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

{
  "type": "minecraft:crafting_shaped",
  "category": "misc",
  "key": {
    "I": "minecraft:iron_ingot",
    "N": "minecraft:iron_nugget"
  },
  "pattern": [
    "N",
    "I",
    "N"
  ],
  "result": {
    "count": 1,
    "id": "minecraft:chain"
  }
}