the closure uses
the values that these variables had at the time the closure was
created. The Continuation attempts to match the remaining
portion (specified by the closure's already-bound arguments) of
the pattern against Input, starting at the
intermediate state given by its State argument. If the match
succeeds, the Continuation returns the final State that it
reached; if the match fails, the Continuation returns
failure.
A Matcher procedure is an internal closure that takes
two arguments—a State and a Continuation—and returns a
MatchResult result. A Matcher attempts to match a middle
subpattern (specified by the closure's already-bound arguments)
of the pattern against Input, starting at the
intermediate state given by its State argument. The Continuation
argument should be a closure that matches the rest of the
pattern. After matching the subpattern of a pattern to obtain a
new State, the Matcher then calls Continuation on that new State
to test if the rest of the pattern can match as well. If it can,
the Matcher returns the State returned by Continuation; if not,
the Matcher may try different choices at its choice points,
repeatedly calling Continuation until it either succeeds or all
possibilities have been exhausted.
An AssertionTester procedure is an internal closure
that takes a State argument and returns a Boolean result. The
assertion tester tests a specific condition (specified by the
closure's already-bound arguments) against the current place in
Input and returns true if the
condition matched or false if not.
If Unicode is true, let
Input be a
List
consisting of the sequence of code points of
str interpreted as a UTF-16 encoded (6.1.4) Unicode string. Otherwise, let Input be a
List
consisting of the sequence of code units that are the
elements of str. Input will be used
throughout the algorithms in
21.2.2. Each element of Input is considered to be a
character.
Let InputLength be the number of characters
contained in Input. This variable will be used
throughout the algorithms in
21.2.2.
Let listIndex be the index into
Input of the character that was obtained from
element index of str.
Let c be a Continuation that always returns its
State argument as a successful MatchResult.
Let cap be a
List
of NcapturingParensundefined values, indexed 1 through
NcapturingParens.
Let x be the State (listIndex,
cap).
Call m(x, c) and return
its result.
Note
A Pattern evaluates (“compiles”) to an internal procedure
value.
RegExpBuiltinExec
can then apply this procedure to a String and an offset within
the String to determine whether the pattern would match
starting at exactly that offset within the String, and, if it
does match, what the values of the capturing parentheses would
be. The algorithms in
21.2.2
are designed so that compiling a pattern may throw a
SyntaxError exception; on the other hand,
once the pattern is successfully compiled, applying the
resulting internal procedure to find a match in a String
cannot throw an exception (except for any host-defined
exceptions that can occur anywhere such as out-of-memory).
Evaluate
Alternative
with argument direction to obtain a Matcher
m1.
Evaluate
Disjunction
with argument direction to obtain a Matcher
m2.
Return an internal Matcher closure that takes two arguments, a
State x and a Continuation c, and
performs the following steps:
Call m1(x, c) and let
r be its result.
If r is not failure,
return r.
Call m2(x, c) and return
its result.
Note
The | regular expression operator separates two
alternatives. The pattern first tries to match the left
Alternative
(followed by the sequel of the regular expression); if it
fails, it tries to match the right
Disjunction
(followed by the sequel of the regular expression). If the
left
Alternative, the right
Disjunction, and the sequel all have choice points, all choices in the
sequel are tried before moving on to the next choice in the
left
Alternative. If choices in the left
Alternative
are exhausted, the right
Disjunction
is tried instead of the left
Alternative. Any capturing parentheses inside a portion of the pattern
skipped by | produce
undefined values instead of Strings. Thus,
for example,
Return an internal Matcher closure that takes two
arguments, a State x and a Continuation
c, and performs the following steps:
Let d be a Continuation that takes a State
argument y and returns the result of
calling m1(y, c).
Call m2(x, d) and
return its result.
Note
Consecutive
Terms try to simultaneously match consecutive portions of
Input. When direction is equal to +1, if
the left
Alternative, the right
Term, and the sequel of the regular expression all have choice
points, all choices in the sequel are tried before moving on
to the next choice in the right
Term, and all choices in the right
Term
are tried before moving on to the next choice in the left
Alternative. When direction is equal to -1, the evaluation
order of
Alternative
and
Term
are reversed.
21.2.2.5Term
With parameter direction.
The production
Term::Assertion
evaluates as follows:
Return an internal Matcher closure that takes two arguments, a
State x and a Continuation c, and
performs the following steps:
Evaluate
Assertion
to obtain an AssertionTester t.
Call t(x) and let r be
the resulting Boolean value.
Evaluate
Atom
with argument direction to obtain a Matcher
m.
Evaluate
Quantifier
to obtain the three results: an
integermin, an
integer
(or ∞) max, and Boolean greedy.
Assert: If max is finite, then max is not
less than min.
Let parenIndex be the number of left-capturing
parentheses in the entire regular expression that occur to the
left of this
Term. This is the total number of
Atom::(GroupSpecifierDisjunction)
Parse Nodes prior to or enclosing this
Term.
Let parenCount be the number of left-capturing
parentheses in
Atom. This is the total number of
Atom::(GroupSpecifierDisjunction)
Parse Nodes enclosed by
Atom.
Return an internal Matcher closure that takes two arguments, a
State x and a Continuation c, and
performs the following steps:
Call
RepeatMatcher(m, min, max,
greedy, x, c,
parenIndex, parenCount) and return
its result.
The abstract operation RepeatMatcher takes eight parameters, a
Matcher m, an
integermin, an
integer
(or ∞) max, a Boolean greedy, a State
x, a Continuation c, an
integerparenIndex, and an
integerparenCount, and performs the following steps:
If max is zero, return
c(x).
Let d be an internal Continuation closure that
takes one State argument y and performs the
following steps:
If min is zero and y's
endIndex is equal to x's
endIndex, return
failure.
If min is zero, let min2 be zero;
otherwise let min2 be min - 1.
If max is ∞, let max2 be ∞;
otherwise let max2 be max - 1.
Call
RepeatMatcher(m, min2, max2,
greedy, y, c,
parenIndex, parenCount) and return
its result.
For each
integerk that satisfies parenIndex <
k and k ≤ parenIndex +
parenCount, set cap[k] to
undefined.
Let e be x's endIndex.
Let xr be the State (e,
cap).
If min is not zero, return
m(xr, d).
If greedy is false, then
Call c(x) and let z be
its result.
If z is not failure,
return z.
Call m(xr, d) and
return its result.
Call m(xr, d) and let
z be its result.
If z is not failure,
return z.
Call c(x) and return its result.
Note 1
An
Atom
followed by a
Quantifier
is repeated the number of times specified by the
Quantifier. A
Quantifier
can be non-greedy, in which case the
Atom
pattern is repeated as few times as possible while still
matching the sequel, or it can be greedy, in which case the
Atom
pattern is repeated as many times as possible while still
matching the sequel. The
Atom
pattern is repeated rather than the input character sequence
that it matches, so different repetitions of the
Atom
can match different input substrings.
Note 2
If the
Atom
and the sequel of the regular expression all have choice
points, the
Atom
is first matched as many (or as few, if non-greedy) times as
possible. All choices in the sequel are tried before moving
on to the next choice in the last repetition of
Atom. All choices in the last (nth) repetition of
Atom
are tried before moving on to the next choice in the
next-to-last (n - 1)st repetition of
Atom; at which point it may turn out that more or fewer
repetitions of
Atom
are now possible; these are exhausted (again, starting with
either as few or as many as possible) before moving on to
the next choice in the (n - 1)st repetition of
Atom
and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a
regular expression that calculates the greatest common
divisor of two numbers (represented in unary notation). The
following example calculates the gcd of 10 and 15:
Step 4 of the RepeatMatcher clears
Atom's captures each time
Atom
is repeated. We can see its behaviour in the regular
expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost
* clears all captured Strings contained in the
quantified
Atom, which in this case includes capture Strings numbered 2,
3, 4, and 5.
Note 4
Step 1 of the RepeatMatcher's d closure states
that, once the minimum number of repetitions has been
satisfied, any more expansions of
Atom
that match the empty character sequence are not considered
for further repetitions. This prevents the regular
expression engine from falling into an infinite loop on
patterns such as:
Evaluate
Disjunction
with argument direction to obtain a Matcher
m.
Let parenIndex be the number of left-capturing
parentheses in the entire regular expression that occur to the
left of this
Atom. This is the total number of
Atom::(GroupSpecifierDisjunction)
Parse Nodes prior to or enclosing this
Atom.
Return an internal Matcher closure that takes two arguments, a
State x and a Continuation c, and
performs the following steps:
Let d be an internal Continuation closure that
takes one State argument y and performs the
following steps:
Return the Matcher that is the result of evaluating
Disjunction
with argument direction.
21.2.2.8.1Runtime Semantics:
CharacterSetMatcher ( A, invert,
direction )
The abstract operation CharacterSetMatcher takes three
arguments, a CharSet A, a Boolean flag
invert, and an
integerdirection, and performs the following steps:
Return an internal Matcher closure that takes two arguments,
a State x and a Continuation c, and
performs the following steps:
The abstract operation Canonicalize takes a character parameter
ch and performs the following steps:
If IgnoreCase is false, return
ch.
If Unicode is true, then
If the file CaseFolding.txt of the Unicode Character
Database provides a simple or common case folding
mapping for ch, return the result of applying
that mapping to ch.
If u does not consist of a single code unit,
return ch.
Let cu be u's single code unit
element.
If the numeric value of ch ≥ 128 and the
numeric value of cu < 128, return
ch.
Return cu.
Note 1
Parentheses of the form (Disjunction) serve both to group the components of the
Disjunction
pattern together and to save the result of the match. The
result can be used either in a backreference (\
followed by a nonzero decimal number), referenced in a
replace String, or returned as part of an array from the
regular expression matching internal procedure. To inhibit
the capturing behaviour of parentheses, use the form
(?:Disjunction) instead.
Note 2
The form (?=Disjunction) specifies a zero-width positive lookahead. In
order for it to succeed, the pattern inside
Disjunction
must match at the current position, but the current position
is not advanced before matching the sequel. If
Disjunction
can match at the current position in several ways, only the
first one is tried. Unlike other regular expression
operators, there is no backtracking into a
(?= form (this unusual behaviour is inherited
from Perl). This only matters when the
Disjunction
contains capturing parentheses and the sequel of the pattern
contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first
b and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead,
consider:
/(?=(a+))a*b\1/.exec("baaabac")
This expression returns
["aba", "a"]
and not:
["aaaba", "a"]
Note 3
The form (?!Disjunction) specifies a zero-width negative lookahead. In
order for it to succeed, the pattern inside
Disjunction
must fail to match at the current position. The current
position is not advanced before matching the sequel.
Disjunction
can contain capturing parentheses, but backreferences to
them only make sense from within
Disjunction
itself. Backreferences to these capturing parentheses from
elsewhere in the pattern always return
undefined because the negative lookahead
must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some
positive number n of a's, a b,
another n a's (specified by the first
\2) and a c. The second
\2 is outside the negative lookahead, so it
matches against undefined and therefore
always succeeds. The whole expression returns the array:
["baaabaac", "ba", undefined, "abaac"]
Note 4
In case-insignificant matches when Unicode is
true, all characters are implicitly
case-folded using the simple mapping provided by the Unicode
standard immediately before they are compared. The simple
mapping always maps to a single code point, so it does not
map, for example, ß (U+00DF) to
SS. It may however map a code point outside the
Basic Latin range to a character within, for example,
ſ (U+017F) to s. Such characters
are not mapped if Unicode is
false. This prevents Unicode code points
such as U+017F and U+212A from matching regular expressions
such as /[a-z]/i, but they will match
/[a-z]/ui.
21.2.2.8.3Runtime Semantics:
UnicodeMatchProperty ( p )
The abstract operation UnicodeMatchProperty takes a parameter
p that is a
List
of Unicode code points and performs the following steps:
Implementations must support the Unicode property names and
aliases listed in
Table 57
and
Table 58. To ensure interoperability, implementations must not support
any other property names or aliases.
Note 1
For example, Script_Extensions (property name) and scx (property alias) are valid, but
script_extensions or Scx aren't.
Note 2
The listed properties form a superset of what
UTS18 RL1.2
requires.
Table 57: Non-binary Unicode property aliases and their
canonical property names
21.2.2.8.4Runtime Semantics:
UnicodeMatchPropertyValue ( p, v )
The abstract operation UnicodeMatchPropertyValue takes two
parameters p and v, each of which is a
List
of Unicode code points, and performs the following steps:
Assert: p is a
List
of Unicode code points that is identical to a
List
of Unicode code points that is a canonical, unaliased
Unicode
property name
listed in the “Canonical
property name” column of
Table 57.
Assert: v is a
List
of Unicode code points that is identical to a
List
of Unicode code points that is a property value or property
value alias for Unicode property p listed in the
“Property value and aliases” column of
Table 59
or
Table 60.
Let value be the canonical property value of
v as given in the “Canonical property value”
column of the corresponding row.
Implementations must support the Unicode property value names
and aliases listed in
Table 59
and
Table 60. To ensure interoperability, implementations must not support
any other property value names or aliases.
Note 1
For example, Xpeo and
Old_Persian are valid
Script_Extensions values, but
xpeo and Old Persian aren't.
Call
CharacterSetMatcher(A, false,
direction) and return its Matcher result.
Note
An escape sequence of the form \ followed by a
nonzero decimal number n matches the result of the
nth set of capturing parentheses
(21.2.2.1). It is an error if the regular expression has fewer than
n capturing parentheses. If the regular expression
has n or more capturing parentheses but the
nth one is
undefined because it has not captured
anything, then the backreference always succeeds.
Let parenIndex be the number of left-capturing
parentheses in the entire regular expression that occur to the
left of the located
GroupSpecifier. This is the total number of
Atom::(GroupSpecifierDisjunction)
Parse Nodes prior to or enclosing the located
GroupSpecifier.
If there exists an
integeri between 0 (inclusive) and
len (exclusive) such that
Canonicalize(s[i]) is not the same character
value as
Canonicalize(Input[g + i]), return
failure.
Return the CapturingGroupNumber of this
DecimalEscape.
Note
If \ is followed by a decimal number
n whose first digit is not 0, then the
escape sequence is considered to be a backreference. It is an
error if n is greater than the total number of
left-capturing parentheses in the entire regular expression.
If ! UnicodeMatchPropertyValue(General_Category, s) is identical to
a
List
of Unicode code points that is the name of a Unicode general
category or general category alias listed in the “Property
value and aliases” column of
Table 59, then
Return the CharSet containing all Unicode code points
whose character database definition includes the property
“General_Category” with value s.
Call
CharacterRange(A, B) and let D be the
resulting CharSet.
Return the union of CharSets D and C.
Note 1
ClassRanges
can expand into a single
ClassAtom
and/or ranges of two
ClassAtom
separated by dashes. In the latter case the
ClassRanges
includes all characters between the first
ClassAtom
and the second
ClassAtom, inclusive; an error occurs if either
ClassAtom
does not represent a single character (for example, if one is
\w) or if the first
ClassAtom's character value is greater than the second
ClassAtom's character value.
Note 2
Even if the pattern ignores case, the case of the two ends of
a range is significant in determining which characters belong
to the range. Thus, for example, the pattern
/[E-F]/i matches only the letters E,
F, e, and f, while the
pattern /[E-f]/i matches all upper and lower-case
letters in the Unicode Basic Latin block as well as the
symbols [, \, ],
^, _, and `.
Note 3
A - character can be treated literally or it can
denote a range. It is treated literally if it is the first or
last character of
ClassRanges, the beginning or end limit of a range specification, or
immediately follows a range specification.
A
ClassAtom
can use any of the escape sequences that are allowed in the
rest of the regular expression except for \b,
\B, and backreferences. Inside a
CharacterClass, \b means the backspace character, while
\B and backreferences raise errors. Using a
backreference inside a
ClassAtom
causes an error.
is the initial value of the "RegExp" property
of the
global object.
creates and initializes a new RegExp object when called as a
function rather than as a
constructor. Thus the function call RegExp(…) is equivalent to
the object creation expression new RegExp(…) with the
same arguments.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
RegExp behaviour must include a
super call to the RegExpconstructor
to create and initialize subclass instances with the necessary
internal slots.
If pattern is supplied using a
StringLiteral, the usual escape sequence substitutions are performed
before the String is processed by RegExp. If pattern must
contain an escape sequence to be recognized by RegExp, any
U+005C (REVERSE SOLIDUS) code points must be escaped within
the
StringLiteral
to prevent them being removed when the contents of the
StringLiteral
are formed.
21.2.3.2Abstract Operations for the
RegExp Constructor
If F contains any code unit other than
"g", "i",
"m", "s",
"u", or "y" or if it
contains the same code unit more than once, throw a
SyntaxError exception.
If F contains "u", let
BMP be false; else let
BMP be true.
If BMP is true, then
Parse P using the grammars in
21.2.1
and interpreting each of its 16-bit elements as a
Unicode BMP code point. UTF-16 decoding is not applied
to the elements. The
goal symbol
for the parse is
Pattern[~U, ~N]. If the result of parsing contains a
GroupName, reparse with the
goal symbolPattern[~U, +N]
and use this result instead. Throw a
SyntaxError exception if
P did not conform to the grammar, if any
elements of P were not matched by the parse,
or if any Early Error conditions exist.
Let patternCharacters be a
List
whose elements are the code unit elements of
P.
Else,
Parse P using the grammars in
21.2.1
and interpreting P as UTF-16 encoded Unicode
code points (6.1.4). The
goal symbol
for the parse is
Pattern[+U, +N]. Throw a SyntaxError exception if
P did not conform to the grammar, if any
elements of P were not matched by the parse,
or if any Early Error conditions exist.
Let patternCharacters be a
List
whose elements are the code points resulting from
applying UTF-16 decoding to P's sequence of
elements.
Set obj.[[OriginalSource]] to P.
Set obj.[[OriginalFlags]] to F.
Set obj.[[RegExpMatcher]] to the internal
procedure that evaluates the above parse of P by
applying the semantics provided in
21.2.2
using patternCharacters as the pattern's
List
of
SourceCharacter
values and F as the flag parameters.
21.2.3.2.4Runtime Semantics:
EscapeRegExpPattern ( P, F )
When the abstract operation EscapeRegExpPattern with arguments
P and F is called, the following occurs:
Let S be a String in the form of a
Pattern[~U]
(Pattern[+U]
if F contains "u") equivalent
to P interpreted as UTF-16 encoded Unicode code
points (6.1.4), in which certain code points are escaped as described
below. S may or may not be identical to
P; however, the internal procedure that would
result from evaluating S as a
Pattern[~U]
(Pattern[+U]
if F contains "u") must behave
identically to the internal procedure given by the
constructed object's [[RegExpMatcher]] internal slot.
Multiple calls to this abstract operation using the same
values for P and F must produce
identical results.
The code points / or any
LineTerminator
occurring in the pattern shall be escaped in S as
necessary to ensure that the
string-concatenation
of "/", S,
"/", and F can be parsed (in
an appropriate lexical context) as a
RegularExpressionLiteral
that behaves identically to the constructed regular
expression. For example, if P is
"/", then S could be
"\/" or "\u002F",
among other possibilities, but not "/",
because /// followed by F would be
parsed as a
SingleLineComment
rather than a
RegularExpressionLiteral. If P is the empty String, this specification
can be met by letting S be
"(?:)".
The initial value of RegExp.prototype is
%RegExp.prototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
21.2.4.2get RegExp [ @@species ]
RegExp[@@species] is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Return the this value.
The value of the "name" property of this
function is "get [Symbol.species]".
Note
RegExp prototype methods normally use their
this object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its
@@species property.
21.2.5Properties of the RegExp Prototype
Object
The RegExp prototype object:
is the intrinsic object %RegExpPrototype%.
is an ordinary object.
is not a RegExp instance and does not have a [[RegExpMatcher]]
internal slot or any of the other internal slots of RegExp
instance objects.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
Note
The RegExp prototype object does not have a
"valueOf" property of its own; however, it
inherits the "valueOf" property from the
Object prototype object.
21.2.5.1RegExp.prototype.constructor
The initial value of RegExp.prototype.constructor is
%RegExp%.
21.2.5.2RegExp.prototype.exec (
string )
Performs a regular expression match of string against
the regular expression and returns an Array object containing the
results of the match, or null if
string did not match.
The String
ToString(string) is searched for an occurrence of the regular
expression pattern as follows:
If a callable "exec" property is not
found this algorithm falls back to attempting to use the
built-in RegExp matching algorithm. This provides compatible
behaviour for code written for prior editions where most
built-in algorithms that use regular expressions did not
perform a dynamic property lookup of
"exec".
21.2.5.2.2Runtime Semantics:
RegExpBuiltinExec ( R, S )
The abstract operation RegExpBuiltinExec with arguments
R and S performs the following steps:
e is an index into the
Input character list, derived from
S, matched by matcher. Let
eUTF be the smallest index into
S that corresponds to the character at
element e of Input. If
e is greater than or equal to the number of
elements in Input, then eUTF is
the number of code units in S.
The value of the "name" property of this
function is "[Symbol.match]".
Note
The @@match property is used by the
IsRegExp
abstract operation to identify objects that have the basic
behaviour of regular expressions. The absence of a @@match
property or the existence of such a property whose value does
not Boolean coerce to true indicates that
the object is not intended to be used as a regular expression
object.
Let replacement be ? GetSubstitution(matched, S,
position, captures,
namedCaptures, replaceValue).
If position ≥ nextSourcePosition,
then
NOTE: position should not normally move
backwards. If it does, it is an indication of an
ill-behaving RegExp subclass or use of an access
triggered side-effect to change the global flag or
other characteristics of rx. In such cases,
the corresponding substitution is ignored.
Set accumulatedResult to the
string-concatenation
of the current value of accumulatedResult,
the substring of S consisting of the code
units from nextSourcePosition (inclusive)
up to position (exclusive), and
replacement.
Set nextSourcePosition to
position + matchLength.
If nextSourcePosition ≥ lengthS, return
accumulatedResult.
Return the
string-concatenation
of accumulatedResult and the substring of
S consisting of the code units from
nextSourcePosition (inclusive) up through the final
code unit of S (inclusive).
The value of the "name" property of this
function is "[Symbol.replace]".
21.2.5.11RegExp.prototype [ @@search ]
( string )
When the @@search method is called with argument
string, the following steps are taken:
Let rx be the this value.
If
Type(rx) is not Object, throw a
TypeError exception.
Returns an Array object into which substrings of the result of
converting string to a String have been stored. The
substrings are determined by searching from left to right for
matches of the this value regular
expression; these occurrences are not part of any substring in
the returned array, but serve to divide up the String value.
The this value may be an empty regular
expression or a regular expression that can match an empty
String. In this case, the regular expression does not match
the empty substring at the beginning or end of the input
String, nor does it match the empty substring at the end of
the previous separator match. (For example, if the regular
expression matches the empty String, the String is split up
into individual code unit elements; the length of the result
array equals the length of the String, and each substring
contains one code unit.) Only the first match at a given index
of the String is considered, even if backtracking could yield
a non-empty-substring match at that index. (For example,
/a*?/[Symbol.split]("ab") evaluates to the array
["a", "b"], while
/a*/[Symbol.split]("ab") evaluates to the array
["","b"].)
If the string is (or converts to) the empty String,
the result depends on whether the regular expression can match
the empty String. If it can, the result array contains no
elements. Otherwise, the result array contains one element,
which is the empty String.
If the regular expression contains capturing parentheses, then
each time separator is matched the results
(including any undefined results) of the
capturing parentheses are spliced into the output array. For
example,
The returned String has the form of a
RegularExpressionLiteral
that evaluates to another RegExp object with the same
behaviour as this object.
21.2.5.17get RegExp.prototype.unicode
RegExp.prototype.unicode is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Let R be the this value.
If
Type(R) is not Object, throw a
TypeError exception.
If R does not have an [[OriginalFlags]] internal
slot, then
If
SameValue(R, %RegExp.prototype%) is
true, return
undefined.
Otherwise, throw a TypeError exception.
Let flags be R.[[OriginalFlags]].
If flags contains the code unit 0x0075 (LATIN SMALL
LETTER U), return true.
Return false.
21.2.6Properties of RegExp Instances
RegExp instances are ordinary objects that inherit properties from
the RegExp prototype object. RegExp instances have internal slots
[[RegExpMatcher]], [[OriginalSource]], and [[OriginalFlags]]. The
value of the [[RegExpMatcher]] internal slot is an
implementation-dependent representation of the
Pattern
of the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were
specified as having the own data properties
"source", "global",
"ignoreCase", and
"multiline". Those properties are now
specified as accessor properties of
RegExp.prototype.
RegExp instances also have the following property:
21.2.6.1lastIndex
The value of the "lastIndex" property specifies
the String index at which to start the next match. It is coerced
to an
integer
when used (see
21.2.5.2.2). This property shall have the attributes { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: false }.
21.2.7RegExp String Iterator Objects
A RegExp String Iterator is an object, that represents a specific
iteration over some specific String instance object, matching
against some specific RegExp instance object. There is not a named
constructor
for RegExp String Iterator objects. Instead, RegExp String Iterator
objects are created by calling certain methods of RegExp instance
objects.
The initial value of the @@toStringTag property is the String
value "RegExp String Iterator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
21.2.7.2Properties of RegExp String
Iterator Instances
RegExp String Iterator instances are ordinary objects that inherit
properties from the
%RegExpStringIteratorPrototype%
intrinsic object. RegExp String Iterator instances are initially
created with the internal slots listed in
Table 61.
Table 61: Internal Slots of RegExp String Iterator Instances
Internal Slot
Description
[[IteratingRegExp]]
The regular expression used for iteration.
IsRegExp([[IteratingRegExp]]) is initially
true.
[[IteratedString]]
The String value being iterated upon.
[[Global]]
A Boolean value to indicate whether the
[[IteratingRegExp]] is global or not.
[[Unicode]]
A Boolean value to indicate whether the
[[IteratingRegExp]] is in Unicode mode or not.
[[Done]]
A Boolean value to indicate whether the iteration is
complete or not.
22Indexed Collections
22.1Array Objects
Array objects are exotic objects that give special treatment to a
certain class of property names. See
9.4.2
for a definition of this special treatment.
also creates and initializes a new Array object when called as a
function rather than as a
constructor. Thus the function call Array(…) is equivalent to
the object creation expression new Array(…) with the
same arguments.
is a single function whose behaviour is overloaded based upon the
number and types of its arguments.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the exotic
Array behaviour must include a
super call to the Arrayconstructor
to initialize subclass instances that are Array exotic objects.
However, most of the Array.prototype methods are
generic methods that are not dependent upon their
this value being an Array
exotic object.
has a "length" property whose value is 1.
22.1.1.1Array ( )
This description applies if and only if the Array
constructor
is called with no arguments.
Let numberOfArgs be the number of arguments passed
to this function call.
The from function is an intentionally generic
factory method; it does not require that its
this value be the Array
constructor. Therefore it can be transferred to or inherited by any
other constructors that may be called with a single numeric
argument.
22.1.2.2Array.isArray (
arg )
The isArray function takes one argument
arg, and performs the following steps:
The items argument is assumed to be a well-formed
rest argument value.
Note 2
The of function is an intentionally generic
factory method; it does not require that its
this value be the Array
constructor. Therefore it can be transferred to or inherited by other
constructors that may be called with a single numeric
argument.
22.1.2.4Array.prototype
The value of Array.prototype is %Array.prototype%,
the intrinsic Array prototype object.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
22.1.2.5get Array [ @@species ]
Array[@@species] is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Return the this value.
The value of the "name" property of this
function is "get [Symbol.species]".
Note
Array prototype methods normally use their
this object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its
@@species property.
22.1.3Properties of the Array Prototype
Object
The Array prototype object:
is the intrinsic object %ArrayPrototype%.
is an Array
exotic object
and has the internal methods specified for such objects.
has a "length" property whose initial value is
0 and whose attributes are { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: false }.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
Note
The Array prototype object is specified to be an Array
exotic object
to ensure compatibility with ECMAScript code that was created
prior to the ECMAScript 2015 specification.
22.1.3.1Array.prototype.concat (
...arguments )
When the concat method is called with zero or more
arguments, it returns an array containing the array elements of
the object followed by the array elements of each argument in
order.
Let items be a
List
whose first element is O and whose subsequent
elements are, in left to right order, the arguments that were
passed to this function invocation.
Repeat, while items is not empty
Remove the first element from items and let
E be the value of the element.
The explicit setting of the
"length" property in step 6 is necessary to
ensure that its value is correct in situations where the
trailing elements of the result Array are not present.
Note 2
The concat function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
22.1.3.1.1Runtime Semantics:
IsConcatSpreadable ( O )
The abstract operation IsConcatSpreadable with argument
O performs the following steps:
The initial value of Array.prototype.constructor is
%Array%.
22.1.3.3Array.prototype.copyWithin (
target, start [ , end ] )
The copyWithin method takes up to three arguments
target, start and end.
Note 1
The end argument is optional with the length of the
this object as its default value. If
target is negative, it is treated as
length + target
where length is the length of the array. If
start is negative, it is treated as
length + start. If end is negative, it is treated as
length + end.
The copyWithin function is intentionally generic;
it does not require that its this value be
an Array object. Therefore it can be transferred to other
kinds of objects for use as a method.
callbackfn should be a function that accepts three
arguments and returns a value that is coercible to the Boolean
value true or false.
every calls callbackfn once for each
element present in the array, in ascending order, until it
finds one where callbackfn returns
false. If such an element is found,
every immediately returns
false. Otherwise, if
callbackfn returned true for all
elements, every will return
true. callbackfn is called only
for elements of the array which actually exist; it is not
called for missing elements of the array.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the
value of the element, the index of the element, and the object
being traversed.
every does not directly mutate the object on
which it is called but the object may be mutated by the calls
to callbackfn.
The range of elements processed by every is set
before the first call to callbackfn. Elements which
are appended to the array after the call to
every begins will not be visited by
callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will
be the value at the time every visits them;
elements that are deleted after the call to
every begins and before being visited are not
visited. every acts like the "for all" quantifier
in mathematics. In particular, for an empty array, it returns
true.
When the every method is called with one or two
arguments, the following steps are taken:
Let testResult be ! ToBoolean(?
Call(callbackfn, thisArg, «
kValue, k, O »)).
If testResult is false,
return false.
Set k to k + 1.
Return true.
Note 2
The every function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
22.1.3.6Array.prototype.fill (
value [ , start [ , end ] ] )
The fill method takes up to three arguments
value, start and end.
Note 1
The start and end arguments are optional
with default values of 0 and the length of the
this object. If start is
negative, it is treated as
length + start
where length is the length of the array. If
end is negative, it is treated as
length + end.
The fill function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
callbackfn should be a function that accepts three
arguments and returns a value that is coercible to the Boolean
value true or false.
filter calls callbackfn once for each
element in the array, in ascending order, and constructs a new
array of all the values for which
callbackfn returns true.
callbackfn is called only for elements of the array
which actually exist; it is not called for missing elements of
the array.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the
value of the element, the index of the element, and the object
being traversed.
filter does not directly mutate the object on
which it is called but the object may be mutated by the calls
to callbackfn.
The range of elements processed by filter is set
before the first call to callbackfn. Elements which
are appended to the array after the call to
filter begins will not be visited by
callbackfn. If existing elements of the array are
changed their value as passed to callbackfn will be
the value at the time filter visits them;
elements that are deleted after the call to
filter begins and before being visited are not
visited.
When the filter method is called with one or two
arguments, the following steps are taken:
The filter function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
The find method is called with one or two arguments,
predicate and thisArg.
Note 1
predicate should be a function that accepts three
arguments and returns a value that is coercible to a Boolean
value. find calls predicate once for
each element of the array, in ascending order, until it finds
one where predicate returns
true. If such an element is found,
find immediately returns that element value.
Otherwise, find returns
undefined.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
predicate. If it is not provided,
undefined is used instead.
predicate is called with three arguments: the value
of the element, the index of the element, and the object being
traversed.
find does not directly mutate the object on which
it is called but the object may be mutated by the calls to
predicate.
The range of elements processed by find is set
before the first call to predicate. Elements that
are appended to the array after the call to
find begins will not be visited by
predicate. If existing elements of the array are
changed, their value as passed to predicate will be
the value at the time that find visits them.
When the find method is called, the following steps
are taken:
Let testResult be ! ToBoolean(?
Call(predicate, thisArg, «
kValue, k, O »)).
If testResult is true,
return kValue.
Set k to k + 1.
Return undefined.
Note 2
The find function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
predicate should be a function that accepts three
arguments and returns a value that is coercible to the Boolean
value true or false.
findIndex calls predicate once for
each element of the array, in ascending order, until it finds
one where predicate returns
true. If such an element is found,
findIndex immediately returns the index of that
element value. Otherwise, findIndex returns -1.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
predicate. If it is not provided,
undefined is used instead.
predicate is called with three arguments: the value
of the element, the index of the element, and the object being
traversed.
findIndex does not directly mutate the object on
which it is called but the object may be mutated by the calls
to predicate.
The range of elements processed by findIndex is
set before the first call to predicate. Elements
that are appended to the array after the call to
findIndex begins will not be visited by
predicate. If existing elements of the array are
changed, their value as passed to predicate will be
the value at the time that findIndex visits them.
When the findIndex method is called with one or two
arguments, the following steps are taken:
Let testResult be ! ToBoolean(?
Call(predicate, thisArg, «
kValue, k, O »)).
If testResult is true,
return k.
Set k to k + 1.
Return -1.
Note 2
The findIndex function is intentionally generic;
it does not require that its this value be
an Array object. Therefore it can be transferred to other
kinds of objects for use as a method.
22.1.3.10Array.prototype.flat ( [
depth ] )
When the flat method is called with zero or one
arguments, the following steps are taken:
callbackfn should be a function that accepts three
arguments. forEach calls
callbackfn once for each element present in the
array, in ascending order. callbackfn is called
only for elements of the array which actually exist; it is not
called for missing elements of the array.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the
value of the element, the index of the element, and the object
being traversed.
forEach does not directly mutate the object on
which it is called but the object may be mutated by the calls
to callbackfn.
When the forEach method is called with one or two
arguments, the following steps are taken:
Perform ? Call(callbackfn, thisArg, «
kValue, k, O »).
Set k to k + 1.
Return undefined.
This function is the %ArrayProto_forEach% intrinsic
object.
Note 2
The forEach function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
includes compares searchElement to the
elements of the array, in ascending order, using the
SameValueZero
algorithm, and if found at any position, returns
true; otherwise,
false is returned.
The optional second argument fromIndex defaults to
0 (i.e. the whole array is searched). If it is greater than or
equal to the length of the array, false is
returned, i.e. the array will not be searched. If it is
negative, it is used as the offset from the end of the array
to compute fromIndex. If the computed index is less
than 0, the whole array will be searched.
When the includes method is called, the following
steps are taken:
Let elementK be the result of ? Get(O, ! ToString(k)).
If
SameValueZero(searchElement, elementK) is
true, return true.
Set k to k + 1.
Return false.
Note 2
The includes function is intentionally generic;
it does not require that its this value be
an Array object. Therefore it can be transferred to other
kinds of objects for use as a method.
Note 3
The includes method intentionally differs from
the similar indexOf method in two ways. First, it
uses the
SameValueZero
algorithm, instead of
Strict Equality Comparison, allowing it to detect NaN array
elements. Second, it does not skip missing array elements,
instead treating them as undefined.
indexOf compares searchElement to the
elements of the array, in ascending order, using the
Strict Equality Comparison
algorithm, and if found at one or more indices, returns the
smallest such index; otherwise, -1 is returned.
The optional second argument fromIndex defaults to
0 (i.e. the whole array is searched). If it is greater than or
equal to the length of the array, -1 is returned, i.e. the
array will not be searched. If it is negative, it is used as
the offset from the end of the array to compute
fromIndex. If the computed index is less than 0,
the whole array will be searched.
When the indexOf method is called with one or two
arguments, the following steps are taken:
The indexOf function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
22.1.3.15Array.prototype.join (
separator )
Note 1
The elements of the array are converted to Strings, and these
Strings are then concatenated, separated by occurrences of the
separator. If no separator is provided, a single
comma is used as the separator.
The join method takes one argument,
separator, and performs the following steps:
The join function is intentionally generic; it
does not require that its this value be an
Array object. Therefore, it can be transferred to other kinds
of objects for use as a method.
lastIndexOf compares searchElement to
the elements of the array in descending order using the
Strict Equality Comparison
algorithm, and if found at one or more indices, returns the
largest such index; otherwise, -1 is returned.
The optional second argument fromIndex defaults to
the array's length minus one (i.e. the whole array is
searched). If it is greater than or equal to the length of the
array, the whole array will be searched. If it is negative, it
is used as the offset from the end of the array to compute
fromIndex. If the computed index is less than 0, -1
is returned.
When the lastIndexOf method is called with one or two
arguments, the following steps are taken:
The lastIndexOf function is intentionally
generic; it does not require that its
this value be an Array object. Therefore it
can be transferred to other kinds of objects for use as a
method.
callbackfn should be a function that accepts three
arguments. map calls callbackfn once
for each element in the array, in ascending order, and
constructs a new Array from the results.
callbackfn is called only for elements of the array
which actually exist; it is not called for missing elements of
the array.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the
value of the element, the index of the element, and the object
being traversed.
map does not directly mutate the object on which
it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by map is set
before the first call to callbackfn. Elements which
are appended to the array after the call to
map begins will not be visited by
callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will
be the value at the time map visits them;
elements that are deleted after the call to
map begins and before being visited are not
visited.
When the map method is called with one or two
arguments, the following steps are taken:
The map function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
22.1.3.19Array.prototype.pop ( )
Note 1
The last element of the array is removed from the array and
returned.
When the pop method is called, the following steps
are taken:
The pop function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
22.1.3.20Array.prototype.push (
...items )
Note 1
The arguments are appended to the end of the array, in the
order in which they appear. The new length of the array is
returned as the result of the call.
When the push method is called with zero or more
arguments, the following steps are taken:
The push function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
callbackfn should be a function that takes four
arguments. reduce calls the callback, as a
function, once for each element after the first element
present in the array, in ascending order.
callbackfn is called with four arguments: the
previousValue (value from the previous call to
callbackfn), the currentValue (value of
the current element), the currentIndex, and the
object being traversed. The first time that callback is
called, the previousValue and
currentValue can be one of two values. If an
initialValue was supplied in the call to
reduce, then previousValue will be
equal to initialValue and
currentValue will be equal to the first value in
the array. If no initialValue was supplied, then
previousValue will be equal to the first value in
the array and currentValue will be equal to the
second. It is a TypeError if the array
contains no elements and initialValue is not
provided.
reduce does not directly mutate the object on
which it is called but the object may be mutated by the calls
to callbackfn.
The range of elements processed by reduce is set
before the first call to callbackfn. Elements that
are appended to the array after the call to
reduce begins will not be visited by
callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will
be the value at the time reduce visits them;
elements that are deleted after the call to
reduce begins and before being visited are not
visited.
When the reduce method is called with one or two
arguments, the following steps are taken:
Set accumulator to ? Call(callbackfn, undefined,
« accumulator, kValue,
k, O »).
Set k to k + 1.
Return accumulator.
Note 2
The reduce function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
callbackfn should be a function that takes four
arguments. reduceRight calls the callback, as a
function, once for each element after the first element
present in the array, in descending order.
callbackfn is called with four arguments: the
previousValue (value from the previous call to
callbackfn), the currentValue (value of
the current element), the currentIndex, and the
object being traversed. The first time the function is called,
the previousValue and currentValue can
be one of two values. If an initialValue was
supplied in the call to reduceRight, then
previousValue will be equal to
initialValue and currentValue will be
equal to the last value in the array. If no
initialValue was supplied, then
previousValue will be equal to the last value in
the array and currentValue will be equal to the
second-to-last value. It is a TypeError if
the array contains no elements and initialValue is
not provided.
reduceRight does not directly mutate the object
on which it is called but the object may be mutated by the
calls to callbackfn.
The range of elements processed by reduceRight is
set before the first call to callbackfn. Elements
that are appended to the array after the call to
reduceRight begins will not be visited by
callbackfn. If existing elements of the array are
changed by callbackfn, their value as passed to
callbackfn will be the value at the time
reduceRight visits them; elements that are
deleted after the call to reduceRight begins and
before being visited are not visited.
When the reduceRight method is called with one or two
arguments, the following steps are taken:
Set accumulator to ? Call(callbackfn, undefined,
« accumulator, kValue,
k, O »).
Set k to k - 1.
Return accumulator.
Note 2
The reduceRight function is intentionally
generic; it does not require that its
this value be an Array object. Therefore it
can be transferred to other kinds of objects for use as a
method.
22.1.3.23Array.prototype.reverse ( )
Note 1
The elements of the array are rearranged so as to reverse
their order. The object is returned as the result of the call.
When the reverse method is called, the following
steps are taken:
Assert: lowerExists and
upperExists are both
false.
No action is required.
Set lower to lower + 1.
Return O.
Note 2
The reverse function is intentionally generic; it
does not require that its this value be an
Array object. Therefore, it can be transferred to other kinds
of objects for use as a method.
22.1.3.24Array.prototype.shift ( )
Note 1
The first element of the array is removed from the array and
returned.
When the shift method is called, the following steps
are taken:
The shift function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
22.1.3.25Array.prototype.slice (
start, end )
Note 1
The slice method takes two arguments,
start and end, and returns an array
containing the elements of the array from element
start up to, but not including, element
end (or through the end of the array if
end is undefined). If
start is negative, it is treated as
length + start
where length is the length of the array. If
end is negative, it is treated as
length + end
where length is the length of the array.
The explicit setting of the
"length" property of the result Array in
step 11 was necessary in previous editions of ECMAScript to
ensure that its length was correct in situations where the
trailing elements of the result Array were not present.
Setting "length" became unnecessary
starting in ES2015 when the result Array was initialized to
its proper length rather than an empty Array but is carried
forward to preserve backward compatibility.
Note 3
The slice function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
callbackfn should be a function that accepts three
arguments and returns a value that is coercible to the Boolean
value true or false.
some calls callbackfn once for each
element present in the array, in ascending order, until it
finds one where callbackfn returns
true. If such an element is found,
some immediately returns true.
Otherwise, some returns false.
callbackfn is called only for elements of the array
which actually exist; it is not called for missing elements of
the array.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the
value of the element, the index of the element, and the object
being traversed.
some does not directly mutate the object on which
it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by some is set
before the first call to callbackfn. Elements that
are appended to the array after the call to
some begins will not be visited by
callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will
be the value at the time that some visits them;
elements that are deleted after the call to
some begins and before being visited are not
visited. some acts like the "exists" quantifier
in mathematics. In particular, for an empty array, it returns
false.
When the some method is called with one or two
arguments, the following steps are taken:
Let testResult be ! ToBoolean(?
Call(callbackfn, thisArg, «
kValue, k, O »)).
If testResult is true,
return true.
Set k to k + 1.
Return false.
Note 2
The some function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
22.1.3.27Array.prototype.sort (
comparefn )
The elements of this array are sorted. The sort must be stable
(that is, elements that compare equal must remain in their
original order). If comparefn is not
undefined, it should be a function that accepts
two arguments x and y and returns a negative
value if x < y, zero if x =
y, or a positive value if x >
y.
Upon entry, the following steps are performed to initialize
evaluation of the sort function:
If comparefn is not
undefined and
IsCallable(comparefn) is false, throw a
TypeError exception.
Let elem be obj.[[GetOwnProperty]](!
ToString(i)).
If elem is undefined, return
true.
Return false.
The sort order is the ordering, after completion of this
function, of the
integer-indexed
property values of obj whose
integer
indexes are less than len. The result of the
sort function is then determined as follows:
If comparefn is not undefined and is
not a consistent comparison function for the elements of this
array (see below), the sort order is implementation-defined. The
sort order is also implementation-defined if
comparefn is undefined and
SortCompare
does not act as a consistent comparison function.
Let proto be obj.[[GetPrototypeOf]](). If
proto is not null and there exists
an
integerj such that all of the conditions below are satisfied
then the sort order is implementation-defined:
Any
integer index
property of obj whose name is a nonnegative
integer
less than len is a
data property
whose [[Configurable]] attribute is false.
The sort order is also implementation-defined if any of the
following conditions are true:
If obj is an
exotic object
(including Proxy exotic objects) whose behaviour for [[Get]],
[[Set]], [[Delete]], and [[GetOwnProperty]] is not the ordinary
object implementation of these internal methods.
If any index property of obj whose name is a
nonnegative
integer
less than len is an
accessor property
or is a
data property
whose [[Writable]] attribute is false.
If comparefn is undefined and the
application of
ToString
to any value passed as an argument to
SortCompare
modifies obj or any object on obj's
prototype chain.
If comparefn is undefined and all
applications of
ToString, to any specific value passed as an argument to
SortCompare, do not produce the same result.
The following steps are taken:
Perform an implementation-dependent sequence of calls to the
Get,
Set,
DeletePropertyOrThrow, and
HasOwnProperty
abstract operation with obj as the first argument,
and to
SortCompare
(described below), such that:
The arguments for calls to
SortCompare
are values returned by a previous call to the
Get
abstract operation, unless the properties accessed by
those previous calls did not exist according to
HasOwnProperty. If both prospective arguments to
SortCompare
correspond to non-existent properties, use
+0 instead of calling
SortCompare. If only the first prospective argument is non-existent
use +1. If only the second prospective argument is
non-existent use -1.
If an
abrupt completion
is returned from any of these operations, it is
immediately returned as the value of this function.
Return obj.
Unless the sort order is specified above to be
implementation-defined, the returned object must have the
following two characteristics:
There must be some mathematical permutation π of the nonnegative
integers less than len, such that for every
nonnegative
integerj less than len, if property
old[j] existed,
then new[π(j)] is
exactly the same value as
old[j]. But if
property old[j] did
not exist, then
new[π(j)] does not
exist.
Then for all nonnegative integers j and k,
each less than len, if
SortCompare(old[j], old[k]) < 0
(see
SortCompare
below), then
new[π(j)] < new[π(k)].
Here the notation
old[j] is used to
refer to the hypothetical result of calling
Get(obj, j)
before this function is executed, and the notation
new[j] to refer to
the hypothetical result of calling
Get(obj, j)
after this function has been executed.
A function comparefn is a consistent comparison
function for a set of values S if all of the
requirements below are met for all values a,
b, and c (possibly the same value) in the
set S: The notation
a <CFb
means
comparefn(a, b) <
0;
a =CFb
means
comparefn(a, b) = 0
(of either sign); and
a >CFb
means
comparefn(a, b) >
0.
Calling comparefn(a, b) always
returns the same value v when given a specific pair
of values a and b as its two arguments.
Furthermore,
Type(v) is Number, and v is not
NaN. Note that this implies that exactly one
of a <CFb,
a =CFb, and
a >CFb will be true for a
given pair of a and b.
Calling comparefn(a, b) does
not modify obj or any object on obj's
prototype chain.
a =CFa (reflexivity)
If a =CFb, then
b =CFa (symmetry)
If a =CFb and
b =CFc, then
a =CFc (transitivity of
=CF)
If a <CFb and
b <CFc, then
a <CFc (transitivity of
<CF)
If a >CFb and
b >CFc, then
a >CFc (transitivity of
>CF)
Note 1
The above conditions are necessary and sufficient to ensure
that comparefn divides the set S into
equivalence classes and that these equivalence classes are
totally ordered.
Note 2
The sort function is intentionally generic; it
does not require that its this value be an
Array object. Therefore, it can be transferred to other kinds
of objects for use as a method.
22.1.3.27.1Runtime Semantics:
SortCompare ( x, y )
The SortCompare abstract operation is called with two arguments
x and y. It also has access to the
comparefn argument passed to the current invocation
of the sort method. The following steps are taken:
If x and y are both
undefined, return +0.
If x is undefined, return 1.
If y is undefined, return -1.
If comparefn is not undefined,
then
Let v be ? ToNumber(?
Call(comparefn, undefined, «
x, y »)).
Because non-existent property values always compare greater
than undefined property values, and
undefined always compares greater than
any other value, undefined property
values always sort to the end of the result, followed by
non-existent property values.
Note 2
Method calls performed by the
ToStringabstract operations
in steps 5 and 7 have the potential to cause SortCompare to
not behave as a consistent comparison function.
When the splice method is called with two or more
arguments start, deleteCount and zero or
more items, the deleteCount elements of
the array starting at
integer indexstart are replaced by the arguments
items. An Array object containing the deleted
elements (if any) is returned.
Let items be a
List
whose elements are, in left to right order, the portion of the
actual argument list starting with the third argument. The
list is empty if fewer than three arguments were passed.
Perform ? Set(O, "length", len -
actualDeleteCount + itemCount,
true).
Return A.
Note 2
The explicit setting of the
"length" property of the result Array in
step 19 was necessary in previous editions of ECMAScript to
ensure that its length was correct in situations where the
trailing elements of the result Array were not present.
Setting "length" became unnecessary
starting in ES2015 when the result Array was initialized to
its proper length rather than an empty Array but is carried
forward to preserve backward compatibility.
Note 3
The splice function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402
Internationalization API must implement the
Array.prototype.toLocaleString method as specified in
the ECMA-402 specification. If an ECMAScript implementation does
not include the ECMA-402 API the following specification of the
toLocaleString method is used.
Note 1
The first edition of ECMA-402 did not include a replacement
specification for the
Array.prototype.toLocaleString method.
The meanings of the optional parameters to this method are defined
in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not use those parameter positions for
anything else.
Let separator be the String value for the
list-separator String appropriate for the host environment's
current locale (this is derived in an implementation-defined
way).
The elements of the array are converted to Strings using their
toLocaleString methods, and these Strings are
then concatenated, separated by occurrences of a separator
String that has been derived in an implementation-defined
locale-specific way. The result of calling this function is
intended to be analogous to the result of
toString, except that the result of this function
is intended to be locale-specific.
Note 3
The toLocaleString function is intentionally
generic; it does not require that its
this value be an Array object. Therefore it
can be transferred to other kinds of objects for use as a
method.
22.1.3.30Array.prototype.toString ( )
When the toString method is called, the following
steps are taken:
The toString function is intentionally generic;
it does not require that its this value be
an Array object. Therefore it can be transferred to other
kinds of objects for use as a method.
22.1.3.31Array.prototype.unshift (
...items )
Note 1
The arguments are prepended to the start of the array, such
that their order within the array is the same as the order in
which they appear in the argument list.
When the unshift method is called with zero or more
arguments item1, item2, etc., the following
steps are taken:
The unshift function is intentionally generic; it
does not require that its this value be an
Array object. Therefore it can be transferred to other kinds
of objects for use as a method.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
Note
The own property names of this object are property names that
were not included as standard properties of
Array.prototype prior to the ECMAScript 2015
specification. These names are ignored for
with statement binding purposes in order to
preserve the behaviour of existing code that might use one of
these names as a binding in an outer scope that is shadowed by
a with statement whose binding object is an Array
object.
22.1.4Properties of Array Instances
Array instances are Array exotic objects and have the internal
methods specified for such objects. Array instances inherit
properties from the Array prototype object.
Array instances have a "length" property, and a
set of enumerable properties with
array index
names.
22.1.4.1length
The "length" property of an Array instance is a
data property
whose value is always numerically greater than the name of every
configurable own property whose name is an
array index.
The "length" property initially has the
attributes { [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]:
false }.
Note
Reducing the value of the "length" property
has the side-effect of deleting own array elements whose
array index
is between the old and new length values. However,
non-configurable properties can not be deleted. Attempting to
set the "length" property of an Array
object to a value that is numerically less than or equal to
the largest numeric own
property name
of an existing non-configurable
array-indexed
property of the array will result in the length being set to a
numeric value that is one greater than that non-configurable
numeric own
property name. See
9.4.2.1.
22.1.5Array Iterator Objects
An Array Iterator is an object, that represents a specific iteration
over some specific Array instance object. There is not a named
constructor
for Array Iterator objects. Instead, Array iterator objects are
created by calling certain methods of Array instance objects.
22.1.5.1CreateArrayIterator (
array, kind )
Several methods of Array objects return Iterator objects. The
abstract operation CreateArrayIterator with arguments
array and kind is used to create such
iterator objects. It performs the following steps:
The initial value of the @@toStringTag property is the String
value "Array Iterator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
22.1.5.3Properties of Array Iterator
Instances
Array Iterator instances are ordinary objects that inherit
properties from the
%ArrayIteratorPrototype%
intrinsic object. Array Iterator instances are initially created
with the internal slots listed in
Table 62.
Table 62: Internal Slots of Array Iterator Instances
The
integer index
of the next element to be examined by this iterator.
[[ArrayLikeIterationKind]]
A String value that identifies what is returned for each
element of the iteration. The possible values are:
key,
value,
key+value.
22.2TypedArray Objects
TypedArray objects present an array-like view of an
underlying binary data buffer (24.1). Each element of a TypedArray instance has the same
underlying binary scalar data type. There is a distinct
TypedArrayconstructor, listed in
Table 63, for each of the supported element types. Each
constructor
in
Table 63
has a corresponding distinct prototype object.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
22.2.2.4get %TypedArray% [ @@species ]
%TypedArray%[@@species] is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Return the this value.
The value of the "name" property of this
function is "get [Symbol.species]".
Note
%TypedArray.prototype%
methods normally use their this object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its
@@species property.
22.2.3Properties of the
%TypedArray.prototype% Object
The %TypedArray.prototype% object:
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
is an ordinary object.
does not have a [[ViewedArrayBuffer]] or any other of the internal
slots that are specific to TypedArray instance objects.
22.2.3.1get
%TypedArray%.prototype.buffer
%TypedArray%.prototype.buffer is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
22.2.3.2get
%TypedArray%.prototype.byteLength
%TypedArray%.prototype.byteLength is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
%TypedArray%.prototype.byteOffset is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
22.2.3.5%TypedArray%.prototype.copyWithin ( target,
start [ , end ] )
The interpretation and use of the arguments of
%TypedArray%.prototype.copyWithin are the same as for
Array.prototype.copyWithin as defined in
22.1.3.3.
%TypedArray%.prototype.every is a distinct function that
implements the same algorithm as
Array.prototype.every as defined in
22.1.3.5
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed properties are not sparse. However, such optimization
must not introduce any observable changes in the specified
behaviour of the algorithm and must take into account the
possibility that calls to callbackfn may cause the
this value to become detached.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.8%TypedArray%.prototype.fill (
value [ , start [ , end ] ] )
The interpretation and use of the arguments of
%TypedArray%.prototype.fill are the same as for
Array.prototype.fill as defined in
22.1.3.6.
%TypedArray%.prototype.find is a distinct function that
implements the same algorithm as
Array.prototype.find as defined in
22.1.3.8
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
predicate may cause the this value
to become detached.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.findIndex is a distinct function that
implements the same algorithm as
Array.prototype.findIndex as defined in
22.1.3.9
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
predicate may cause the this value
to become detached.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.forEach is a distinct function that
implements the same algorithm as
Array.prototype.forEach as defined in
22.1.3.12
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
callbackfn may cause the this value
to become detached.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.includes is a distinct function that
implements the same algorithm as
Array.prototype.includes as defined in
22.1.3.13
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.indexOf is a distinct function that
implements the same algorithm as
Array.prototype.indexOf as defined in
22.1.3.14
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.join is a distinct function that
implements the same algorithm as
Array.prototype.join as defined in
22.1.3.15
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.lastIndexOf is a distinct function that
implements the same algorithm as
Array.prototype.lastIndexOf as defined in
22.1.3.17
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.18get
%TypedArray%.prototype.length
%TypedArray%.prototype.length is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
%TypedArray%.prototype.reduce is a distinct function that
implements the same algorithm as
Array.prototype.reduce as defined in
22.1.3.21
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
callbackfn may cause the this value
to become detached.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.reduceRight is a distinct function that
implements the same algorithm as
Array.prototype.reduceRight as defined in
22.1.3.22
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
callbackfn may cause the this value
to become detached.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.22%TypedArray%.prototype.reverse ( )
%TypedArray%.prototype.reverse is a distinct function that
implements the same algorithm as
Array.prototype.reverse as defined in
22.1.3.23
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
Sets multiple values in this TypedArray, reading the
values from the object array. The optional
offset value indicates the first element index in
this TypedArray where values are written. If omitted,
it is assumed to be 0.
Assert: array is any
ECMAScript language value
other than an Object with a [[TypedArrayName]] internal
slot. If it is such an Object, the definition in
22.2.3.23.2
applies.
Sets multiple values in this TypedArray, reading the
values from the typedArray argument object. The
optional offset value indicates the first element
index in this TypedArray where values are written. If
omitted, it is assumed to be 0.
Assert: typedArray has a [[TypedArrayName]] internal
slot. If it does not, the definition in
22.2.3.23.1
applies.
Set srcByteIndex to
srcByteIndex + srcElementSize.
Set targetByteIndex to
targetByteIndex +
targetElementSize.
Return undefined.
22.2.3.24%TypedArray%.prototype.slice
( start, end )
The interpretation and use of the arguments of
%TypedArray%.prototype.slice are the same as for
Array.prototype.slice as defined in
22.1.3.25. The following steps are taken:
%TypedArray%.prototype.some is a distinct function that
implements the same algorithm as
Array.prototype.some as defined in
22.1.3.26
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
callbackfn may cause the this value
to become detached.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.sort is a distinct function that, except
as described below, implements the same requirements as those of
Array.prototype.sort as defined in
22.1.3.27. The implementation of the
%TypedArray%.prototype.sort specification may be optimized with
the knowledge that the this value is an object
that has a fixed length and whose
integer-indexed
properties are not sparse. The only internal methods of the
this object that the algorithm may call are
[[Get]] and [[Set]].
This function is not generic. The this value
must be an object with a [[TypedArrayName]] internal slot.
Upon entry, the following steps are performed to initialize
evaluation of the sort function. These steps are used
instead of the entry steps in
22.1.3.27:
If comparefn is not
undefined and
IsCallable(comparefn) is false, throw a
TypeError exception.
The implementation-defined sort order condition for exotic objects
is not applied by
%TypedArray%.prototype.sort.
The following version of
SortCompare
is used by
%TypedArray%.prototype.sort. It performs a numeric comparison
rather than the string comparison used in
22.1.3.27.
SortCompare
has access to the comparefn and
buffer values of the current invocation of the
sort method.
When the TypedArray
SortCompare
abstract operation is called with two arguments x and
y, the following steps are taken:
Assert: Both
Type(x) and
Type(y) are Number or both are BigInt.
If comparefn is not undefined,
then
Let v be ? ToNumber(?
Call(comparefn, undefined, «
x, y »)).
If
IsDetachedBuffer(buffer) is true, throw a
TypeError exception.
If v is NaN, return
+0.
Return v.
If x and y are both
NaN, return +0.
If x is NaN, return 1.
If y is NaN, return -1.
If x < y, return -1.
If x > y, return 1.
If x is -0 and y is
+0, return -1.
If x is +0 and y is
-0, return 1.
Return +0.
Note
Because NaN always compares greater than
any other value, NaN property values always
sort to the end of the result when comparefn is not
provided.
22.2.3.27%TypedArray%.prototype.subarray ( begin,
end )
Returns a new TypedArray object whose element type is
the same as this TypedArray and whose ArrayBuffer is
the same as the ArrayBuffer of this TypedArray,
referencing the elements at begin, inclusive, up to
end, exclusive. If either begin or
end is negative, it refers to an index from the end of
the array, as opposed to from the beginning.
%TypedArray%.prototype.toLocaleString is a distinct function
that implements the same algorithm as
Array.prototype.toLocaleString as defined in
22.1.3.29
except that the this object's [[ArrayLength]]
internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm
may be optimized with the knowledge that the
this value is an object that has a fixed length
and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the this value prior to
evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402
Internationalization API this function is based upon the
algorithm for Array.prototype.toLocaleString that
is in the ECMA-402 specification.
%TypedArray%.prototype[@@toStringTag] is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
is an intrinsic object that has the structure described below,
differing only in the name used as the
constructor
name instead of TypedArray, in
Table 63.
is a single function whose behaviour is overloaded based upon the
number and types of its arguments. The actual behaviour of a call
of TypedArray depends upon the number and kind of
arguments that are passed to it.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
TypedArray behaviour must include a
super call to the TypedArrayconstructor
to create and initialize the subclass instance with the internal
state necessary to support the
%TypedArray%.prototype built-in methods.
has a "length" property whose value is 3.
22.2.4.1TypedArray ( )
This description applies only if the
TypedArray function is called with no arguments.
If NewTarget is undefined, throw a
TypeError exception.
Let constructorName be the String value of the
Constructor
Name value specified in
Table 63
for this TypedArrayconstructor.
The abstract operation AllocateTypedArray with arguments
constructorName, newTarget,
defaultProto and optional argument
length is used to validate and create an instance of
a TypedArray
constructor. constructorName is required to be the name of a
TypedArray
constructor
in
Table 63. If the length argument is passed, an ArrayBuffer
of that length is also allocated and associated with the new
TypedArray instance. AllocateTypedArray provides common
semantics that is used by all of the
TypedArray overloads. AllocateTypedArray performs the
following steps:
22.2.4.2.2Runtime Semantics:
AllocateTypedArrayBuffer ( O, length )
The abstract operation AllocateTypedArrayBuffer with arguments
O and length allocates and associates an
ArrayBuffer with the TypedArray instance O. It
performs the following steps:
Assert: O is an Object that has a
[[ViewedArrayBuffer]] internal slot.
This description applies only if the
TypedArray function is called with at least one
argument and the Type of the first argument is Object and that
object has a [[TypedArrayName]] internal slot.
TypedArray called with argument
typedArray performs the following steps:
Assert:
Type(typedArray) is Object and
typedArray has a [[TypedArrayName]] internal slot.
If NewTarget is undefined, throw a
TypeError exception.
Let constructorName be the String value of the
Constructor
Name value specified in
Table 63
for this TypedArrayconstructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget,
"%TypedArray.prototype%").
Let srcArray be typedArray.
Let srcData be
srcArray.[[ViewedArrayBuffer]].
If
IsDetachedBuffer(srcData) is true, throw a
TypeError exception.
Let elementType be the Element Type value in
Table 63
for constructorName.
Let elementLength be
srcArray.[[ArrayLength]].
Let srcName be the String value of
srcArray.[[TypedArrayName]].
Let srcType be the Element Type value in
Table 63
for srcName.
Let srcElementSize be the Element Size value
specified in
Table 63
for srcName.
Let srcByteOffset be
srcArray.[[ByteOffset]].
Let elementSize be the Element Size value specified
in
Table 63
for constructorName.
Set srcByteIndex to
srcByteIndex + srcElementSize.
Set targetByteIndex to
targetByteIndex + elementSize.
Set count to count - 1.
Set O.[[ViewedArrayBuffer]] to data.
Set O.[[ByteLength]] to byteLength.
Set O.[[ByteOffset]] to 0.
Set O.[[ArrayLength]] to elementLength.
Return O.
22.2.4.4TypedArray (
object )
This description applies only if the
TypedArray function is called with at least one
argument and the Type of the first argument is Object and that
object does not have either a [[TypedArrayName]] or an
[[ArrayBufferData]] internal slot.
TypedArray called with argument
object performs the following steps:
Assert:
Type(object) is Object and object does not
have either a [[TypedArrayName]] or an [[ArrayBufferData]]
internal slot.
If NewTarget is undefined, throw a
TypeError exception.
Let constructorName be the String value of the
Constructor
Name value specified in
Table 63
for this TypedArrayconstructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget,
"%TypedArray.prototype%").
Let usingIterator be ? GetMethod(object, @@iterator).
This description applies only if the
TypedArray function is called with at least one
argument and the Type of the first argument is Object and that
object has an [[ArrayBufferData]] internal slot.
TypedArray called with at least one argument
buffer performs the following steps:
Assert:
Type(buffer) is Object and buffer has an
[[ArrayBufferData]] internal slot.
If NewTarget is undefined, throw a
TypeError exception.
Let constructorName be the String value of the
Constructor
Name value specified in
Table 63
for this TypedArrayconstructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget,
"%TypedArray.prototype%").
Let elementSize be the Element Size value specified
in
Table 63
for constructorName.
The abstract operation TypedArrayCreate with arguments
constructor and argumentList is used to
specify the creation of a new TypedArray object using a
constructor
function. It performs the following steps:
Let newTypedArray be ? Construct(constructor, argumentList).
The abstract operation TypedArraySpeciesCreate with arguments
exemplar and argumentList is used to specify
the creation of a new TypedArray object using a
constructor
function that is derived from exemplar. It performs the
following steps:
Assert: exemplar is an Object that has
[[TypedArrayName]] and [[ContentType]] internal slots.
Let defaultConstructor be the intrinsic object
listed in column one of
Table 63
for exemplar.[[TypedArrayName]].
does not have a [[ViewedArrayBuffer]] or any other of the internal
slots that are specific to TypedArray instance objects.
22.2.6.1TypedArray.prototype.BYTES_PER_ELEMENT
The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size
value specified in
Table 63
for TypedArray.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
22.2.6.2TypedArray.prototype.constructor
The initial value of a TypedArray.prototype.constructor is the corresponding
%TypedArray%
intrinsic object.
22.2.7Properties of
TypedArray Instances
TypedArray instances are
Integer-Indexed exotic objects. Each TypedArray instance inherits properties from the
corresponding TypedArray prototype object. Each
TypedArray instance has the following internal slots:
[[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]],
[[ByteOffset]], and [[ArrayLength]].
23Keyed Collections
23.1Map Objects
Map objects are collections of key/value pairs where both the keys and
values may be arbitrary ECMAScript language values. A distinct key
value may only occur in one key/value pair within the Map's
collection. Distinct key values are discriminated using the
SameValueZero
comparison algorithm.
Map object must be implemented using either hash tables or other
mechanisms that, on average, provide access times that are sublinear
on the number of elements in the collection. The data structures used
in this Map objects specification is only intended to describe the
required observable semantics of Map objects. It is not intended to be
a viable implementation model.
is the initial value of the "Map" property of
the
global object.
creates and initializes a new Map object when called as a
constructor.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Map behaviour must include a super call
to the Mapconstructor
to create and initialize the subclass instance with the internal
state necessary to support the Map.prototype built-in
methods.
23.1.1.1Map ( [ iterable ]
)
When the Map function is called with optional
argument iterable, the following steps are taken:
If NewTarget is undefined, throw a
TypeError exception.
If the parameter iterable is present, it is
expected to be an object that implements an @@iterator method
that returns an iterator object that produces a two element
array-like object
whose first element is a value that will be used as a Map key
and whose second element is the value to associate with that
key.
The abstract operation AddEntriesFromIterable accepts a
target object, an iterable of entries, and
an adder function to be invoked, with
target as the receiver.
If
IsCallable(adder) is false, throw a
TypeError exception.
Assert: iterable is present, and is neither
undefined nor null.
The parameter iterable is expected to be an object
that implements an @@iterator method that returns an iterator
object that produces a two element
array-like object
whose first element is a value that will be used as a Map key
and whose second element is the value to associate with that
key.
The initial value of Map.prototype is
%Map.prototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
23.1.2.2get Map [ @@species ]
Map[@@species] is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Return the this value.
The value of the "name" property of this
function is "get [Symbol.species]".
Note
Methods that create derived collection objects should call
@@species to determine the
constructor
to use to create the derived objects. Subclass
constructor
may over-ride @@species to change the default
constructor
assignment.
23.1.3Properties of the Map Prototype
Object
The Map prototype object:
is the intrinsic object %MapPrototype%.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
For each
Record
{ [[Key]], [[Value]] } p that is an element of
entries, do
Set p.[[Key]] to empty.
Set p.[[Value]] to
empty.
Return undefined.
Note
The existing [[MapData]]
List
is preserved because there may be existing Map Iterator
objects that are suspended midway through iterating over that
List.
23.1.3.2Map.prototype.constructor
The initial value of Map.prototype.constructor is
%Map%.
For each
Record
{ [[Key]], [[Value]] } p that is an element of
entries, do
If p.[[Key]] is not
empty and
SameValueZero(p.[[Key]], key) is
true, then
Set p.[[Key]] to
empty.
Set p.[[Value]] to
empty.
Return true.
Return false.
Note
The value empty is used as a
specification device to indicate that an entry has been
deleted. Actual implementations may take other actions such as
physically removing the entry from internal data structures.
For each
Record
{ [[Key]], [[Value]] } e that is an element of
entries, in original key insertion order, do
If e.[[Key]] is not
empty, then
Perform ? Call(callbackfn, thisArg, «
e.[[Value]], e.[[Key]],
M »).
Return undefined.
Note
callbackfn should be a function that accepts three
arguments. forEach calls
callbackfn once for each key/value pair present in
the map object, in key insertion order.
callbackfn is called only for keys of the map which
actually exist; it is not called for keys that have been
deleted from the map.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the
value of the item, the key of the item, and the Map object
being traversed.
forEach does not directly mutate the object on
which it is called but the object may be mutated by the calls
to callbackfn. Each entry of a map's [[MapData]] is
only visited once. New keys added after the call to
forEach begins are visited. A key will be
revisited if it is deleted after it has been visited and then
re-added before the forEach call completes. Keys
that are deleted after the call to forEach begins
and before being visited are not visited unless the key is
added again before the forEach call completes.
The initial value of the @@iterator property is the same
function object
as the initial value of the "entries" property.
23.1.3.13Map.prototype [ @@toStringTag
]
The initial value of the @@toStringTag property is the String
value "Map".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
23.1.4Properties of Map Instances
Map instances are ordinary objects that inherit properties from the
Map prototype. Map instances also have a [[MapData]] internal slot.
23.1.5Map Iterator Objects
A Map Iterator is an object, that represents a specific iteration
over some specific Map instance object. There is not a named
constructor
for Map Iterator objects. Instead, map iterator objects are created
by calling certain methods of Map instance objects.
23.1.5.1CreateMapIterator (
map, kind )
Several methods of Map objects return Iterator objects. The
abstract operation CreateMapIterator with arguments
map and kind is used to create such iterator
objects. It performs the following steps:
The initial value of the @@toStringTag property is the String
value "Map Iterator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
23.1.5.3Properties of Map Iterator
Instances
Map Iterator instances are ordinary objects that inherit
properties from the
%MapIteratorPrototype%
intrinsic object. Map Iterator instances are initially created
with the internal slots described in
Table 64.
Table 64: Internal Slots of Map Iterator Instances
Internal Slot
Description
[[IteratedMap]]
The Map object that is being iterated.
[[MapNextIndex]]
The
integer index
of the next [[MapData]] element to be examined by this
iterator.
[[MapIterationKind]]
A String value that identifies what is returned for each
element of the iteration. The possible values are:
key,
value,
key+value.
23.2Set Objects
Set objects are collections of ECMAScript language values. A distinct
value may only occur once as an element of a Set's collection.
Distinct values are discriminated using the
SameValueZero
comparison algorithm.
Set objects must be implemented using either hash tables or other
mechanisms that, on average, provide access times that are sublinear
on the number of elements in the collection. The data structures used
in this Set objects specification is only intended to describe the
required observable semantics of Set objects. It is not intended to be
a viable implementation model.
is the initial value of the "Set" property of
the
global object.
creates and initializes a new Set object when called as a
constructor.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Set behaviour must include a super call
to the Setconstructor
to create and initialize the subclass instance with the internal
state necessary to support the Set.prototype built-in
methods.
23.2.1.1Set ( [ iterable ]
)
When the Set function is called with optional
argument iterable, the following steps are taken:
If NewTarget is undefined, throw a
TypeError exception.
The initial value of Set.prototype is the intrinsic
%SetPrototype%
object.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
23.2.2.2get Set [ @@species ]
Set[@@species] is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Return the this value.
The value of the "name" property of this
function is "get [Symbol.species]".
Note
Methods that create derived collection objects should call
@@species to determine the
constructor
to use to create the derived objects. Subclass
constructor
may over-ride @@species to change the default
constructor
assignment.
23.2.3Properties of the Set Prototype
Object
The Set prototype object:
is the intrinsic object %SetPrototype%.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
Replace the element of entries whose value is
e with an element whose value is
empty.
Return undefined.
Note
The existing [[SetData]]
List
is preserved because there may be existing Set Iterator
objects that are suspended midway through iterating over that
List.
23.2.3.3Set.prototype.constructor
The initial value of Set.prototype.constructor is
%Set%.
If e is not empty and
SameValueZero(e, value) is
true, then
Replace the element of entries whose value
is e with an element whose value is
empty.
Return true.
Return false.
Note
The value empty is used as a
specification device to indicate that an entry has been
deleted. Actual implementations may take other actions such as
physically removing the entry from internal data structures.
callbackfn should be a function that accepts three
arguments. forEach calls
callbackfn once for each value present in the set
object, in value insertion order. callbackfn is
called only for values of the Set which actually exist; it is
not called for keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used
as the this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the
first two arguments are a value contained in the Set. The same
value is passed for both arguments. The Set object being
traversed is passed as the third argument.
The callbackfn is called with three arguments to be
consistent with the call back functions used by
forEach methods for Map and Array. For Sets, each
item value is considered to be both the key and the value.
forEach does not directly mutate the object on
which it is called but the object may be mutated by the calls
to callbackfn.
Each value is normally visited only once. However, a value
will be revisited if it is deleted after it has been visited
and then re-added before the forEach call
completes. Values that are deleted after the call to
forEach begins and before being visited are not
visited unless the value is added again before the
forEach call completes. New values added after
the call to forEach begins are visited.
The initial value of the @@iterator property is the same
function object
as the initial value of the "values" property.
23.2.3.12Set.prototype [ @@toStringTag
]
The initial value of the @@toStringTag property is the String
value "Set".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
23.2.4Properties of Set Instances
Set instances are ordinary objects that inherit properties from the
Set prototype. Set instances also have a [[SetData]] internal slot.
23.2.5Set Iterator Objects
A Set Iterator is an ordinary object, with the structure defined
below, that represents a specific iteration over some specific Set
instance object. There is not a named
constructor
for Set Iterator objects. Instead, set iterator objects are created
by calling certain methods of Set instance objects.
23.2.5.1CreateSetIterator (
set, kind )
Several methods of Set objects return Iterator objects. The
abstract operation CreateSetIterator with arguments
set and kind is used to create such iterator
objects. It performs the following steps:
The initial value of the @@toStringTag property is the String
value "Set Iterator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
23.2.5.3Properties of Set Iterator
Instances
Set Iterator instances are ordinary objects that inherit
properties from the
%SetIteratorPrototype%
intrinsic object. Set Iterator instances are initially created
with the internal slots specified in
Table 65.
Table 65: Internal Slots of Set Iterator Instances
Internal Slot
Description
[[IteratedSet]]
The Set object that is being iterated.
[[SetNextIndex]]
The
integer index
of the next [[SetData]] element to be examined by this
iterator.
[[SetIterationKind]]
A String value that identifies what is returned for each
element of the iteration. The possible values are
value and
key+value.
23.3WeakMap Objects
WeakMap objects are collections of key/value pairs where the keys are
objects and values may be arbitrary ECMAScript language values. A
WeakMap may be queried to see if it contains a key/value pair with a
specific key, but no mechanism is provided for enumerating the objects
it holds as keys. If an object that is being used as the key of a
WeakMap key/value pair is only reachable by following a chain of
references that start within that WeakMap, then that key/value pair is
inaccessible and is automatically removed from the WeakMap. WeakMap
implementations must detect and remove such key/value pairs and any
associated resources.
An implementation may impose an arbitrarily determined latency between
the time a key/value pair of a WeakMap becomes inaccessible and the
time when the key/value pair is removed from the WeakMap. If this
latency was observable to ECMAScript program, it would be a source of
indeterminacy that could impact program execution. For that reason, an
ECMAScript implementation must not provide any means to observe a key
of a WeakMap that does not require the observer to present the
observed key.
WeakMap objects must be implemented using either hash tables or other
mechanisms that, on average, provide access times that are sublinear
on the number of key/value pairs in the collection. The data structure
used in this WeakMap objects specification are only intended to
describe the required observable semantics of WeakMap objects. It is
not intended to be a viable implementation model.
Note
WeakMap and WeakSets are intended to provide mechanisms for
dynamically associating state with an object in a manner that does
not “leak” memory resources if, in the absence of the WeakMap or
WeakSet, the object otherwise became inaccessible and subject to
resource reclamation by the implementation's garbage collection
mechanisms. This characteristic can be achieved by using an
inverted per-object mapping of weak map instances to keys.
Alternatively each weak map may internally store its key to value
mappings but this approach requires coordination between the
WeakMap or WeakSet implementation and the garbage collector. The
following references describe mechanism that may be useful to
implementations of WeakMap and WeakSets:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In
Proceedings of the 12th ACM SIGPLAN conference on
Object-oriented programming, systems, languages, and
applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183,
http://doi.acm.org/10.1145/263698.263733.
is the initial value of the "WeakMap" property
of the
global object.
creates and initializes a new WeakMap object when called as a
constructor.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
WeakMap behaviour must include a
super call to the WeakMapconstructor
to create and initialize the subclass instance with the internal
state necessary to support the
WeakMap.prototype built-in methods.
23.3.1.1WeakMap ( [
iterable ] )
When the WeakMap function is called with optional
argument iterable, the following steps are taken:
If NewTarget is undefined, throw a
TypeError exception.
If the parameter iterable is present, it is
expected to be an object that implements an @@iterator method
that returns an iterator object that produces a two element
array-like object
whose first element is a value that will be used as a WeakMap
key and whose second element is the value to associate with
that key.
For each
Record
{ [[Key]], [[Value]] } p that is an element of
entries, do
If p.[[Key]] is not
empty and
SameValue(p.[[Key]], key) is
true, then
Set p.[[Key]] to
empty.
Set p.[[Value]] to
empty.
Return true.
Return false.
Note
The value empty is used as a
specification device to indicate that an entry has been
deleted. Actual implementations may take other actions such as
physically removing the entry from internal data structures.
Let entries be the
List
that is M.[[WeakMapData]].
If
Type(key) is not Object, throw a
TypeError exception.
For each
Record
{ [[Key]], [[Value]] } p that is an element of
entries, do
If p.[[Key]] is not
empty and
SameValue(p.[[Key]], key) is
true, then
Set p.[[Value]] to value.
Return M.
Let p be the
Record
{ [[Key]]: key, [[Value]]: value }.
Append p as the last element of entries.
Return M.
23.3.3.6WeakMap.prototype [
@@toStringTag ]
The initial value of the @@toStringTag property is the String
value "WeakMap".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
23.3.4Properties of WeakMap Instances
WeakMap instances are ordinary objects that inherit properties from
the WeakMap prototype. WeakMap instances also have a [[WeakMapData]]
internal slot.
23.4WeakSet Objects
WeakSet objects are collections of objects. A distinct object may only
occur once as an element of a WeakSet's collection. A WeakSet may be
queried to see if it contains a specific object, but no mechanism is
provided for enumerating the objects it holds. If an object that is
contained by a WeakSet is only reachable by following a chain of
references that start within that WeakSet, then that object is
inaccessible and is automatically removed from the WeakSet. WeakSet
implementations must detect and remove such objects and any associated
resources.
An implementation may impose an arbitrarily determined latency between
the time an object contained in a WeakSet becomes inaccessible and the
time when the object is removed from the WeakSet. If this latency was
observable to ECMAScript program, it would be a source of
indeterminacy that could impact program execution. For that reason, an
ECMAScript implementation must not provide any means to determine if a
WeakSet contains a particular object that does not require the
observer to present the observed object.
WeakSet objects must be implemented using either hash tables or other
mechanisms that, on average, provide access times that are sublinear
on the number of elements in the collection. The data structure used
in this WeakSet objects specification is only intended to describe the
required observable semantics of WeakSet objects. It is not intended
to be a viable implementation model.
is the initial value of the "WeakSet" property
of the
global object.
creates and initializes a new WeakSet object when called as a
constructor.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
WeakSet behaviour must include a
super call to the WeakSetconstructor
to create and initialize the subclass instance with the internal
state necessary to support the
WeakSet.prototype built-in methods.
23.4.1.1WeakSet ( [
iterable ] )
When the WeakSet function is called with optional
argument iterable, the following steps are taken:
If NewTarget is undefined, throw a
TypeError exception.
Let entries be the
List
that is S.[[WeakSetData]].
For each e that is an element of
entries, do
If e is not empty and
SameValue(e, value) is
true, then
Replace the element of entries whose value
is e with an element whose value is
empty.
Return true.
Return false.
Note
The value empty is used as a
specification device to indicate that an entry has been
deleted. Actual implementations may take other actions such as
physically removing the entry from internal data structures.
If e is not empty and
SameValue(e, value) is
true, return true.
Return false.
23.4.3.5WeakSet.prototype [
@@toStringTag ]
The initial value of the @@toStringTag property is the String
value "WeakSet".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
23.4.4Properties of WeakSet Instances
WeakSet instances are ordinary objects that inherit properties from
the WeakSet prototype. WeakSet instances also have a [[WeakSetData]]
internal slot.
The abstract operation AllocateArrayBuffer with arguments
constructor and byteLength is used to create
an ArrayBuffer object. It performs the following steps:
Let obj be ? OrdinaryCreateFromConstructor(constructor,
"%ArrayBuffer.prototype%", «
[[ArrayBufferData]], [[ArrayBufferByteLength]],
[[ArrayBufferDetachKey]] »).
Detaching an ArrayBuffer instance disassociates the
Data Block
used as its backing store from the instance and sets the byte
length of the buffer to 0. No operations defined by this
specification use the DetachArrayBuffer abstract operation.
However, an ECMAScript implementation or host environment may
define such operations.
The abstract operation CloneArrayBuffer takes four parameters, an
ArrayBuffer srcBuffer, an
integer
offset srcByteOffset, an
integer
length srcLength, and a
constructor
function cloneConstructor. It creates a new ArrayBuffer
whose data is a copy of srcBuffer's data over the range
starting at srcByteOffset and continuing for
srcLength bytes. This operation performs the following
steps:
Assert:
Type(srcBuffer) is Object and it has an
[[ArrayBufferData]] internal slot.
The abstract operation IsUnsignedElementType verifies if the
argument type is an unsigned TypedArray element type.
This operation performs the following steps:
If type is Uint8,
Uint8C, Uint16,
Uint32, or
BigUint64, return
true.
Return false.
24.1.1.6IsUnclampedIntegerElementType
( type )
The abstract operation IsUnclampedIntegerElementType verifies if
the argument type is an
Integer
TypedArray element type not including
Uint8C. This operation performs the
following steps:
If type is Int8,
Uint8, Int16,
Uint16, Int32,
or Uint32, return
true.
Return false.
24.1.1.7IsBigIntElementType (
type )
The abstract operation IsBigIntElementType verifies if the
argument type is a BigInt TypedArray element type. This
operation performs the following steps:
If type is BigUint64 or
BigInt64, return
true.
Return false.
24.1.1.8IsNoTearConfiguration (
type, order )
The abstract operation IsNoTearConfiguration with arguments
type and order performs the following steps:
The abstract operation RawBytesToNumeric takes three parameters, a
TypedArray element type type, a
ListrawBytes, and a Boolean isLittleEndian. This
operation performs the following steps:
Let elementSize be the Element Size value specified
in
Table 63
for Element Type type.
If isLittleEndian is false,
reverse the order of the elements of rawBytes.
If type is Float32, then
Let value be the byte elements of
rawBytes concatenated and interpreted as a
little-endian bit string encoding of an IEEE 754-2008
binary32 value.
If value is an IEEE 754-2008 binary32 NaN
value, return the NaNNumber value.
Return the
Number value
that corresponds to value.
If type is Float64, then
Let value be the byte elements of
rawBytes concatenated and interpreted as a
little-endian bit string encoding of an IEEE 754-2008
binary64 value.
If value is an IEEE 754-2008 binary64 NaN
value, return the NaNNumber value.
Return the
Number value
that corresponds to value.
Let intValue be the byte elements of
rawBytes concatenated and interpreted as a bit
string encoding of an unsigned little-endian binary
number.
Else,
Let intValue be the byte elements of
rawBytes concatenated and interpreted as a bit
string encoding of a binary little-endian 2's complement
number of bit length elementSize × 8.
If ! IsBigIntElementType(type) is true, return the
BigInt value that corresponds to intValue.
Otherwise, return the
Number value
that corresponds to intValue.
The abstract operation GetValueFromBuffer takes six parameters, an
ArrayBuffer or SharedArrayBuffer arrayBuffer, an
integerbyteIndex, a TypedArray element type type, a
Boolean isTypedArray, order which is one of
(SeqCst, Unordered),
and optionally a Boolean isLittleEndian. This operation
performs the following steps:
Let eventList be the [[EventList]] field of the
element in execution.[[EventsRecords]] whose
[[AgentSignifier]] is
AgentSignifier().
If isTypedArray is true and
IsNoTearConfiguration(type, order) is
true, let noTear be
true; otherwise let
noTear be false.
Let rawValue be a
List
of length elementSize of nondeterministically
chosen byte values.
NOTE: In implementations, rawValue is the
result of a non-atomic or atomic read instruction on the
underlying hardware. The nondeterminism is a semantic
prescription of the
memory model
to describe observable behaviour of hardware with weak
consistency.
Let readEvent be
ReadSharedMemory
{ [[Order]]: order, [[NoTear]]:
noTear, [[Block]]: block,
[[ByteIndex]]: byteIndex, [[ElementSize]]:
elementSize }.
Append readEvent to eventList.
Append
Chosen Value Record
{ [[Event]]: readEvent, [[ChosenValue]]:
rawValue } to
execution.[[ChosenValues]].
Else, let rawValue be a
List
of elementSize containing, in order, the
elementSize sequence of bytes starting with
block[byteIndex].
If isLittleEndian is not present, set
isLittleEndian to the value of the [[LittleEndian]]
field of the
surrounding agent's
Agent Record.
The abstract operation NumericToRawBytes takes three parameters, a
TypedArray element type type, a BigInt or a Number
value, and a Boolean isLittleEndian. This
operation performs the following steps:
If type is Float32, then
Let rawBytes be a
List
containing the 4 bytes that are the result of converting
value to IEEE 754-2008 binary32 format using
roundTiesToEven mode. If isLittleEndian is
false, the bytes are arranged in big
endian order. Otherwise, the bytes are arranged in little
endian order. If value is
NaN, rawBytes may be set to
any implementation chosen IEEE 754-2008 binary32 format
Not-a-Number encoding. An implementation must always
choose the same encoding for each implementation
distinguishable NaN value.
Else if type is Float64,
then
Let rawBytes be a
List
containing the 8 bytes that are the IEEE 754-2008 binary64
format encoding of value. If
isLittleEndian is false, the
bytes are arranged in big endian order. Otherwise, the
bytes are arranged in little endian order. If
value is NaN,
rawBytes may be set to any implementation
chosen IEEE 754-2008 binary64 format Not-a-Number
encoding. An implementation must always choose the same
encoding for each implementation distinguishable
NaN value.
Else,
Let n be the Element Size value specified in
Table 63
for Element Type type.
Let convOp be the abstract operation named in
the Conversion Operation column in
Table 63
for Element Type type.
Let intValue be
convOp(value) treated as a
mathematical value, whether the result is a BigInt or Number.
Let rawBytes be a
List
containing the n-byte binary encoding of
intValue. If isLittleEndian is
false, the bytes are ordered in big
endian order. Otherwise, the bytes are ordered in
little endian order.
Else,
Let rawBytes be a
List
containing the n-byte binary 2's complement
encoding of intValue. If
isLittleEndian is false,
the bytes are ordered in big endian order. Otherwise,
the bytes are ordered in little endian order.
The abstract operation SetValueInBuffer takes seven parameters, an
ArrayBuffer or SharedArrayBuffer arrayBuffer, an
integerbyteIndex, a TypedArray element type type, a
Number or BigInt value, a Boolean
isTypedArray, order which is one of
(SeqCst, Unordered,
Init), and optionally a Boolean
isLittleEndian. This operation performs the following
steps:
The abstract operation GetModifySetValueInBuffer takes six
parameters, a SharedArrayBuffer arrayBuffer, a
nonnegative
integerbyteIndex, a TypedArray element type type, a
Number or BigInt value, a semantic function
op, and optionally a Boolean isLittleEndian.
This operation performs the following steps:
Let eventList be the [[EventList]] field of the
element in execution.[[EventsRecords]] whose
[[AgentSignifier]] is
AgentSignifier().
Let rawBytesRead be a
List
of length elementSize of nondeterministically
chosen byte values.
NOTE: In implementations, rawBytesRead is the
result of a load-link, of a load-exclusive, or of an operand
of a read-modify-write instruction on the underlying hardware.
The nondeterminism is a semantic prescription of the
memory model
to describe observable behaviour of hardware with weak
consistency.
Let rmwEvent be
ReadModifyWriteSharedMemory
{ [[Order]]: SeqCst, [[NoTear]]:
true, [[Block]]: block,
[[ByteIndex]]: byteIndex, [[ElementSize]]:
elementSize, [[Payload]]: rawBytes,
[[ModifyOp]]: op }.
Append rmwEvent to eventList.
Append
Chosen Value Record
{ [[Event]]: rmwEvent, [[ChosenValue]]:
rawBytesRead } to
execution.[[ChosenValues]].
is the initial value of the
"ArrayBuffer" property of the
global object.
creates and initializes a new ArrayBuffer object when called as a
constructor.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
ArrayBuffer behaviour must include a
super call to the ArrayBufferconstructor
to create and initialize subclass instances with the internal
state necessary to support the
ArrayBuffer.prototype built-in methods.
24.1.2.1ArrayBuffer (
length )
When the ArrayBuffer function is called with argument
length, the following steps are taken:
If NewTarget is undefined, throw a
TypeError exception.
If arg has a [[ViewedArrayBuffer]] internal slot,
return true.
Return false.
24.1.3.2ArrayBuffer.prototype
The initial value of ArrayBuffer.prototype is
%ArrayBuffer.prototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
24.1.3.3get ArrayBuffer [ @@species ]
ArrayBuffer[@@species] is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Return the this value.
The value of the "name" property of this
function is "get [Symbol.species]".
Note
ArrayBuffer prototype methods normally use their
this object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its
@@species property.
24.1.4Properties of the ArrayBuffer
Prototype Object
The ArrayBuffer prototype object:
is the intrinsic object %ArrayBufferPrototype%.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
is an ordinary object.
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]]
internal slot.
24.1.4.1get
ArrayBuffer.prototype.byteLength
ArrayBuffer.prototype.byteLength is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
The initial value of the @@toStringTag property is the String
value "ArrayBuffer".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
24.1.5Properties of ArrayBuffer
Instances
ArrayBuffer instances inherit properties from the ArrayBuffer
prototype object. ArrayBuffer instances each have an
[[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]]
internal slot, and an [[ArrayBufferDetachKey]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is
null are considered to be detached and all
operators to access or modify data contained in the ArrayBuffer
instance will fail.
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a
value other than undefined need to have all
DetachArrayBuffer
calls passing that same "detach key" as an argument, otherwise a
TypeError will result. This internal slot is only ever set by
certain embedding environments, not by algorithms in this
specification.
24.2SharedArrayBuffer Objects
24.2.1Abstract Operations for
SharedArrayBuffer Objects
The abstract operation AllocateSharedArrayBuffer with arguments
constructor and byteLength is used to create
a SharedArrayBuffer object. It performs the following steps:
Let obj be ? OrdinaryCreateFromConstructor(constructor,
"%SharedArrayBuffer.prototype%", «
[[ArrayBufferData]], [[ArrayBufferByteLength]] »).
is the initial value of the
"SharedArrayBuffer" property of the
global object.
creates and initializes a new SharedArrayBuffer object when called
as a
constructor.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
SharedArrayBuffer behaviour must include a
super call to the SharedArrayBufferconstructor
to create and initialize subclass instances with the internal
state necessary to support the
SharedArrayBuffer.prototype built-in methods.
Note
Unlike an ArrayBuffer, a
SharedArrayBuffer cannot become detached, and its
internal [[ArrayBufferData]] slot is never
null.
24.2.2.1SharedArrayBuffer ( [
length ] )
When the SharedArrayBuffer function is called with
optional argument length, the following steps are
taken:
If NewTarget is undefined, throw a
TypeError exception.
SharedArrayBuffer.prototype.byteLength is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
The initial value of the @@toStringTag property is the String
value "SharedArrayBuffer".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
24.2.5Properties of SharedArrayBuffer
Instances
SharedArrayBuffer instances inherit properties from the
SharedArrayBuffer prototype object. SharedArrayBuffer instances each
have an [[ArrayBufferData]] internal slot and an
[[ArrayBufferByteLength]] internal slot.
Note
SharedArrayBuffer instances, unlike ArrayBuffer instances, are
never detached.
24.3DataView Objects
24.3.1Abstract Operations For DataView
Objects
24.3.1.1GetViewValue (
view, requestIndex,
isLittleEndian, type )
The abstract operation GetViewValue with arguments
view, requestIndex,
isLittleEndian, and type is used by
functions on DataView instances to retrieve values from the view's
buffer. It performs the following steps:
24.3.1.2SetViewValue (
view, requestIndex,
isLittleEndian, type, value )
The abstract operation SetViewValue with arguments
view, requestIndex,
isLittleEndian, type, and
value is used by functions on DataView instances to
store values into the view's buffer. It performs the following
steps:
is the initial value of the "DataView" property
of the
global object.
creates and initializes a new DataView object when called as a
constructor.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
DataView behaviour must include a
super call to the DataViewconstructor
to create and initialize subclass instances with the internal
state necessary to support the
DataView.prototype built-in methods.
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
24.3.4.2get
DataView.prototype.byteLength
DataView.prototype.byteLength is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
If
IsDetachedBuffer(buffer) is true, throw a
TypeError exception.
Let size be O.[[ByteLength]].
Return size.
24.3.4.3get
DataView.prototype.byteOffset
DataView.prototype.byteOffset is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
The initial value of the @@toStringTag property is the String
value "DataView".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
24.3.5Properties of DataView Instances
DataView instances are ordinary objects that inherit properties from
the DataView prototype object. DataView instances each have
[[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and
[[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within
this specification. The simple presence of that internal slot is
used within the specification to identify objects created using
the DataViewconstructor.
24.4The Atomics Object
The Atomics object:
is the intrinsic object %Atomics%.
is the initial value of the "Atomics" property of
the
global object.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
does not have a [[Construct]] internal method; it cannot be used as
a
constructor
with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a
function.
The Atomics object provides functions that operate indivisibly
(atomically) on shared memory array cells as well as functions that
let agents wait for and dispatch primitive events. When used with
discipline, the Atomics functions allow multi-agent
programs that communicate through shared memory to execute in a
well-understood order even on parallel CPUs. The rules that govern
shared-memory communication are provided by the
memory model, defined below.
Note
For informative guidelines for programming and implementing shared
memory in ECMAScript, please see the notes at the end of the
memory model
section.
The abstract operation ValidateSharedIntegerTypedArray takes one
argument typedArray and an optional Boolean
waitable. It performs the following steps:
If waitable is not present, set
waitable to false.
If accessIndex ≥ length, throw a
RangeError exception.
Return accessIndex.
24.4.1.3GetWaiterList (
block, i )
A WaiterList is a semantic object that contains an
ordered list of those agents that are waiting on a location
(block, i) in shared memory;
block is a
Shared Data Block
and i a byte offset into the memory of
block. A WaiterList object also optionally contains a
Synchronize event
denoting the previous leaving of its critical section.
Initially a WaiterList object has an empty list and no
Synchronize event.
The
agent cluster
has a store of WaiterList objects; the store is indexed by
(block, i). WaiterLists are
agent-independent: a lookup in the store of WaiterLists by
(block, i) will result in the same
WaiterList object in any
agent
in the
agent cluster.
Each WaiterList has a critical section that controls
exclusive access to that WaiterList during evaluation. Only a
single
agent
may enter a WaiterList's critical section at one time. Entering
and leaving a WaiterList's critical section is controlled by the
abstract operationsEnterCriticalSection
and
LeaveCriticalSection. Operations on a WaiterList—adding and removing waiting agents,
traversing the list of agents, suspending and notifying agents on
the list, setting and retrieving the
Synchronize event—may only be performed by agents that have entered the
WaiterList's critical section.
The abstract operation GetWaiterList takes two arguments, a
Shared Data Blockblock and a nonnegative
integeri. It performs the following steps:
Append (leaveEvent, enterEvent) to
eventsRecord.[[AgentSynchronizesWith]].
EnterCriticalSection has contention when an
agent
attempting to enter the
critical section
must wait for another
agent
to leave it. When there is no contention, FIFO order of
EnterCriticalSection calls is observable. When there is
contention, an implementation may choose an arbitrary order but
may not cause an
agent
to wait indefinitely.
24.4.1.5LeaveCriticalSection (
WL )
The abstract operation LeaveCriticalSection takes one argument, a
WaiterListWL. It performs the following steps:
The abstract operation Suspend takes three arguments, a
WaiterListWL, an
agent
signifier W, and a nonnegative, non-NaN
Number timeout. It performs the following steps:
Perform
LeaveCriticalSection(WL) and suspend W for up to
timeout milliseconds, performing the combined
operation in such a way that a notification that arrives after
the
critical section
is exited but before the suspension takes effect is not lost.
W can notify either because the timeout expired or
because it was notified explicitly by another
agent
calling
NotifyWaiter(WL, W), and not for any other reasons
at all.
The embedding may delay notifying W, e.g. for
resource management reasons, but W must eventually
be notified in order to guarantee forward progress.
24.4.1.11AtomicReadModifyWrite (
typedArray, index, value,
op )
The abstract operation AtomicReadModifyWrite takes four arguments,
typedArray, index, value, and a
pure combining operation op. The pure combining
operation op takes two
List
of byte values arguments and returns a
List
of byte values. The operation atomically loads a value, combines
it with another value, and stores the result of the combination.
It returns the loaded value. It performs the following steps:
The abstract operation AtomicLoad takes two arguments,
typedArray, index. The operation atomically
loads a value and returns the loaded value. It performs the
following steps:
Let add denote a semantic function of two
List
of byte values arguments that applies the addition operation to the
Number values corresponding to the
List
of byte values arguments and returns a
List
of byte values corresponding to the result of that operation.
Let and denote a semantic function of two
List
of byte values arguments that applies the bitwise-and operation
element-wise to the two arguments and returns a
List
of byte values corresponding to the result of that operation.
Let expectedBytes be
NumericToRawBytes(elementType, expected,
isLittleEndian).
Let elementSize be the Element Size value specified
in
Table 63
for arrayTypeName.
Let offset be typedArray.[[ByteOffset]].
Let indexedPosition be (i ×
elementSize) + offset.
Let compareExchange denote a semantic function of
two
List
of byte values arguments that returns the second argument if the
first argument is element-wise equal to
expectedBytes.
Atomics.isLockFree() is an optimization primitive.
The intuition is that if the atomic step of an atomic primitive
(compareExchange, load,
store, add, sub,
and, or, xor, or
exchange) on a datum of size n bytes
will be performed without the calling
agent
acquiring a lock outside the n bytes comprising the
datum, then Atomics.isLockFree(n) will
return true. High-performance algorithms will
use Atomics.isLockFree to determine whether to use
locks or atomic operations in critical sections. If an atomic
primitive is not lock-free then it is often more efficient for
an algorithm to provide its own locking.
Atomics.isLockFree(4) always returns
true as that can be supported on all known
relevant hardware. Being able to assume this will generally
simplify programs.
Regardless of the value of Atomics.isLockFree, all
atomic operations are guaranteed to be atomic. For example, they
will never have a visible operation take place in the middle of
the operation (e.g., "tearing").
Let or denote a semantic function of two
List
of byte values arguments that applies the bitwise-or operation
element-wise to the two arguments and returns a
List
of byte values corresponding to the result of that operation.
Let subtract denote a semantic function of two
List
of byte values arguments that applies the subtraction operation to
the Number values corresponding to the
List
of byte values arguments and returns a
List
of byte values corresponding to the result of that operation.
Let xor denote a semantic function of two
List
of byte values arguments that applies the bitwise-xor operation
element-wise to the two arguments and returns a
List
of byte values corresponding to the result of that operation.
The initial value of the @@toStringTag property is the String value
"Atomics".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.5The JSON Object
The JSON object:
is the intrinsic object %JSON%.
is the initial value of the "JSON" property of
the
global object.
is an ordinary object.
contains two functions, parse and
stringify, that are used to parse and construct JSON
texts.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
does not have a [[Construct]] internal method; it cannot be used as
a
constructor
with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a
function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON
interchange format used in this specification is exactly that
described by ECMA-404. Conforming implementations of
JSON.parse and JSON.stringify must support
the exact interchange format described in the ECMA-404 specification
without any deletions or extensions to the format.
24.5.1JSON.parse ( text [ ,
reviver ] )
The parse function parses a JSON text (a JSON-formatted
String) and produces an ECMAScript value. The JSON format represents
literals, arrays, and objects with a syntax similar to the syntax
for ECMAScript literals, Array Initializers, and Object
Initializers. After parsing, JSON objects are realized as ECMAScript
objects. JSON arrays are realized as ECMAScript Array instances.
JSON strings, numbers, booleans, and null are realized as ECMAScript
Strings, Numbers, Booleans, and null.
The optional reviver parameter is a function that takes
two parameters, key and value. It can filter
and transform the results. It is called with each of the
key/value pairs produced by the parse, and its
return value is used instead of the original value. If it returns
what it received, the structure is not modified. If it returns
undefined then the property is deleted from the
result.
Parse JText interpreted as UTF-16 encoded Unicode
points (6.1.4) as a JSON text as specified in ECMA-404. Throw a
SyntaxError exception if JText is
not a valid JSON text as defined in that specification.
Let completion be the result of parsing and
evaluating scriptText as if it was the source text of
an ECMAScript
Script. The extended PropertyDefinitionEvaluation semantics defined
in
B.3.1
must not be used during the evaluation.
Let unfiltered be completion.[[Value]].
Assert: unfiltered is either a String, Number, Boolean,
Null, or an Object that is defined by either an
ArrayLiteral
or an
ObjectLiteral.
This function is the %JSONParse% intrinsic object.
The "length" property of the
parse function is 2.
Note
Valid JSON text is a subset of the ECMAScript
PrimaryExpression
syntax as modified by Step 4 above. Step 2 verifies that
JText conforms to that subset, and step 6 verifies
that that parsing and evaluation returns a value of an
appropriate type.
24.5.1.1Runtime Semantics:
InternalizeJSONProperty ( holder, name )
The abstract operation InternalizeJSONProperty is a recursive
abstract operation that takes two parameters: a
holder object and the String name of a
property in that object. InternalizeJSONProperty uses the value of
reviver that was originally passed to the above parse
function.
Note 1
This algorithm intentionally does not throw an exception if
either [[Delete]] or
CreateDataProperty
return false.
It is not permitted for a conforming implementation of
JSON.parse to extend the JSON grammars. If an
implementation wishes to support a modified or extended JSON
interchange format it must do so by defining a different parse
function.
Note 2
In the case where there are duplicate name Strings within an
object, lexically preceding values for the same key shall be
overwritten.
24.5.2JSON.stringify (
value [ , replacer [ , space ] ] )
The stringify function returns a String in UTF-16
encoded JSON format representing an ECMAScript value, or
undefined. It can take three parameters. The
value parameter is an ECMAScript value, which is usually
an object or array, although it can also be a String, Boolean,
Number or null. The optional
replacer parameter is either a function that alters the
way objects and arrays are stringified, or an array of Strings and
Numbers that acts as an inclusion list for selecting the object
properties that will be stringified. The optional
space parameter is a String or Number that allows the
result to have white space injected into it to improve human
readability.
This function is the %JSONStringify% intrinsic object.
The "length" property of the
stringify function is 3.
Note 1
JSON structures are allowed to be nested to any depth, but they
must be acyclic. If value is or contains a cyclic
structure, then the stringify function must throw a
TypeError exception. This is an example of a
value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The null value is rendered in JSON text as
the String "null".
The undefined value is not rendered.
The true value is rendered in JSON text as
the String "true".
The false value is rendered in JSON text as
the String "false".
Note 3
String values are wrapped in QUOTATION MARK (")
code units. The code units " and \ are
escaped with \ prefixes. Control characters code
units are replaced with escape sequences \uHHHH, or
with the shorter forms, \b (BACKSPACE),
\f (FORM FEED), \n (LINE FEED),
\r (CARRIAGE RETURN), \t (CHARACTER
TABULATION).
Note 4
Finite numbers are stringified as if by calling
ToString(number). NaN and
Infinity regardless of sign are represented
as the String "null".
Note 5
Values that do not have a JSON representation (such as
undefined and functions) do not produce a
String. Instead they produce the
undefined value. In arrays these values are
represented as the String "null". In objects
an unrepresentable value causes the property to be excluded from
stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by
zero or more properties, separated with a U+002C (COMMA), closed
with a U+007D (RIGHT CURLY BRACKET). A property is a quoted
String representing the key or
property name, a U+003A (COLON), and then the stringified property value. An
array is rendered as an opening U+005B (LEFT SQUARE BRACKET
followed by zero or more values, separated with a U+002C
(COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).
The abstract operation SerializeJSONProperty with arguments
key, and holder has access to
ReplacerFunction from the invocation of the
stringify method. Its algorithm is as follows:
24.5.2.2Runtime Semantics:
QuoteJSONString ( value )
The abstract operation QuoteJSONString with argument
value wraps a String value in QUOTATION MARK code units
and escapes certain other code units within it.
This operation interprets a String value as a sequence of UTF-16
encoded code points, as described in
6.1.4.
Let product be the String value consisting solely
of the code unit 0x0022 (QUOTATION MARK).
Let cpList be a
List
containing in order the code points of value when
interpreted as a sequence of UTF-16 encoded code points as
described in
6.1.4.
For each code point C in cpList, do
If C is listed in the “Code Point” column of
Table 66, then
Set product to the
string-concatenation
of product and the escape sequence for
C as specified in the “Escape Sequence”
column of the corresponding row.
Else if C has a numeric value less than 0x0020
(SPACE), or if C has the same numeric value as
a
leading surrogate
or
trailing surrogate, then
Let unit be the code unit whose numeric
value is that of C.
the String representation of n, formatted as a
four-digit lowercase hexadecimal number, padded to the
left with zeroes if necessary
24.5.2.4Runtime Semantics:
SerializeJSONObject ( value )
The abstract operation SerializeJSONObject with argument
value serializes an object. It has access to the
stack, indent, gap, and
PropertyList values of the current invocation of the
stringify method.
If stack contains value, throw a
TypeError exception because the structure
is cyclical.
Let properties be the String value formed
by concatenating all the element Strings of
partial with each adjacent pair of Strings
separated with the code unit 0x002C (COMMA). A comma
is not inserted either before the first String or
after the last String.
Let separator be the
string-concatenation
of the code unit 0x002C (COMMA), the code unit 0x000A
(LINE FEED), and indent.
Let properties be the String value formed
by concatenating all the element Strings of
partial with each adjacent pair of Strings
separated with separator. The
separator String is not inserted either
before the first String or after the last String.
Let final be the
string-concatenation
of "{", the code unit 0x000A (LINE
FEED), indent, properties, the
code unit 0x000A (LINE FEED), stepback, and
"}".
Remove the last element of stack.
Set indent to stepback.
Return final.
24.5.2.5Runtime Semantics:
SerializeJSONArray ( value )
The abstract operation SerializeJSONArray with argument
value serializes an array. It has access to the
stack, indent, and gap values of
the current invocation of the stringify method.
If stack contains value, throw a
TypeError exception because the structure
is cyclical.
Let properties be the String value formed
by concatenating all the element Strings of
partial with each adjacent pair of Strings
separated with the code unit 0x002C (COMMA). A comma
is not inserted either before the first String or
after the last String.
Let separator be the
string-concatenation
of the code unit 0x002C (COMMA), the code unit 0x000A
(LINE FEED), and indent.
Let properties be the String value formed
by concatenating all the element Strings of
partial with each adjacent pair of Strings
separated with separator. The
separator String is not inserted either
before the first String or after the last String.
Let final be the
string-concatenation
of "[", the code unit 0x000A (LINE
FEED), indent, properties, the
code unit 0x000A (LINE FEED), stepback, and
"]".
Remove the last element of stack.
Set indent to stepback.
Return final.
Note
The representation of arrays includes only the elements
between zero and
array.length - 1
inclusive. Properties whose keys are not
array indexes
are excluded from the stringification. An array is stringified
as an opening LEFT SQUARE BRACKET, elements separated by
COMMA, and a closing RIGHT SQUARE BRACKET.
24.5.3JSON [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"JSON".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
25Control Abstraction Objects
25.1Iteration
25.1.1Common Iteration Interfaces
An interface is a set of property keys whose associated values match
a specific specification. Any object that provides all the
properties as described by an interface's specification
conforms to that interface. An interface is not represented
by a distinct object. There may be many separately implemented
objects that conform to any interface. An individual object may
conform to multiple interfaces.
25.1.1.1The Iterable Interface
The Iterable interface includes the property described in
Table 67:
Table 67: Iterable Interface Required Properties
Property
Value
Requirements
@@iterator
A function that returns an Iterator object.
The returned object must conform to the
Iterator interface.
25.1.1.2The Iterator Interface
An object that implements the Iterator interface must
include the property in
Table 68. Such objects may also implement the properties in
Table 69.
Table 68: Iterator Interface Required Properties
Property
Value
Requirements
"next"
A function that returns an IteratorResult object.
The returned object must conform to the
IteratorResult interface. If a previous call to
the next method of an Iterator has
returned an IteratorResult object whose
"done" property is
true, then all subsequent calls to
the next method of that object should also
return an IteratorResult object whose
"done" property is
true. However, this requirement is
not enforced.
Note 1
Arguments may be passed to the next function but their
interpretation and validity is dependent upon the target
Iterator. The for-of statement and other common users
of Iterators do not pass any arguments, so
Iterator objects that expect to be used in such a
manner must be prepared to deal with being called with no
arguments.
Table 69: Iterator Interface Optional Properties
Property
Value
Requirements
"return"
A function that returns an IteratorResult object.
The returned object must conform to the
IteratorResult interface. Invoking this method
notifies the Iterator object that the caller does
not intend to make any more next method
calls to the Iterator. The returned
IteratorResult object will typically have a
"done" property whose value is
true, and a
"value" property with the value
passed as the argument of the
return method. However, this requirement is
not enforced.
"throw"
A function that returns an IteratorResult object.
The returned object must conform to the
IteratorResult interface. Invoking this method
notifies the Iterator object that the caller has
detected an error condition. The argument may be used to
identify the error condition and typically will be an
exception object. A typical response is to
throw the value passed as the argument. If
the method does not throw, the returned
IteratorResult object will typically have a
"done" property whose value is
true.
Note 2
Typically callers of these methods should check for their
existence before invoking them. Certain ECMAScript language
features including for-of,
yield*, and array destructuring call these
methods after performing an existence check. Most ECMAScript
library functions that accept Iterable objects as
arguments also conditionally call them.
25.1.1.3The
AsyncIterable Interface
The AsyncIterable interface includes the properties
described in
Table 70:
The returned object must conform to the
AsyncIterator interface.
25.1.1.4The
AsyncIterator Interface
An object that implements the AsyncIterator interface must
include the properties in
Table 71. Such objects may also implement the properties in
Table 72.
A function that returns a promise for an
IteratorResult object.
The returned promise, when fulfilled, must fulfill
with an object which conforms to the
IteratorResult interface. If a previous call to
the next method of an
AsyncIterator has returned a promise for an
IteratorResult object whose
"done" property is
true, then all subsequent calls to
the next method of that object should
also return a promise for an
IteratorResult object whose
"done" property is
true. However, this requirement is
not enforced.
Additionally, the IteratorResult object that
serves as a fulfillment value should have a
"value" property whose value is not
a promise (or "thenable"). However, this requirement
is also not enforced.
Note 1
Arguments may be passed to the next function but their
interpretation and validity is dependent upon the target
AsyncIterator. The
for-await-of statement
and other common users of AsyncIterators do not pass
any arguments, so AsyncIterator objects that expect to
be used in such a manner must be prepared to deal with being
called with no arguments.
A function that returns a promise for an
IteratorResult object.
The returned promise, when fulfilled, must fulfill
with an object which conforms to the
IteratorResult interface. Invoking this method
notifies the AsyncIterator object that the
caller does not intend to make any more
next method calls to the
AsyncIterator. The returned promise will
fulfill with an IteratorResult object which
will typically have a
"done" property whose value is
true, and a
"value" property with the value
passed as the argument of the
return method. However, this requirement
is not enforced.
Additionally, the IteratorResult object that
serves as a fulfillment value should have a
"value" property whose value is not
a promise (or "thenable"). If the argument value is
used in the typical manner, then if it is a rejected
promise, a promise rejected with the same reason
should be returned; if it is a fulfilled promise, then
its fulfillment value should be used as the
"value" property of the returned
promise's IteratorResult object fulfillment
value. However, these requirements are also not
enforced.
"throw"
A function that returns a promise for an
IteratorResult object.
The returned promise, when fulfilled, must fulfill
with an object which conforms to the
IteratorResult interface. Invoking this method
notifies the AsyncIterator object that the
caller has detected an error condition. The argument
may be used to identify the error condition and
typically will be an exception object. A typical
response is to return a rejected promise which rejects
with the value passed as the argument.
If the returned promise is fulfilled, the
IteratorResult fulfillment value will typically
have a "done" property whose value
is true. Additionally, it should
have a "value" property whose value
is not a promise (or "thenable"), but this requirement
is not enforced.
Note 2
Typically callers of these methods should check for their
existence before invoking them. Certain ECMAScript language
features including for-await-of
and yield* call these methods after performing an
existence check.
25.1.1.5The
IteratorResult Interface
The IteratorResult interface includes the properties listed
in
Table 73:
Table 73: IteratorResult Interface Properties
Property
Value
Requirements
"done"
Either true or
false.
This is the result status of an iteratornext method call. If the end of the
iterator was reached "done" is
true. If the end was not reached
"done" is
false and a value is available. If a
"done" property (either own or
inherited) does not exist, it is consider to have the
value false.
If done is false, this is the current
iteration element value. If done is
true, this is the return value of the
iterator, if it supplied one. If the iterator does not
have a return value, "value" is
undefined. In that case, the
"value" property may be absent from
the conforming object if it does not inherit an explicit
"value" property.
25.1.2The %IteratorPrototype% Object
The %IteratorPrototype% object:
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
is an ordinary object.
Note
All objects defined in this specification that implement the
Iterator interface also inherit from %IteratorPrototype%.
ECMAScript code may also define objects that inherit from
%IteratorPrototype%. The %IteratorPrototype% object provides a
place where additional methods that are applicable to all
iterator objects may be added.
The following expression is one way that ECMAScript code can
access the %IteratorPrototype% object:
The value of the "name" property of this
function is "[Symbol.iterator]".
25.1.3The %AsyncIteratorPrototype%
Object
The %AsyncIteratorPrototype% object:
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
is an ordinary object.
Note
All objects defined in this specification that implement the
AsyncIterator interface also inherit from
%AsyncIteratorPrototype%. ECMAScript code may also define
objects that inherit from %AsyncIteratorPrototype%. The
%AsyncIteratorPrototype% object provides a place where
additional methods that are applicable to all async iterator
objects may be added.
The value of the "name" property of this
function is "[Symbol.asyncIterator]".
25.1.4Async-from-Sync Iterator Objects
An Async-from-Sync Iterator object is an async iterator that adapts
a specific synchronous iterator. There is not a named
constructor
for Async-from-Sync Iterator objects. Instead, Async-from-Sync
iterator objects are created by the
CreateAsyncFromSyncIterator
abstract operation as needed.
The abstract operation CreateAsyncFromSyncIterator is used to
create an async iterator
Record
from a synchronous iterator
Record. It performs the following steps:
25.1.4.2.4Async-from-Sync Iterator
Value Unwrap Functions
An async-from-sync iterator value unwrap function is an
anonymous built-in function that is used by
AsyncFromSyncIteratorContinuation
when processing the "value" property of an
IteratorResult object, in order to wait for its value if
it is a promise and re-package the result in a new "unwrapped"
IteratorResult object. Each async-from-sync iterator
value unwrap function has a [[Done]] internal slot.
When an async-from-sync iterator value unwrap function is called
with argument value, the following steps are taken:
25.1.4.3Properties of Async-from-Sync
Iterator Instances
Async-from-Sync Iterator instances are ordinary objects that
inherit properties from the
%AsyncFromSyncIteratorPrototype%
intrinsic object. Async-from-Sync Iterator instances are initially
created with the internal slots listed in
Table 74. Async-from-Sync Iterator instances are not directly observable
from ECMAScript code.
Table 74: Internal Slots of Async-from-Sync Iterator Instances
Internal Slot
Description
[[SyncIteratorRecord]]
A
Record, of the type returned by
GetIterator, representing the original synchronous iterator which
is being adapted.
creates and initializes a new GeneratorFunction object when called
as a function rather than as a
constructor. Thus the function call GeneratorFunction (…) is
equivalent to the object creation expression
new GeneratorFunction (…) with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
GeneratorFunction behaviour must include a
super call to the GeneratorFunctionconstructor
to create and initialize subclass instances with the internal
slots necessary for built-in GeneratorFunction behaviour. All
ECMAScript syntactic forms for defining generator function objects
create direct instances of GeneratorFunction. There
is no syntactic means to create instances of
GeneratorFunction subclasses.
25.2.1.1GeneratorFunction (
p1, p2, … , pn, body )
The last argument specifies the body (executable code) of a
generator function; any preceding arguments specify formal
parameters.
When the GeneratorFunction function is called with
some arguments p1, p2, … , pn,
body (where n might be 0, that is, there are
no “p” arguments, and where body might also
not be provided), the following steps are taken:
The initial value of the @@toStringTag property is the String
value "GeneratorFunction".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
25.2.4GeneratorFunction Instances
Every GeneratorFunction instance is an ECMAScript
function object
and has the internal slots listed in
Table 29. The value of the [[IsClassConstructor]] internal slot for all
such instances is false.
Each GeneratorFunction instance has the following own properties:
25.2.4.1length
The specification for the "length" property of
Function instances given in
19.2.4.1
also applies to GeneratorFunction instances.
25.2.4.2name
The specification for the "name" property of
Function instances given in
19.2.4.2
also applies to GeneratorFunction instances.
25.2.4.3prototype
Whenever a GeneratorFunction instance is created another ordinary
object is also created and is the initial value of the generator
function's "prototype" property. The value of
the prototype property is used to initialize the [[Prototype]]
internal slot of a newly created Generator object when the
generator
function object
is invoked using [[Call]].
This property has the attributes { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
Unlike Function instances, the object that is the value of the
a GeneratorFunction's "prototype" property
does not have a "constructor" property
whose value is the GeneratorFunction instance.
creates and initializes a new AsyncGeneratorFunction object when
called as a function rather than as a
constructor. Thus the function call
AsyncGeneratorFunction (...) is equivalent to the
object creation expression
new AsyncGeneratorFunction (...) with the same
arguments.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
AsyncGeneratorFunction behaviour must include a
super call to the
AsyncGeneratorFunctionconstructor
to create and initialize subclass instances with the internal
slots necessary for built-in AsyncGeneratorFunction behaviour. All
ECMAScript syntactic forms for defining async generator function
objects create direct instances of
AsyncGeneratorFunction. There is no syntactic means
to create instances of
AsyncGeneratorFunction subclasses.
25.3.1.1AsyncGeneratorFunction (
p1, p2, … , pn, body )
The last argument specifies the body (executable code) of an async
generator function; any preceding arguments specify formal
parameters.
When the AsyncGeneratorFunction function is called
with some arguments p1, p2, … ,
pn, body (where n might be 0,
that is, there are no "p" arguments, and where
body might also not be provided), the following steps
are taken:
The initial value of the @@toStringTag property is the String
value "AsyncGeneratorFunction".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
25.3.4AsyncGeneratorFunction Instances
Every AsyncGeneratorFunction instance is an ECMAScript
function object
and has the internal slots listed in
Table 29. The value of the [[IsClassConstructor]] internal slot for all
such instances is false.
Each AsyncGeneratorFunction instance has the following own
properties:
25.3.4.1length
The value of the "length" property is an
integer
that indicates the typical number of arguments expected by the
AsyncGeneratorFunction. However, the language permits the function
to be invoked with some other number of arguments. The behaviour
of an AsyncGeneratorFunction when invoked on a number of arguments
other than the number specified by its
"length" property depends on the function.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
25.3.4.2name
The specification for the "name" property of
Function instances given in
19.2.4.2
also applies to AsyncGeneratorFunction instances.
25.3.4.3prototype
Whenever an AsyncGeneratorFunction instance is created another
ordinary object is also created and is the initial value of the
async generator function's
"prototype" property. The value of the
prototype property is used to initialize the [[Prototype]]
internal slot of a newly created AsyncGenerator object when the
generator
function object
is invoked using [[Call]].
This property has the attributes { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
Unlike function instances, the object that is the value of the
an AsyncGeneratorFunction's
"prototype" property does not have a
"constructor" property whose value is the
AsyncGeneratorFunction instance.
25.4Generator Objects
A Generator object is an instance of a generator function and conforms
to both the Iterator and Iterable interfaces.
Generator instances directly inherit properties from the object that
is the value of the "prototype" property of the
Generator function that created the instance. Generator instances
indirectly inherit properties from the Generator Prototype intrinsic,
%Generator.prototype%.
25.4.1Properties of the Generator
Prototype Object
The Generator prototype object:
is the intrinsic object %GeneratorPrototype%.
is the initial value of the
"prototype" property of
%Generator%
(the GeneratorFunction.prototype).
is an ordinary object.
is not a Generator instance and does not have a [[GeneratorState]]
internal slot.
Once a generator enters the
completed state it never leaves it
and its associated
execution context
is never resumed. Any execution state associated with
generator can be discarded at this point.
If result.[[Type]] is
normal, let
resultValue be undefined.
Else if result.[[Type]] is
return, let
resultValue be result.[[Value]].
Resume the suspended evaluation of genContext using
NormalCompletion(value) as the result of the operation that
suspended it. Let result be the value returned by
the resumed computation.
Once a generator enters the
completed state it never leaves it
and its associated
execution context
is never resumed. Any execution state associated with
generator can be discarded at this point.
Resume the suspended evaluation of genContext using
abruptCompletion as the result of the operation
that suspended it. Let result be the completion
record returned by the resumed computation.
Set the code evaluation state of genContext such
that when evaluation is resumed with a
CompletionresumptionValue the following steps will be
performed:
Return resumptionValue.
NOTE: This returns to the evaluation of the
YieldExpression
that originally called this abstract operation.
NOTE: This returns to the evaluation of the operation that had
most previously resumed evaluation of genContext.
25.5AsyncGenerator Objects
An AsyncGenerator object is an instance of an async generator function
and conforms to both the AsyncIterator and AsyncIterable interfaces.
AsyncGenerator instances directly inherit properties from the object
that is the value of the "prototype" property of
the AsyncGenerator function that created the instance. AsyncGenerator
instances indirectly inherit properties from the AsyncGenerator
Prototype intrinsic, %AsyncGenerator.prototype%.
25.5.1Properties of the AsyncGenerator
Prototype Object
The AsyncGenerator prototype object:
is the intrinsic object %AsyncGeneratorPrototype%.
is the initial value of the
"prototype" property of
%AsyncGenerator%
(the AsyncGeneratorFunction.prototype).
is an ordinary object.
is not an AsyncGenerator instance and does not have an
[[AsyncGeneratorState]] internal slot.
The initial value of the @@toStringTag property is the String
value "AsyncGenerator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
25.5.2Properties of AsyncGenerator
Instances
AsyncGenerator instances are initially created with the internal
slots described below:
Table 76: Internal Slots of AsyncGenerator Instances
Internal Slot
Description
[[AsyncGeneratorState]]
The current execution state of the async generator. The
possible values are: undefined,
suspendedStart,
suspendedYield,
executing,
awaiting-return, and
completed.
[[AsyncGeneratorContext]]
The
execution context
that is used when executing the code of this async
generator.
[[AsyncGeneratorQueue]]
A
List
of AsyncGeneratorRequest records which represent requests
to resume the async generator.
25.5.3AsyncGenerator Abstract Operations
25.5.3.1AsyncGeneratorRequest Records
The AsyncGeneratorRequest is a
Record
value used to store information about how an async generator
should be resumed and contains capabilities for fulfilling or
rejecting the corresponding promise.
Resume the suspended evaluation of genContext using
completion as the result of the operation that
suspended it. Let result be the completion record
returned by the resumed computation.
Set the code evaluation state of genContext such
that when evaluation is resumed with a
CompletionresumptionValue the following steps will be
performed:
If resumptionValue.[[Type]] is not
return, return
Completion(resumptionValue).
NOTE: When one of the above steps returns, it returns to
the evaluation of the
YieldExpression
production that originally called this abstract operation.
NOTE: This returns to the evaluation of the operation that had
most previously resumed evaluation of genContext.
25.6Promise Objects
A Promise is an object that is used as a placeholder for the eventual
results of a deferred (and possibly asynchronous) computation.
Any Promise object is in one of three mutually exclusive states:
fulfilled, rejected, and pending:
A promise p is fulfilled if
p.then(f, r) will immediately enqueue a
Job
to call the function f.
A promise p is rejected if
p.then(f, r) will immediately enqueue a
Job
to call the function r.
A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if
it is either fulfilled or rejected.
A promise is resolved if it is settled or if it has been
“locked in” to match the state of another promise. Attempting to
resolve or reject a resolved promise has no effect. A promise is
unresolved if it is not resolved. An unresolved promise is
always in the pending state. A resolved promise may be pending,
fulfilled or rejected.
25.6.1Promise Abstract Operations
25.6.1.1PromiseCapability Records
A PromiseCapability is a
Record
value used to encapsulate a promise object along with the
functions that are capable of resolving or rejecting that promise
object. PromiseCapability Records are produced by the
NewPromiseCapability
abstract operation.
PromiseCapability Records have the fields listed in
Table 78.
The PromiseReaction is a
Record
value used to store information about how a promise should react
when it becomes resolved or rejected with a given value.
PromiseReaction records are created by the
PerformPromiseThen
abstract operation, and are used by a
PromiseReactionJob.
PromiseReaction records have the fields listed in
Table 79.
The function that should be applied to the incoming
value, and whose return value will govern what happens
to the derived promise. If [[Handler]] is
undefined, a function that depends on
the value of [[Type]] will be used instead.
25.6.1.3CreateResolvingFunctions (
promise )
When CreateResolvingFunctions is performed with argument
promise, the following steps are taken:
Let alreadyResolved be the
Record
{ [[Value]]: false }.
The abstract operation NewPromiseCapability takes a
constructor
function, and attempts to use that
constructor
function in the fashion of the built-in Promiseconstructor
to create a Promise object and extract its resolve and reject
functions. The promise plus the resolve and reject functions are
used to initialize a new PromiseCapability
Record
which is returned as the value of this abstract operation.
If
IsConstructor(C) is false, throw a
TypeError exception.
NOTE: C is assumed to be a
constructor
function that supports the parameter conventions of the
Promiseconstructor
(see
25.6.3.1).
Let promiseCapability be the PromiseCapability {
[[Promise]]: undefined, [[Resolve]]:
undefined, [[Reject]]:
undefined }.
If
IsCallable(promiseCapability.[[Resolve]]) is
false, throw a
TypeError exception.
If
IsCallable(promiseCapability.[[Reject]]) is
false, throw a
TypeError exception.
Set promiseCapability.[[Promise]] to
promise.
Return promiseCapability.
Note
This abstract operation supports Promise subclassing, as it is
generic on any
constructor
that calls a passed executor function argument in the same way
as the Promise
constructor. It is used to generalize static methods of the Promise
constructor
to any subclass.
25.6.1.5.1GetCapabilitiesExecutor
Functions
A GetCapabilitiesExecutor function is an anonymous built-in
function that has a [[Capability]] internal slot.
When a GetCapabilitiesExecutor function is called with arguments
resolve and reject, the following steps
are taken:
The abstract operation TriggerPromiseReactions takes a collection
of PromiseReactionRecords and enqueues a new
Job
for each record. Each such
Job
processes the [[Type]] and [[Handler]] of the
PromiseReactionRecord, and if the [[Handler]] is a function, calls
it passing the given argument. If the [[Handler]] is
undefined, the behaviour is determined by the
[[Type]].
For each reaction in reactions, in
original insertion order, do
HostPromiseRejectionTracker is an implementation-defined abstract
operation that allows host environments to track promise
rejections.
An implementation of HostPromiseRejectionTracker must complete
normally in all cases. The default implementation of
HostPromiseRejectionTracker is to unconditionally return an empty
normal completion.
Note 1
HostPromiseRejectionTracker is called in two scenarios:
When a promise is rejected without any handlers, it is
called with its operation argument set to
"reject".
When a handler is added to a rejected promise for the first
time, it is called with its operation argument
set to "handle".
A typical implementation of HostPromiseRejectionTracker might
try to notify developers of unhandled rejections, while also
being careful to notify them if such previous notifications
are later invalidated by new handlers being attached.
Note 2
If operation is "handle", an
implementation should not hold a reference to
promise in a way that would interfere with garbage
collection. An implementation may hold a reference to
promise if operation is
"reject", since it is expected that
rejections will be rare and not on hot code paths.
25.6.2Promise Jobs
25.6.2.1PromiseReactionJob (
reaction, argument )
The job PromiseReactionJob with parameters reaction and
argument applies the appropriate handler to the
incoming value, and uses the handler's return value to resolve or
reject the derived promise associated with that handler.
This
Job
uses the supplied thenable and its then method to
resolve the given promise. This process must take place as a
Job
to ensure that the evaluation of the then method
occurs after evaluation of any surrounding code has completed.
is the initial value of the "Promise" property
of the
global object.
creates and initializes a new Promise object when called as a
constructor.
is not intended to be called as a function and will throw an
exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Promise behaviour must include a
super call to the Promiseconstructor
to create and initialize the subclass instance with the internal
state necessary to support the Promise and
Promise.prototype built-in methods.
25.6.3.1Promise (
executor )
When the Promise function is called with argument
executor, the following steps are taken:
If NewTarget is undefined, throw a
TypeError exception.
If
IsCallable(executor) is false, throw a
TypeError exception.
Let promise be ? OrdinaryCreateFromConstructor(NewTarget, "%Promise.prototype%", «
[[PromiseState]], [[PromiseResult]],
[[PromiseFulfillReactions]], [[PromiseRejectReactions]],
[[PromiseIsHandled]] »).
Set promise.[[PromiseState]] to
pending.
Set promise.[[PromiseFulfillReactions]] to a new
empty
List.
Set promise.[[PromiseRejectReactions]] to a new
empty
List.
The executor argument must be a
function object. It is called for initiating and reporting completion of the
possibly deferred action represented by this Promise object.
The executor is called with two arguments:
resolve and reject. These are functions
that may be used by the executor function to report
eventual completion or failure of the deferred computation.
Returning from the executor function does not mean that the
deferred action has been completed but only that the request
to eventually perform the deferred action has been accepted.
The resolve function that is passed to an
executor function accepts a single argument. The
executor code may eventually call the
resolve function to indicate that it wishes to
resolve the associated Promise object. The argument passed to
the resolve function represents the eventual value
of the deferred action and can be either the actual
fulfillment value or another Promise object which will provide
the value if it is fulfilled.
The reject function that is passed to an
executor function accepts a single argument. The
executor code may eventually call the
reject function to indicate that the associated
Promise is rejected and will never be fulfilled. The argument
passed to the reject function is used as the
rejection value of the promise. Typically it will be an
Error object.
The resolve and reject functions passed to an
executor function by the Promise
constructor
have the capability to actually resolve and reject the
associated promise. Subclasses may have different
constructor
behaviour that passes in customized values for resolve and
reject.
The all function returns a new promise which is
fulfilled with an array of fulfillment values for the passed
promises, or rejects with the reason of the first passed promise
that rejects. It resolves all elements of the passed iterable to
promises as it runs this algorithm.
When the PerformPromiseAll abstract operation is called with
arguments iteratorRecord, constructor, and
resultCapability, the following steps are taken:
A Promise.all resolve element function is an
anonymous built-in function that is used to resolve a specific
Promise.all element. Each
Promise.all resolve element function has [[Index]],
[[Values]], [[Capability]], [[RemainingElements]], and
[[AlreadyCalled]] internal slots.
When a Promise.all resolve element function is
called with argument x, the following steps are
taken:
The "length" property of a
Promise.all resolve element function is 1.
25.6.4.2Promise.allSettled (
iterable )
The allSettled function returns a promise that is
fulfilled with an array of promise state snapshots, but only after
all the original promises have settled, i.e. become either
fulfilled or rejected. It resolves all elements of the passed
iterable to promises as it runs this algorithm.
When the PerformPromiseAllSettled abstract operation is called
with arguments iteratorRecord,
constructor, and resultCapability, the
following steps are taken:
25.6.4.2.2Promise.allSettled Resolve Element Functions
A Promise.allSettled resolve element function is an
anonymous built-in function that is used to resolve a specific
Promise.allSettled element. Each
Promise.allSettled resolve element function has
[[Index]], [[Values]], [[Capability]], [[RemainingElements]],
and [[AlreadyCalled]] internal slots.
When a Promise.allSettled resolve element function
is called with argument x, the following steps are
taken:
The "length" property of a
Promise.allSettled resolve element function is 1.
25.6.4.2.3Promise.allSettled Reject Element Functions
A Promise.allSettled reject element function is an
anonymous built-in function that is used to reject a specific
Promise.allSettled element. Each
Promise.allSettled reject element function has
[[Index]], [[Values]], [[Capability]], [[RemainingElements]],
and [[AlreadyCalled]] internal slots.
When a Promise.allSettled reject element function
is called with argument x, the following steps are
taken:
The "length" property of a
Promise.allSettled reject element function is 1.
25.6.4.3Promise.prototype
The initial value of Promise.prototype is
%Promise.prototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
25.6.4.4Promise.race (
iterable )
The race function returns a new promise which is
settled in the same way as the first passed promise to settle. It
resolves all elements of the passed iterable to
promises as it runs this algorithm.
If the iterable argument is empty or if none of the
promises in iterable ever settle then the pending
promise returned by this method will never be settled.
Note 2
The race function expects its
this value to be a
constructor
function that supports the parameter conventions of the
Promiseconstructor. It also expects that its this value
provides a resolve method.
When the PerformPromiseRace abstract operation is called with
arguments iteratorRecord, constructor, and
resultCapability, the following steps are taken:
Perform ? Call(promiseCapability.[[Reject]],
undefined, « r »).
Return promiseCapability.[[Promise]].
This function is the %Promise_reject% intrinsic object.
Note
The reject function expects its
this value to be a
constructor
function that supports the parameter conventions of the
Promiseconstructor.
25.6.4.6Promise.resolve (
x )
The resolve function returns either a new promise
resolved with the passed argument, or the argument itself if the
argument is a promise produced by this
constructor.
Let C be the this value.
If
Type(C) is not Object, throw a
TypeError exception.
Perform ? Call(promiseCapability.[[Resolve]],
undefined, « x »).
Return promiseCapability.[[Promise]].
25.6.4.7get Promise [ @@species ]
Promise[@@species] is an
accessor property
whose set accessor function is undefined. Its
get accessor function performs the following steps:
Return the this value.
The value of the "name" property of this
function is "get [Symbol.species]".
Note
Promise prototype methods normally use their
this object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its
@@species property.
25.6.5Properties of the Promise
Prototype Object
The Promise prototype object:
is the intrinsic object %PromisePrototype%.
has a [[Prototype]] internal slot whose value is
%Object.prototype%.
is an ordinary object.
does not have a [[PromiseState]] internal slot or any of the other
internal slots of Promise instances.
25.6.5.1Promise.prototype.catch (
onRejected )
When the catch method is called with argument
onRejected, the following steps are taken:
A Then Finally function is an anonymous built-in function that
has a [[Constructor]] and an [[OnFinally]] internal slot. The
value of the [[Constructor]] internal slot is a
Promise-like
constructorfunction object, and the value of the [[OnFinally]] internal slot is a
function object.
When a Then Finally function is called with argument
value, the following steps are taken:
The "length" property of a Then Finally
function is 1.
25.6.5.3.2Catch Finally Functions
A Catch Finally function is an anonymous built-in function that
has a [[Constructor]] and an [[OnFinally]] internal slot. The
value of the [[Constructor]] internal slot is a
Promise-like
constructorfunction object, and the value of the [[OnFinally]] internal slot is a
function object.
When a Catch Finally function is called with argument
reason, the following steps are taken:
The abstract operation PerformPromiseThen performs the “then”
operation on promise using onFulfilled and
onRejected as its settlement actions. If
resultCapability is passed, the result is stored by
updating resultCapability's promise. (If it is not
passed, then PerformPromiseThen is being called by a
specification-internal operation where the result does not
matter.)
The initial value of the @@toStringTag property is the String
value "Promise".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
25.6.6Properties of Promise Instances
Promise instances are ordinary objects that inherit properties from
the Promise prototype object (the intrinsic, %Promise.prototype%).
Promise instances are initially created with the internal slots
described in
Table 80.
Table 80: Internal Slots of Promise Instances
Internal Slot
Description
[[PromiseState]]
One of pending,
fulfilled, or
rejected. Governs how a promise
will react to incoming calls to its
then method.
[[PromiseResult]]
The value with which the promise has been fulfilled or
rejected, if any. Only meaningful if [[PromiseState]] is
not pending.
[[PromiseFulfillReactions]]
A
List
of PromiseReaction records to be processed when/if the
promise transitions from the
pending state to the
fulfilled state.
[[PromiseRejectReactions]]
A
List
of PromiseReaction records to be processed when/if the
promise transitions from the
pending state to the
rejected state.
[[PromiseIsHandled]]
A boolean indicating whether the promise has ever had a
fulfillment or rejection handler; used in unhandled
rejection tracking.
creates and initializes a new AsyncFunction object when called as
a function rather than as a
constructor. Thus the function call AsyncFunction(…) is
equivalent to the object creation expression
new AsyncFunction(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass
constructors that intend to inherit the specified AsyncFunction
behaviour must include a super call to the
AsyncFunctionconstructor
to create and initialize a subclass instance with the internal
slots necessary for built-in async function behaviour.
25.7.1.1AsyncFunction ( p1,
p2, … , pn, body )
The last argument specifies the body (executable code) of an async
function. Any preceding arguments specify formal parameters.
When the AsyncFunction function is called with some
arguments p1, p2, … , pn,
body (where n might be 0, that is, there are
no p arguments, and where body might also
not be provided), the following steps are taken:
The initial value of
AsyncFunction.prototype.constructor is
%AsyncFunction%
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
25.7.3.2AsyncFunction.prototype [
@@toStringTag ]
The initial value of the @@toStringTag property is the string
value "AsyncFunction".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
25.7.4AsyncFunction Instances
Every AsyncFunction instance is an ECMAScript
function object
and has the internal slots listed in
Table 29. The value of the [[IsClassConstructor]] internal slot for all
such instances is false. AsyncFunction instances
are not constructors and do not have a [[Construct]] internal
method. AsyncFunction instances do not have a prototype property as
they are not constructable.
Each AsyncFunction instance has the following own properties:
25.7.4.1length
The specification for the "length" property of
Function instances given in
19.2.4.1
also applies to AsyncFunction instances.
25.7.4.2name
The specification for the "name" property of
Function instances given in
19.2.4.2
also applies to AsyncFunction instances.
Assert: result is a normal completion with a value of
undefined. The possible sources of
completion values are
Await
or, if the async function doesn't await anything, the step 3.g
above.
Return.
26Reflection
26.1The Reflect Object
The Reflect object:
is the intrinsic object %Reflect%.
is the initial value of the "Reflect" property of
the
global object.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %Object.prototype%.
does not have a "prototype" property because
proxy exotic objects do not have a [[Prototype]] internal slot
that requires initialization.
has the following properties:
26.2.2.1Proxy.revocable (
target, handler )
The Proxy.revocable function is used to create a
revocable Proxy object. When Proxy.revocable is
called with arguments target and handler,
the following steps are taken:
The "length" property of a Proxy revocation
function is 0.
26.3Module Namespace Objects
A Module Namespace Object is a module namespace
exotic object
that provides runtime property-based access to a module's exported
bindings. There is no
constructor
function for Module Namespace Objects. Instead, such an object is
created for each module that is imported by an
ImportDeclaration
that includes a
NameSpaceImport.
In addition to the properties specified in
9.4.6
each Module Namespace Object has the following own property:
26.3.1@@toStringTag
The initial value of the @@toStringTag property is the String value
"Module".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
27Memory Model
The memory consistency model, or memory model, specifies the
possible orderings of
Shared Data Block
events, arising via accessing TypedArray instances backed by a
SharedArrayBuffer and via methods on the Atomics object. When the
program has no data races (defined below), the ordering of events
appears as sequentially consistent, i.e., as an interleaving of actions
from each
agent. When the program has data races, shared memory operations may appear
sequentially inconsistent. For example, programs may exhibit
causality-violating behaviour and other astonishments. These
astonishments arise from compiler transforms and the design of CPUs
(e.g., out-of-order execution and speculation). The memory model defines
both the precise conditions under which a program exhibits sequentially
consistent behaviour as well as the possible values read from data
races. To wit, there is no undefined behaviour.
The memory model is defined as relational constraints on events
introduced by
abstract operations
on SharedArrayBuffer or by methods on the Atomics object during an
evaluation.
Note
This section provides an axiomatic model on events introduced by the
abstract operations
on SharedArrayBuffers. It bears stressing that the model is not
expressible algorithmically, unlike the rest of this specification.
The nondeterministic introduction of events by
abstract operations
is the interface between the operational semantics of ECMAScript
evaluation and the axiomatic semantics of the memory model. The
semantics of these events is defined by considering graphs of all
events in an evaluation. These are neither Static Semantics nor
Runtime Semantics. There is no demonstrated algorithmic
implementation, but instead a set of constraints that determine if a
particular event graph is allowed or disallowed.
27.1Memory Model Fundamentals
Shared memory accesses (reads and writes) are divided into two groups,
atomic accesses and data accesses, defined below. Atomic accesses are
sequentially consistent, i.e., there is a strict total ordering of
events agreed upon by all agents in an
agent cluster. Non-atomic accesses do not have a strict total ordering agreed upon
by all agents, i.e., unordered.
Note 1
No orderings weaker than sequentially consistent and stronger than
unordered, such as release-acquire, are supported.
A Shared Data Block event is either a
ReadSharedMemory, WriteSharedMemory, or
ReadModifyWriteSharedMemory
Record.
The
List
of byte values to be passed to [[ModifyOp]].
[[ModifyOp]]
A semantic function
A pure semantic function that returns a modified
List
of byte values from a read
List
of byte values and [[Payload]].
These events are introduced by
abstract operations
or by methods on the Atomics object.
Some operations may also introduce Synchronize events. A
Synchronize event has no fields, and exists purely to
directly constrain the permitted orderings of other events.
In addition to
Shared Data Block
and Synchronize events, there are host-specific events.
Let the range of a ReadSharedMemory, WriteSharedMemory, or
ReadModifyWriteSharedMemory event be the Set of contiguous integers
from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two
events' ranges are equal when the events have the same [[Block]], and
the ranges are element-wise equal. Two events' ranges are overlapping
when the events have the same [[Block]], the ranges are not equal and
their intersection is non-empty. Two events' ranges are disjoint when
the events do not have the same [[Block]] or their ranges are neither
equal nor overlapping.
Note 2
Examples of host-specific synchronizing events that should be
accounted for are: sending a SharedArrayBuffer from one
agent
to another (e.g., by postMessage in a browser),
starting and stopping agents, and communicating within the
agent cluster
via channels other than shared memory. It is assumed those events
are appended to
agent-order
during evaluation like the other SharedArrayBuffer events.
Events are ordered within candidate executions by the relations
defined below.
27.2Agent Events Records
An Agent Events Record is a
Record
with the following fields.
For a
candidate executionexecution, execution.[[AgentOrder]] is a
Relation
on events that satisfies the following.
For each pair (E, D) in
EventSet(execution), (E, D) is in
execution.[[AgentOrder]] if there is some
Agent Events Recordaer in execution.[[EventsRecords]] such that
E and D are in aer.[[EventList]]
and E is before D in
List
order of aer.[[EventList]].
Note
Each
agent
introduces events in a per-agentstrict total order
during the evaluation. This is the union of those strict total
orders.
Each event W with index i in
Ws has R.[[ByteIndex]] + i in
its range.
R is not in Ws.
27.6.3reads-from
For a
candidate executionexecution, execution.[[ReadsFrom]] is the
least
Relation
on events that satisfies the following.
For each pair (R, W) in
SharedDataBlockEventSet(execution), (R, W) is in
execution.[[ReadsFrom]] if W is in
execution.[[ReadsBytesFrom]](R).
27.6.4host-synchronizes-with
For a
candidate executionexecution, execution.[[HostSynchronizesWith]]
is a host-provided
strict partial order
on host-specific events that satisfies at least the following.
If (E, D) is in
execution.[[HostSynchronizesWith]], E and
D are in
HostEventSet(execution).
There is no cycle in the union of
execution.[[HostSynchronizesWith]] and
execution.[[AgentOrder]].
Note 1
For two host-specific events E and D,
E host-synchronizes-with D implies
Ehappens-beforeD.
Note 2
The host-synchronizes-with relation allows the host to provide
additional synchronization mechanisms, such as
postMessage between HTML workers.
27.6.5synchronizes-with
For a
candidate executionexecution, execution.[[SynchronizesWith]] is
the least
Relation
on events that satisfies the following.
For each pair (R, W) in
execution.[[ReadsFrom]], (W, R)
is in execution.[[SynchronizesWith]] if
R.[[Order]] is SeqCst,
W.[[Order]] is SeqCst, and
R and W have equal ranges.
For each element eventsRecord of
execution.[[EventsRecords]], the following is true.
For each pair (S, Sw) in
eventsRecord.[[AgentSynchronizesWith]],
(S, Sw) is in
execution.[[SynchronizesWith]].
For each pair (E, D) in
execution.[[HostSynchronizesWith]], (E,
D) is in execution.[[SynchronizesWith]].
Note 1
Owing to convention, write events synchronizes-with read events,
instead of read events synchronizes-with write events.
Note 2
Init events do not participate in
synchronizes-with, and are instead constrained directly by
happens-before.
Note 3
Not all SeqCst events related by
reads-from
are related by synchronizes-with. Only events that also have
equal ranges are related by synchronizes-with.
For a
candidate executionexecution, execution.[[HappensBefore]] is the
least
Relation
on events that satisfies the following.
For each pair (E, D) in
execution.[[AgentOrder]], (E, D)
is in execution.[[HappensBefore]].
For each pair (E, D) in
execution.[[SynchronizesWith]], (E,
D) is in execution.[[HappensBefore]].
For each pair (E, D) in
SharedDataBlockEventSet(execution), (E, D) is in
execution.[[HappensBefore]] if E.[[Order]]
is Init and E and
D have overlapping ranges.
For each pair (E, D) in
EventSet(execution), (E, D) is in
execution.[[HappensBefore]] if there is an event
F such that the pairs (E, F) and
(F, D) are in
execution.[[HappensBefore]].
Note
Because happens-before is a superset of
agent-order, candidate executions are consistent with the single-thread
evaluation semantics of ECMAScript.
27.7Properties of Valid Executions
27.7.1Valid Chosen Reads
A
candidate executionexecution has valid chosen reads if the following
abstract operation returns true.
If there is a
WriteSharedMemory
or
ReadModifyWriteSharedMemory
event V that has byteLocation in
its range such that the pairs (W,
V) and (V, R) are in
execution.[[HappensBefore]], then
Return false.
Set byteLocation to byteLocation +
1.
Return true.
27.7.3Tear Free Reads
A
candidate executionexecution has tear free reads if the following abstract
operation returns true.
Assert: The remainder of dividing R.[[ByteIndex]]
by R.[[ElementSize]] is 0.
For each event W such that (R,
W) is in execution.[[ReadsFrom]]
and W.[[NoTear]] is true,
do
If R and W have equal ranges,
and there is an event V such that
V and W have equal ranges,
V.[[NoTear]] is true,
W is not V, and (R,
V) is in
execution.[[ReadsFrom]], then
Return false.
Return true.
Note
An event's [[NoTear]] field is true when that
event was introduced via accessing an
integer
TypedArray, and false when introduced via
accessing a floating point TypedArray or DataView.
Intuitively, this requirement says when a memory range is
accessed in an aligned fashion via an
integer
TypedArray, a single write event on that range must "win" when
in a data race with other write events with equal ranges. More
precisely, this requirement says an aligned read event cannot
read a value composed of bytes from multiple, different write
events all with equal ranges. It is possible, however, for an
aligned read event to read from multiple write events with
overlapping ranges.
For each pair (E, D) in
execution.[[HappensBefore]], (E,
D) is in memory-order.
For each pair (R, W) in
execution.[[ReadsFrom]], there is no
WriteSharedMemory
or
ReadModifyWriteSharedMemory
event V in
SharedDataBlockEventSet(execution) such that V.[[Order]] is
SeqCst, the pairs (W,
V) and (V, R) are in
memory-order, and any of the following conditions are true.
The pair (W, R) is in
execution.[[SynchronizesWith]], and
V and R have equal ranges.
The pairs (W, R) and (V,
R) are in execution.[[HappensBefore]],
W.[[Order]] is SeqCst, and
W and V have equal ranges.
The pairs (W, R) and (W,
V) are in execution.[[HappensBefore]],
R.[[Order]] is SeqCst, and
V and R have equal ranges.
Note 1
This clause additionally constrains
SeqCst events on equal ranges.
This clause together with the forward progress guarantee on
agents ensure the liveness condition that
SeqCst writes become visible to
SeqCst reads with equal range in
finite time.
A
candidate execution
has sequentially consistent atomics if a memory-order exists.
Note 3
While memory-order includes all events in
EventSet(execution), those that are not constrained by
happens-before
or
synchronizes-with
are allowed to occur anywhere in the order.
27.7.5Valid Executions
A
candidate executionexecution is a valid execution (or simply an execution)
if all of the following are true.
If either (E, D) or (D,
E) is in execution.[[ReadsFrom]],
then
Return true.
Return false.
27.9Data Races
For an execution execution, two events E and
D in
SharedDataBlockEventSet(execution) are in a data race if the following abstract
operation returns true.
If E and D are in a race in
execution, then
If E.[[Order]] is not
SeqCst or D.[[Order]] is not
SeqCst, then
Return true.
If E and D have overlapping ranges, then
Return true.
Return false.
27.10Data Race Freedom
An execution execution is data race free if there are no
two events in
SharedDataBlockEventSet(execution) that are in a data race.
A program is data race free if all its executions are data race free.
The
memory model
guarantees sequential consistency of all events for data race free
programs.
27.11Shared Memory Guidelines
Note 1
The following are guidelines for ECMAScript programmers working
with shared memory.
We recommend programs be kept data race free, i.e., make it so
that it is impossible for there to be concurrent non-atomic
operations on the same memory location. Data race free programs
have interleaving semantics where each step in the evaluation
semantics of each
agent
are interleaved with each other. For data race free programs, it
is not necessary to understand the details of the
memory model. The details are unlikely to build intuition that will help one
to better write ECMAScript.
More generally, even if a program is not data race free it may
have predictable behaviour, so long as atomic operations are not
involved in any data races and the operations that race all have
the same access size. The simplest way to arrange for atomics not
to be involved in races is to ensure that different memory cells
are used by atomic and non-atomic operations and that atomic
accesses of different sizes are not used to access the same cells
at the same time. Effectively, the program should treat shared
memory as strongly typed as much as possible. One still cannot
depend on the ordering and timing of non-atomic accesses that
race, but if memory is treated as strongly typed the racing
accesses will not "tear" (bits of their values will not be mixed).
Note 2
The following are guidelines for ECMAScript implementers writing
compiler transformations for programs using shared memory.
It is desirable to allow most program transformations that are
valid in a single-agent
setting in a multi-agent
setting, to ensure that the performance of each
agent
in a multi-agent
program is as good as it would be in a single-agent
setting. Frequently these transformations are hard to judge. We
outline some rules about program transformations that are intended
to be taken as normative (in that they are implied by the
memory model
or stronger than what the
memory model
implies) but which are likely not exhaustive. These rules are
intended to apply to program transformations that precede the
introductions of the events that make up the
agent-order.
Let an agent-order slice be the subset of the
agent-order
pertaining to a single
agent.
Let possible read values of a read event be the set of
all values of
ValueOfReadEvent
for that event across all valid executions.
Any transformation of an agent-order slice that is valid in the
absence of shared memory is valid in the presence of shared
memory, with the following exceptions.
Atomics are carved in stone: Program transformations
must not cause the SeqCst events in an
agent-order slice to be reordered with its
Unordered operations, nor its
SeqCst operations to be reordered with
each other, nor may a program transformation remove a
SeqCst operation from the
agent-order.
(In practice, the prohibition on reorderings forces a compiler
to assume that every SeqCst operation
is a synchronization and included in the final
memory-order, which it would usually have to assume anyway in the absence
of inter-agent
program analysis. It also forces the compiler to assume that
every call where the callee's effects on the
memory-order
are unknown may contain
SeqCst operations.)
Reads must be stable: Any given shared memory read
must only observe a single value in an execution.
(For example, if what is semantically a single read in the
program is executed multiple times then the program is
subsequently allowed to observe only one of the values read. A
transformation known as rematerialization can violate this
rule.)
Writes must be stable: All observable writes to
shared memory must follow from program semantics in an
execution.
(For example, a transformation may not introduce certain
observable writes, such as by using read-modify-write
operations on a larger location to write a smaller datum,
writing a value to memory that the program could not have
written, or writing a just-read value back to the location it
was read from, if that location could have been overwritten by
another
agent
after the read.)
Possible read values must be nonempty: Program
transformations cannot cause the possible read values of a
shared memory read to become empty.
(Counterintuitively, this rule in effect restricts
transformations on writes, because writes have force in
memory model
insofar as to be read by read events. For example, writes may
be moved and coalesced and sometimes reordered between two
SeqCst operations, but the
transformation may not remove every write that updates a
location; some write must be preserved.)
Examples of transformations that remain valid are: merging
multiple non-atomic reads from the same location, reordering
non-atomic reads, introducing speculative non-atomic reads,
merging multiple non-atomic writes to the same location,
reordering non-atomic writes to different locations, and hoisting
non-atomic reads out of loops even if that affects termination.
Note in general that aliased TypedArrays make it hard to prove
that locations are different.
Note 3
The following are guidelines for ECMAScript implementers
generating machine code for shared memory accesses.
For architectures with memory models no weaker than those of ARM
or Power, non-atomic stores and loads may be compiled to bare
stores and loads on the target architecture. Atomic stores and
loads may be compiled down to instructions that guarantee
sequential consistency. If no such instructions exist, memory
barriers are to be employed, such as placing barriers on both
sides of a bare store or load. Read-modify-write operations may be
compiled to read-modify-write instructions on the target
architecture, such as LOCK-prefixed instructions on
x86, load-exclusive/store-exclusive instructions on ARM, and
load-link/store-conditional instructions on Power.
Specifically, the
memory model
is intended to allow code generation as follows.
Every atomic operation in the program is assumed to be
necessary.
Atomic operations are never rearranged with each other or with
non-atomic operations.
Functions are always assumed to perform atomic operations.
Atomic operations are never implemented as read-modify-write
operations on larger data, but as non-lock-free atomics if the
platform does not have atomic operations of the appropriate
size. (We already assume that every platform has normal memory
access operations of every interesting size.)
Naive code generation uses these patterns:
Regular loads and stores compile to single load and store
instructions.
Lock-free atomic loads and stores compile to a full
(sequentially consistent) fence, a regular load or store, and a
full fence.
Lock-free atomic read-modify-write accesses compile to a full
fence, an atomic read-modify-write instruction sequence, and a
full fence.
Non-lock-free atomics compile to a spinlock acquire, a full
fence, a series of non-atomic load and store instructions, a
full fence, and a spinlock release.
That mapping is correct so long as an atomic operation on an
address range does not race with a non-atomic write or with an
atomic operation of different size. However, that is all we need:
the
memory model
effectively demotes the atomic operations involved in a race to
non-atomic status. On the other hand, the naive mapping is quite
strong: it allows atomic operations to be used as sequentially
consistent fences, which the
memory model
does not actually guarantee.
A number of local improvements to those basic patterns are also
intended to be legal:
There are obvious platform-dependent improvements that remove
redundant fences. For example, on x86 the fences around
lock-free atomic loads and stores can always be omitted except
for the fence following a store, and no fence is needed for
lock-free read-modify-write instructions, as these all use
LOCK-prefixed instructions. On many platforms there are fences
of several strengths, and weaker fences can be used in certain
contexts without destroying sequential consistency.
Most modern platforms support lock-free atomics for all the data
sizes required by ECMAScript atomics. Should non-lock-free
atomics be needed, the fences surrounding the body of the atomic
operation can usually be folded into the lock and unlock steps.
The simplest solution for non-lock-free atomics is to have a
single lock word per SharedArrayBuffer.
There are also more complicated platform-dependent local
improvements, requiring some code analysis. For example, two
back-to-back fences often have the same effect as a single
fence, so if code is generated for two atomic operations in
sequence, only a single fence need separate them. On x86, even a
single fence separating atomic stores can be omitted, as the
fence following a store is only needed to separate the store
from a subsequent load.
The ECMAScript language syntax and semantics defined in this annex are
required when the ECMAScript host is a web browser. The content of this
annex is normative but optional if the ECMAScript host is not a web
browser.
Note
This annex describes various legacy features and other
characteristics of web browser based ECMAScript implementations. All
of the language features and behaviours specified in this annex have
one or more undesirable characteristics and in the absence of legacy
usage would be removed from this specification. However, the usage
of these features by large numbers of existing web pages means that
web browsers must continue to support them. The specifications in
this annex define the requirements for interoperable implementations
of these legacy features.
These features are not considered part of the core ECMAScript
language. Programmers should not use or assume the existence of
these features and behaviours when writing new ECMAScript code.
ECMAScript implementations are discouraged from implementing these
features unless the implementation is part of a web browser or is
required to run the same legacy ECMAScript code that web browsers
encounter.
B.1Additional Syntax
B.1.1Numeric Literals
The syntax and semantics of
11.8.3
is extended as follows except that this extension is not allowed for
strict mode code:
The syntax and semantics of
11.4
is extended as follows except that this extension is not allowed
when parsing source code using the
goal symbolModule:
The syntax of
21.2.1
is modified and extended as follows. These changes introduce
ambiguities that are broken by the ordering of grammar productions
and by contextual information. When parsing using the following
grammar, each alternative is considered only if previous production
alternatives do not match.
This alternative pattern grammar and semantics only changes the
syntax and semantics of BMP patterns. The following grammar
extensions include productions parameterized with the [U] parameter.
However, none of these extensions change the syntax of Unicode
patterns recognized when parsing with the [U] parameter present on
the
goal symbol.
It is a Syntax Error if IsCharacterClass of the first
ClassAtom
is true or IsCharacterClass of the second
ClassAtom
is true
and this production has a [U] parameter.
Atom (21.2.2.8) evaluation rules for the
Atom
productions except for
Atom::PatternCharacter
are also used for the
ExtendedAtom
productions, but with
ExtendedAtom
substituted for
Atom. The following evaluation rules, with parameter
direction, are also added:
The production
ExtendedAtom::\[lookahead = c]
evaluates as follows:
Let A be the CharSet containing the single
character \ U+005C (REVERSE SOLIDUS).
Call
CharacterSetMatcher(A, false,
direction) and return its Matcher result.
The escape function is a property of the
global object. It computes a new version of a String value in which certain
code units have been replaced by a hexadecimal escape sequence.
For those code units being replaced whose value is
0x00FF or less, a two-digit escape sequence of the
form %xx is used. For those characters
being replaced whose code unit value is greater than
0x00FF, a four-digit escape sequence of the form
%uxxxx is used.
The escape function is the
%escape% intrinsic object. When the
escape function is called with one argument
string, the following steps are taken:
The encoding is partly based on the encoding described in RFC
1738, but the entire encoding specified in this standard is
described above without regard to the contents of RFC 1738.
This encoding does not reflect changes to RFC 1738 made by RFC
3986.
B.2.1.2unescape ( string )
The unescape function is a property of the
global object. It computes a new version of a String value in which each
escape sequence of the sort that might be introduced by the
escape function is replaced with the code unit that
it represents.
The unescape function is the
%unescape% intrinsic object. When the
unescape function is called with one argument
string, the following steps are taken:
If k ≤ length - 6 and the code
unit at index k + 1 within
string is the code unit 0x0075 (LATIN SMALL
LETTER U) and the four code units at indices
k + 2, k + 3, k + 4,
and k + 5 within string are all
hexadecimal digits, then
Set c to the code unit whose value is
the
integer
represented by the four hexadecimal digits at
indices k + 2, k + 3,
k + 4, and k + 5 within
string.
Set k to k + 5.
Else if k ≤ length - 3 and the
two code units at indices k + 1 and
k + 2 within string are both
hexadecimal digits, then
Set c to the code unit whose value is
the
integer
represented by two zeroes plus the two hexadecimal
digits at indices k + 1 and
k + 2 within string.
B.2.2Additional Properties of the
Object.prototype Object
B.2.2.1Object.prototype.__proto__
Object.prototype.__proto__ is an
accessor property
with attributes { [[Enumerable]]: false,
[[Configurable]]: true }. The [[Get]] and
[[Set]] attributes are defined as follows:
B.2.2.1.1get
Object.prototype.__proto__
The value of the [[Get]] attribute is a built-in function that
requires no arguments. It performs the following steps:
B.2.3Additional Properties of the
String.prototype Object
B.2.3.1String.prototype.substr (
start, length )
The substr method takes two arguments,
start and length, and returns a substring of
the result of converting the this object to a
String, starting from index start and running for
length code units (or through the end of the String if
length is undefined). If
start is negative, it is treated as
sourceLength + start
where sourceLength is the length of the String. The
result is a String value, not a String object. The following steps
are taken:
If length is undefined, let
end be +∞; otherwise let
end be ? ToInteger(length).
Let size be the number of code units in
S.
If intStart < 0, set intStart to
max(size + intStart, 0).
Let resultLength be
min(max(end, 0), size - intStart).
If resultLength ≤ 0, return the empty String
"".
Return the String value containing
resultLength consecutive code units from
S beginning with the code unit at index
intStart.
Note
The substr function is intentionally generic; it
does not require that its this value be a
String object. Therefore it can be transferred to other kinds
of objects for use as a method.
B.2.3.2String.prototype.anchor (
name )
When the anchor method is called with argument
name, the following steps are taken:
B.2.3.2.1Runtime Semantics:
CreateHTML ( string, tag,
attribute, value )
The abstract operation CreateHTML is called with arguments
string, tag, attribute, and
value. The arguments tag and
attribute must be String values. The following steps
are taken:
Let escapedV be the String value that is the
same as V except that each occurrence of the
code unit 0x0022 (QUOTATION MARK) in V has
been replaced with the six code unit sequence
""".
The property "trimStart" is preferred. The
"trimLeft" property is provided principally
for compatibility with old code. It is recommended that the
"trimStart" property be used in new
ECMAScript code.
The initial value of the "trimLeft" property is
the same
function object
as the initial value of the
String.prototype.trimStart property.
B.2.3.16String.prototype.trimRight ( )
Note
The property "trimEnd" is preferred. The
"trimRight" property is provided
principally for compatibility with old code. It is recommended
that the "trimEnd" property be used in new
ECMAScript code.
The initial value of the "trimRight" property
is the same
function object
as the initial value of the
String.prototype.trimEnd property.
B.2.4Additional Properties of the
Date.prototype Object
B.2.4.1Date.prototype.getYear ( )
Note
The getFullYear method is preferred for nearly
all purposes, because it avoids the “year 2000 problem.”
When the getYear method is called with no arguments,
the following steps are taken:
Return the value of the [[DateValue]] internal slot of
this Date object.
B.2.4.3Date.prototype.toGMTString ( )
Note
The property "toUTCString" is preferred.
The "toGMTString" property is provided
principally for compatibility with old code. It is recommended
that the "toUTCString" property be used in
new ECMAScript code.
The
function object
that is the initial value of
Date.prototype.toGMTString is the same
function object
that is the initial value of
Date.prototype.toUTCString.
B.2.5Additional Properties of the
RegExp.prototype Object
The compile method completely reinitializes the
this object RegExp with a new pattern and
flags. An implementation may interpret use of this method as
an assertion that the resulting RegExp object will be used
multiple times and hence is a candidate for extra
optimization.
B.3Other Additional Features
B.3.1__proto__ Property Names in Object
Initializers
Prior to ECMAScript 2015, the specification of
LabelledStatement
did not allow for the association of a statement label with a
FunctionDeclaration. However, a labelled
FunctionDeclaration
was an allowable extension for
non-strict code
and most browser-hosted ECMAScript implementations supported that
extension. In ECMAScript 2015, the grammar productions for
LabelledStatement
permits use of
FunctionDeclaration
as a
LabelledItem
but
13.13.1
includes an Early Error rule that produces a Syntax Error if that
occurs. For web browser compatibility, that rule is modified with
the addition of the highlighted text:
B.3.3Block-Level Function Declarations
Web Legacy Compatibility Semantics
Prior to ECMAScript 2015, the ECMAScript specification did not
define the occurrence of a
FunctionDeclaration
as an element of a
Block
statement's
StatementList. However, support for that form of
FunctionDeclaration
was an allowable extension and most browser-hosted ECMAScript
implementations permitted them. Unfortunately, the semantics of such
declarations differ among those implementations. Because of these
semantic differences, existing web ECMAScript code that uses
Block
level function declarations is only portable among browser
implementation if the usage only depends upon the semantic
intersection of all of the browser implementations for such
declarations. The following are the use cases that fall within that
intersection semantics:
A function is declared and only referenced within a single block
One or more
FunctionDeclarations whose
BindingIdentifier
is the name f occur within the function code of an
enclosing function g and that declaration is nested
within a
Block.
No other declaration of f that is not a
var declaration occurs within the function code
of g
A function is declared and possibly used within a single
Block
but also referenced by an inner function definition that is not
contained within that same
Block.
One or more
FunctionDeclarations whose
BindingIdentifier
is the name f occur within the function code of an
enclosing function g and that declaration is nested
within a
Block.
No other declaration of f that is not a
var declaration occurs within the function code
of g
There is at least one occurrence of f as an
IdentifierReference
within another function h that is nested within
g and no other declaration of f shadows
the references to f from within h.
All invocations of h occur after the declaration of
f has been evaluated.
A function is declared and possibly used within a single block
but also referenced within subsequent blocks.
One or more
FunctionDeclaration
whose
BindingIdentifier
is the name f occur within the function code of an
enclosing function g and that declaration is nested
within a
Block.
No other declaration of f that is not a
var declaration occurs within the function code
of g
There is at least one occurrence of f as an
IdentifierReference
within the function code of g that lexically
follows the
Block
containing the declaration of f.
The first use case is interoperable with the semantics of
Block
level function declarations provided by ECMAScript 2015. Any
pre-existing ECMAScript code that employs that use case will operate
using the Block level function declarations semantics defined by
clauses 9, 13, and 14 of this specification.
ECMAScript 2015 interoperability for the second and third use cases
requires the following extensions to the clause
9, clause
14, clause
18.2.1
and clause
15.1.11
semantics.
If an ECMAScript implementation has a mechanism for reporting
diagnostic warning messages, a warning should be produced when code
contains a
FunctionDeclaration
for which these compatibility semantics are applied and introduce
observable differences from non-compatibility semantics. For
example, if a var binding is not introduced because its introduction
would create an
early error, a warning message should not be produced.
B.3.3.1Changes to
FunctionDeclarationInstantiation
NOTE: A var binding for F is only
instantiated here if it is neither a
VarDeclaredName, the name of a formal parameter,
or another
FunctionDeclaration.
If initializedBindings does not contain
F and F is not
"arguments", then
It is a Syntax Error if the LexicallyDeclaredNames of
StatementList
contains any duplicate entries,
unless the source code matching this production is not
strict mode code
and the duplicate entries are only bound by
FunctionDeclarations.
B.3.3.5Changes to
switch Statement Static Semantics: Early Errors
For web browser compatibility, that rule is modified with the
addition of the highlighted text:
It is a Syntax Error if the LexicallyDeclaredNames of
CaseBlock
contains any duplicate entries,
unless the source code matching this production is not
strict mode code
and the duplicate entries are only bound by
FunctionDeclarations.
This production only applies when parsing
non-strict code. Code matching this production is processed as if each matching
occurrence of
FunctionDeclaration[?Yield, ?Await, ~Default]
was the sole
StatementListItem
of a
BlockStatement
occupying that position in the source code. The semantics of such a
synthetic
BlockStatement
includes the web legacy compatibility semantics specified in
B.3.3.
B.3.5VariableStatements in Catch Blocks
The content of subclause
13.15.1
is replaced with the following:
The
Block
of a
Catch
clause may contain var declarations that bind a
name that is also bound by the
CatchParameter. At runtime, such bindings are instantiated in the
VariableDeclarationEnvironment. They do not shadow the
same-named bindings introduced by the
CatchParameter
and hence the
Initializer
for such var declarations will assign to the
corresponding catch parameter rather than the
var binding.
This modified behaviour also applies to var and
function declarations introduced by
direct eval
calls contained within the
Block
of a
Catch
clause. This change is accomplished by modifying the algorithm of
18.2.1.3
as follows:
Step 5.d.ii.2.a.i is replaced by:
If thisEnvRec is not the
Environment Record
for a
Catch
clause, throw a SyntaxError exception.
Objects with an [[IsHTMLDDA]] internal slot are never created by
this specification. However, the
document.all object
in web browsers is a host-created
exotic object
with this slot that exists for web compatibility purposes. There
are no other known examples of this type of object and
implementations should not create any with the exception of
document.all.
B.3.7.1Changes to ToBoolean
The result column in
Table 10
for an argument type of Object is replaced with the following
algorithm:
implements, interface, let,
package, private, protected,
public, static, and yield are
reserved words within
strict mode code. (11.6.2).
A conforming implementation, when processing
strict mode code, must not extend, as described in
B.1.1, the syntax of
NumericLiteral
to include LegacyOctalIntegerLiteral, nor extend the
syntax of
DecimalIntegerLiteral
to include NonOctalDecimalIntegerLiteral.
A conforming implementation, when processing
strict mode code, may not extend the syntax of
EscapeSequence
to include LegacyOctalEscapeSequence as described in
B.1.2.
Assignment to an undeclared identifier or otherwise unresolvable
reference does not create a property in the
global object. When a simple assignment occurs within
strict mode code, its
LeftHandSideExpression
must not evaluate to an unresolvable
Reference. If it does a ReferenceError exception is thrown
(6.2.4.9). The
LeftHandSideExpression
also may not be a reference to a
data property
with the attribute value { [[Writable]]: false },
to an
accessor property
with the attribute value { [[Set]]: undefined },
nor to a non-existent property of an object whose [[Extensible]]
internal slot has the value false. In these cases a
TypeError exception is thrown (12.15).
Arguments objects for strict functions define a non-configurable
accessor property"callee" which throws a
TypeError exception on access (9.4.4.6).
Arguments objects for strict functions do not dynamically share their
array-indexed
property values with the corresponding formal parameter bindings of
their functions. (9.4.4).
For strict functions, if an arguments object is created the binding of
the local identifier arguments to the arguments object is
immutable and hence may not be the target of an assignment expression.
(9.2.10).
Strict mode eval code cannot instantiate variables or functions in the
variable environment of the caller to eval. Instead, a new variable
environment is created and that environment is used for declaration
binding instantiation for the eval code (18.2.1).
If this is evaluated within
strict mode code, then the this value is not coerced to an object.
A this value of undefined or
null is not converted to the
global object
and primitive values are not converted to wrapper objects. The
this value passed via a function call (including
calls made using Function.prototype.apply and
Function.prototype.call) do not coerce the passed
this value to an object (9.2.1.2,
19.2.3.1,
19.2.3.3).
When a delete operator occurs within
strict mode code, a SyntaxError is thrown if its
UnaryExpression
is a direct reference to a variable, function argument, or function
name (12.5.3.1).
When a delete operator occurs within
strict mode code, a TypeError is thrown if the property to be
deleted has the attribute { [[Configurable]]:
false } (12.5.3.2).
An implementation may not extend, beyond that defined in this
specification, the meanings within strict functions of properties
named "caller" or "arguments" of
function instances.
DCorrections and Clarifications in
ECMAScript 2015 with Possible Compatibility Impact
8.1.1.4.15-8.1.1.4.18
Edition 5 and 5.1 used a property existence test to determine whether a
global object
property corresponding to a new global declaration already existed.
ECMAScript 2015 uses an own property existence test. This corresponds to
what has been most commonly implemented by web browsers.
9.4.2.1: The 5th Edition moved the capture of the current array
length prior to the
integer
conversion of the
array index
or new length value. However, the captured length value could become
invalid if the conversion process has the side-effect of changing the
array length. ECMAScript 2015 specifies that the current array length
must be captured after the possible occurrence of such side-effects.
20.4.1.14: Previous editions permitted the
TimeClip
abstract operation to return either +0 or
-0 as the representation of a 0
time value. ECMAScript 2015 specifies that +0 always returned.
This means that for ECMAScript 2015 the
time value
of a Date object is never observably -0 and methods
that return time values never return -0.
20.4.1.15: If a UTC offset representation is not present, the local time zone is
used. Edition 5.1 incorrectly stated that a missing time zone should be
interpreted as "z".
20.4.4.36: If the year cannot be represented using the Date Time String Format
specified in
20.4.1.15
a RangeError exception is thrown. Previous editions did not specify the
behaviour for that case.
20.4.4.41: Previous editions did not specify the value returned by
Date.prototype.toString when
this time value
is NaN. ECMAScript 2015 specifies the result to be
the String value "Invalid Date".
21.2.3.1,
21.2.3.2.4: Any LineTerminator code points in the value of the
"source" property of a RegExp instance must be
expressed using an escape sequence. Edition 5.1 only required the
escaping of /.
21.2.5.7,
21.2.5.10: In previous editions, the specifications for
String.prototype.match and
String.prototype.replace was incorrect for cases where the
pattern argument was a RegExp value whose global flag is
set. The previous specifications stated that for each attempt to match
the pattern, if lastIndex did not change it should be
incremented by 1. The correct behaviour is that
lastIndex should be incremented by one only if the pattern
matched the empty string.
22.1.3.27,
22.1.3.27.1: Previous editions did not specify how a NaN value
returned by a comparefn was interpreted by
Array.prototype.sort. ECMAScript 2015 specifies that such
as value is treated as if +0 was returned from the
comparefn. ECMAScript 2015 also specifies that
ToNumber
is applied to the result returned by a comparefn. In previous
editions, the effect of a comparefn result that is not a
Number value
was implementation-dependent. In practice, implementations call
ToNumber.
EAdditions and Changes That Introduce
Incompatibilities with Prior Editions
6.2.4: In ECMAScript 2015, Function calls are not allowed to return a
Reference
value.
8.2: In ECMAScript 2018, Template objects are canonicalized based on
Parse Node
(source location), instead of across all occurrences of that template
literal or tagged template in a
Realm
in previous editions.
11.2: In ECMASCript 2016, Unicode 8.0.0 or higher is mandated, as opposed
to ECMAScript 2015 which mandated Unicode 5.1. In particular, this
caused U+180E MONGOLIAN VOWEL SEPARATOR, which was in the
Space_Separator (Zs) category and thus treated
as whitespace in ECMAScript 2015, to be moved to the
Format (Cf) category (as of Unicode 6.3.0).
This causes whitespace-sensitive methods to behave differently. For
example, "\u180E".trim().length was 0 in
previous editions, but 1 in ECMAScript 2016 and later.
Additionally, ECMAScript 2017 mandated always using the latest version
of the Unicode standard.
11.6: In ECMAScript 2015, the valid code points for an
IdentifierName
are specified in terms of the Unicode properties “ID_Start” and
“ID_Continue”. In previous editions, the valid
IdentifierName
or
Identifier
code points were specified by enumerating various Unicode code point
categories.
11.9.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at
the end of a do-while statement if the semicolon is missing. This change
aligns the specification with the actual behaviour of most existing
implementations.
12.2.6.1: In ECMAScript 2015, it is no longer an
early error
to have duplicate property names in Object Initializers.
12.15.1: In ECMAScript 2015,
strict mode code
containing an assignment to an immutable binding such as the function
name of a
FunctionExpression
does not produce an
early error. Instead it produces a runtime error.
13.2: In ECMAScript 2015, a
StatementList
beginning with the token let followed by the input elements
LineTerminator
then
Identifier
is the start of a
LexicalDeclaration. In previous editions, automatic semicolon insertion would always
insert a semicolon before the
Identifier
input element.
13.6.7: In ECMAScript 2015, the normal completion value of an
IfStatement
is never the value empty. If no
Statement
part is evaluated or if the evaluated
Statement
part produces a normal completion whose value is
empty, the completion value of the
IfStatement
is undefined.
13.7: In ECMAScript 2015, if the ( token of a for statement is
immediately followed by the token sequence let [ then the
let is treated as the start of a
LexicalDeclaration. In previous editions such a token sequence would be the start of an
Expression.
13.7: In ECMAScript 2015, if the ( token of a for-in statement is
immediately followed by the token sequence let [ then the
let is treated as the start of a
ForDeclaration. In previous editions such a token sequence would be the start of an
LeftHandSideExpression.
13.7: Prior to ECMAScript 2015, an initialization expression could appear
as part of the
VariableDeclaration
that precedes the inkeyword. In ECMAScript 2015, the
ForBinding
in that same position does not allow the occurrence of such an
initializer. In ECMAScript 2017, such an initializer is permitted only
in
non-strict code.
13.7: In ECMAScript 2015, the completion value of an
IterationStatement
is never the value empty. If the
Statement
part of an
IterationStatement
is not evaluated or if the final evaluation of the
Statement
part produces a completion whose value is empty,
the completion value of the
IterationStatement
is undefined.
13.11.7: In ECMAScript 2015, the normal completion value of a
WithStatement
is never the value empty. If evaluation of the
Statement
part of a
WithStatement
produces a normal completion whose value is
empty, the completion value of the
WithStatement
is undefined.
13.15: In ECMAScript 2015, it is an
early error
for a
Catch clause
to contain a var declaration for the same
Identifier
that appears as the
Catch clause
parameter. In previous editions, such a variable declaration would be
instantiated in the enclosing variable environment but the declaration's
Initializer
value would be assigned to the
Catch
parameter.
13.15,
18.2.1.3: In ECMAScript 2015, a runtime SyntaxError is
thrown if a
Catch clause
evaluates a non-strict direct eval whose eval code includes
a var or FunctionDeclaration declaration that
binds the same
Identifier
that appears as the
Catch clause
parameter.
13.15.8: In ECMAScript 2015, the completion value of a
TryStatement
is never the value empty. If the
Block part of
a
TryStatement
evaluates to a normal completion whose value is
empty, the completion value of the
TryStatement
is undefined. If the
Block part of
a
TryStatement
evaluates to a throw completion and it has a
Catch part
that evaluates to a normal completion whose value is
empty, the completion value of the
TryStatement
is undefined if there is no
Finally
clause or if its
Finally
clause evaluates to an empty normal completion.
14.3.8
In ECMAScript 2015, the function objects that are created as the values
of the [[Get]] or [[Set]] attribute of accessor properties in an
ObjectLiteral
are not
constructor
functions and they do not have a "prototype" own
property. In the previous edition, they were constructors and had a
"prototype" property.
19.1.2.6: In ECMAScript 2015, if the argument to Object.freeze is
not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object
argument always causes a TypeError to be thrown.
19.1.2.8: In ECMAScript 2015, if the argument to
Object.getOwnPropertyDescriptor is not an object an attempt
is made to coerce the argument using
ToObject. If the coercion is successful the result is used in place of the
original argument value. In the previous edition, a non-object argument
always causes a TypeError to be thrown.
19.1.2.10: In ECMAScript 2015, if the argument to
Object.getOwnPropertyNames is not an object an attempt is
made to coerce the argument using
ToObject. If the coercion is successful the result is used in place of the
original argument value. In the previous edition, a non-object argument
always causes a TypeError to be thrown.
19.1.2.12: In ECMAScript 2015, if the argument to
Object.getPrototypeOf is not an object an attempt is made
to coerce the argument using
ToObject. If the coercion is successful the result is used in place of the
original argument value. In the previous edition, a non-object argument
always causes a TypeError to be thrown.
19.1.2.14: In ECMAScript 2015, if the argument to
Object.isExtensible is not an object it is treated as if it
was a non-extensible ordinary object with no own properties. In the
previous edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.15: In ECMAScript 2015, if the argument to
Object.isFrozen is not an object it is treated as if it was
a non-extensible ordinary object with no own properties. In the previous
edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.16: In ECMAScript 2015, if the argument to
Object.isSealed is not an object it is treated as if it was
a non-extensible ordinary object with no own properties. In the previous
edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.17: In ECMAScript 2015, if the argument to Object.keys is
not an object an attempt is made to coerce the argument using
ToObject. If the coercion is successful the result is used in place of the
original argument value. In the previous edition, a non-object argument
always causes a TypeError to be thrown.
19.1.2.18: In ECMAScript 2015, if the argument to
Object.preventExtensions is not an object it is treated as
if it was a non-extensible ordinary object with no own properties. In
the previous edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.20: In ECMAScript 2015, if the argument to Object.seal is
not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object
argument always causes a TypeError to be thrown.
19.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a
bound function
is set to the [[GetPrototypeOf]] value of its target function. In the
previous edition, [[Prototype]] was always set to
%Function.prototype%.
19.2.4.1: In ECMAScript 2015, the "length" property of
function instances is configurable. In previous editions it was
non-configurable.
19.5.6.2: In ECMAScript 2015, the [[Prototype]] internal slot of a
NativeErrorconstructor
is the Error
constructor. In previous editions it was the Function prototype object.
20.4.4
In ECMAScript 2015, the Date prototype object is not a Date instance. In
previous editions it was a Date instance whose TimeValue was
NaN.
21.1.3.10
In ECMAScript 2015, the
String.prototype.localeCompare function must treat Strings
that are canonically equivalent according to the Unicode standard as
being identical. In previous editions implementations were permitted to
ignore canonical equivalence and could instead use a bit-wise
comparison.
21.1.3.25
and
21.1.3.27
In ECMAScript 2015, lowercase/upper conversion processing operates on
code points. In previous editions such the conversion processing was
only applied to individual code units. The only affected code points are
those in the Deseret block of Unicode.
21.1.3.28
In ECMAScript 2015, the String.prototype.trim method is
defined to recognize white space code points that may exists outside of
the Unicode BMP. However, as of Unicode 7 no such code points are
defined. In previous editions such code points would not have been
recognized as white space.
21.2.3.1
In ECMAScript 2015, If the pattern argument is a RegExp
instance and the flags argument is not
undefined, a new RegExp instance is created just like
pattern except that pattern's flags are replaced
by the argument flags. In previous editions a
TypeError exception was thrown when
pattern was a RegExp instance and flags was not
undefined.
21.2.5
In ECMAScript 2015, the RegExp prototype object is not a RegExp
instance. In previous editions it was a RegExp instance whose pattern is
the empty string.
21.2.5
In ECMAScript 2015, "source",
"global", "ignoreCase", and
"multiline" are accessor properties defined on the
RegExp prototype object. In previous editions they were data properties
defined on RegExp instances.
24.4.12: In ECMAScript 2019, Atomics.wake has been renamed to
Atomics.notify to prevent confusion with
Atomics.wait.
25.1.4.4,
25.5.3.5: In ECMAScript 2019, the number of Jobs enqueued by
await was reduced, which could create an observable
difference in resolution order between a then() call and an
await expression.
FColophon
This specification is authored on
GitHub in a plaintext
source format called
Ecmarkup. Ecmarkup is
an HTML and Markdown dialect that provides a framework and toolset for
authoring ECMAScript specifications in plaintext and processing the
specification into a full-featured HTML rendering that follows the
editorial conventions for this document. Ecmarkup builds on and
integrates a number of other formats and technologies including
Grammarkdown for
defining syntax and
Ecmarkdown for
authoring algorithm steps. PDF renderings of this specification are
produced by printing the HTML rendering to a PDF.
Prior editions of this specification were authored using Word—the
Ecmarkup source text that formed the basis of this edition was produced
by converting the ECMAScript 2015 Word document to Ecmarkup using an
automated conversion tool.
GBibliography
IEEE Std 754-2008: IEEE Standard for Floating-Point Arithmetic.
Institute of Electrical and Electronic Engineers, New York (2008)
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