How does group by nested array with parent and child?

I have problem with array JSON. I think it nested group by the first is group by _id and second one is group by itemId I do not know how to solve it,Please help me

 let data=[
   {
    "_id" : "013",
    "items" : [
        {
            "itemId" : "4",
            "stockIn" : 50,
            "stockOut" : 0
        },
        {
            "itemId" : "9",
            "stockIn" : 100,
            "stockOut" : 0
        }
    ]
  },
  {
    "_id" : "013",
    "items" : [
        {
            "itemId" : "3",
            "noFreeQtyCount" : 20,
            "freeQtyCount" : 0
        },
        {
            "itemId" : "9",
            "noFreeQtyCount" : 30,
            "freeQtyCount" : 0
        }
    ]
  }
 ]

This is my data that I want

[
   {
    "_id" : "013",
    "items" : [
        {
            "itemId" : "3",
            "noFreeQtyCount" : 20,
            "freeQtyCount" : 0
        },
        {
            "itemId" : "4",
            "stockIn" : 50,
            "stockOut" : 0
        },
        {
            "itemId" : "9",
            "stockIn" : 100,
            "stockOut" : 0
            "noFreeQtyCount" : 30,
            "freeQtyCount" : 0
        }
    ]
  },
 ]

This my script please help

 const groupBy = data.reduce((acc, cur) => {
acc[cur._id] ? acc[cur._id].items = [...acc[cur._id].items, ...cur.items] : acc[cur._id] = cur;
return acc;
 }, {});

const output = groupBy;
print(output);

>Solution :

You can use Object.key(), Array.prototype.map() and Object.assign() accomplish the grouping you need.

Refer the code below:

    function groupByItemId(data) {
  const mergedData = {};

  data.forEach(entry => {
    const _id = entry["_id"];
    const items = entry["items"];

    if (!mergedData[_id]) {
      mergedData[_id] = {};
    }

    items.forEach(item => {
      const itemId = item["itemId"];

      if (!mergedData[_id][itemId]) {
        mergedData[_id][itemId] = {};
      }

      Object.assign(mergedData[_id][itemId], item);
    });
  });

  const result = Object.keys(mergedData).map(_id => {
    return {"_id": _id, "items": Object.values(mergedData[_id])};
  });

  return result;
}

const inputData = [
  {
    "_id": "013",
    "items": [
      {"itemId": "4", "stockIn": 50, "stockOut": 0},
      {"itemId": "9", "stockIn": 100, "stockOut": 0},
    ],
  },
  {
    "_id": "013",
    "items": [
      {"itemId": "3", "noFreeQtyCount": 20, "freeQtyCount": 0},
      {"itemId": "9", "noFreeQtyCount": 30, "freeQtyCount": 0},
    ],
  },
];

const result = groupByItemId(inputData);
console.log(result);