minecraft-assets/data/minecraft/recipes/chain.json

{
  "type": "minecraft:crafting_shaped",
  "category": "misc",
  "key": {
    "I": {
      "item": "minecraft:iron_ingot"
    },
    "N": {
      "item": "minecraft:iron_nugget"
    }
  },
  "pattern": [
    "N",
    "I",
    "N"
  ],
  "result": {
    "item": "minecraft:chain"
  }
}