JSON array index by

json-array-index-by · version 1.0.0 · JSON & JSONL · free, no key needed

Convert an array of objects into a single object indexed by a specific key.

Use when you need to: json array index by · index json array by key · convert array to map by key.

Supported

• json array index by
• index json array by key
• convert array to map by key

Not supported

• multiple keys
• deep indexing

Behavior

• Creates a single JSON object where keys are the stringified values of the specified index field.
• If multiple objects have the same key, the behavior is determined by on_duplicate ("last", "first", or "throw").
• Result keys are ordered by first appearance.
• Missing or null keys are treated as the string "undefined" or "null".

Input

• array (array of object, required): max items 10000
• key (string, required): min length 1
• on_duplicate (one of "last", "first", "throw", optional): default "last"

Output

• result (object, required)
• count (integer, required)

Limits

• max array items: 10000
• max output items: 10000

Example

Request input:

{
  "array": [
    {
      "id": "a",
      "val": 1
    },
    {
      "id": "b",
      "val": 2
    },
    {
      "id": "a",
      "val": 3
    }
  ],
  "key": "id",
  "on_duplicate": "last"
}

Response:

{
  "result": {
    "result": {
      "a": {
        "id": "a",
        "val": 3
      },
      "b": {
        "id": "b",
        "val": 2
      }
    },
    "count": 2
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

{
  "id": "json-array-index-by",
  "version": "1.0.0",
  "input": {
    "array": [
      {
        "id": "a",
        "val": 1
      },
      {
        "id": "b",
        "val": 2
      },
      {
        "id": "a",
        "val": 3
      }
    ],
    "key": "id",
    "on_duplicate": "last"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/json-array-index-by/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"array":[{"id":"a","val":1},{"id":"b","val":2},{"id":"a","val":3}],"key":"id","on_duplicate":"last"}'

The machine-readable contract is at /v1/tools/json-array-index-by/versions/1.0.0.

CLI

node cli.mjs run json-array-index-by 1.0.0 --input input.json --base-url https://computefirst.net

Get the client at /clients/cli/.

Related tools

• Array to JSONL: Serialize an array of JSON-safe values as JSON Lines text.
• JSON array distinct by: Deduplicate a JSON array of objects based on specific keys.
• JSON array group count: Group an array of JSON objects by exact keys and return the count.
• JSONL to array: Parse strict JSON Lines text into an array of JSON-safe values.
• JSON array anti join: Anti join two JSON arrays of objects on exact keys.
• JSON array filter equals: Filter JSON array objects where a key equals a specific value.