Transform an array into an object in JSONata

Question

Here's my input data:

$array := [
  {
    "thing": "one",
    "stuff": {
      "type": "car",
      "engine": "vrooom"
    }
  },
  {
    "thing": "two",
    "stuff": {
      "type": "truck",
      "engine": "vrooom"
    }
  },
  {
      "thing": "three",
      "stuff": {
          "type": "car",
          "engine": "vroom"
      }
  }
];

Ultimately I want the output to look like this:

{
  "car": [
    {
      "thing": "one",
      "stuff": {
        "type": "car",
        "engine": "vroom"
      }
    },
    {
      "thing": "three",
      "stuff": {
        "type": "car",
        "engine": "vroom"
      }
    }
  ],
  "truck": [
    {
      "thing": "two",
      "stuff": {
        "type": "truck",
        "engine": "vrooom"
      }
    },
  ]
}

I've tried like this:

$reduce($array, function($accumulator, $value, $index){ 
  $merge([$accumulator, {$value.stuff.type: [$accumulator.$value, $value]}]) 
}, {} )

but the output looks like this:

{
  "car": [
    {
      "thing": "three",
      "stuff": {
        "type": "car",
        "engine": "vroom"
      }
    },
    {
      "thing": "three",
      "stuff": {
        "type": "car",
        "engine": "vroom"
      }
    }
  ],
  "truck": [
    {
      "thing": "two",
      "stuff": {
        "type": "truck",
        "engine": "vrooom"
      }
    },
    {
      "thing": "two",
      "stuff": {
        "type": "truck",
        "engine": "vrooom"
      }
    }
  ]
}

I feel like it has something to do with $accumulator.$value but I'm not sure how to get the syntax proper.

Here's a playground to try: https://try.jsonata.org/gM7o-EOV4

Answer 1

Try this:

${
    stuff.type: [$]
}

See https://try.jsonata.org/VoX6C9N8V