Reducer function for a nested array with conditions

I’m struggling to write a reducer function for the nested object that I have.

{
    "queryResult": [{
        "__typename": "Parent",
        "_id": "ABC",
        "items": [{
            "__typename": "Child",
            "_id": "123",
            "subitems": [{
                "__typename": "Grandchild",
                "_id": "abc",
            }, {
                "__typename": "Grandchild",
                "_id": "def",
            }, {
                "__typename": "Grandchild",
                "_id": "ghi",
            }, {
                "__typename": "Grandchild",
                "_id": "jkl",
            }, {
                "__typename": "Grandchild",
                "_id": "mno",
            }, {
                "__typename": "Grandchild",
                "_id": "pqr",
            }]
        }, {
            "__typename": "Child",
            "_id": "456",
            "subitems": [{
                "__typename": "Grandchild",
                "_id": "aaa",
            }, {
                "__typename": "Grandchild",
                "_id": "bbb",
            }, {
                "__typename": "Grandchild",
                "_id": "ccc",
            }, {
                "__typename": "Grandchild",
                "_id": "ddd",
            }]
        }, {
            "__typename": "Child",
            "_id": "789",
            "subitems": [{
                "__typename": "Grandchild",
                "_id": "eee",
            }]
        }]
    }]
}

queryResult can have many Parents. Every Parent has items, and every item has subitems with their .ids.

How to write a reducer that would reduce the queryResult by taking in the parentId and the itemId and returning the array of subitems? For example, for parentId = ABC and for the itemId = 456 I need a result that looks like:

 "subitems": [{
  "__typename": "Grandchild",
  "_id": "aaa",
}, {
  "__typename": "Grandchild",
  "_id": "bbb",
}, {
  "__typename": "Grandchild",
  "_id": "ccc",
}, {
  "__typename": "Grandchild",
  "_id": "ddd",
}]

Note: all the IDs are random, there is no logic there whatsoever.

Answer

You can reduce queryResult to a map of parentId-itemId as the key and subitems as the value as follows:

const data = {
  "queryResult": [
    {
      "__typename": "Parent",
      "_id": "ABC",
      "items": [
        {
          "__typename": "Child",
          "_id": "123",
          "subitems": [
            { "__typename": "Grandchild", "_id": "abc" }, 
            { "__typename": "Grandchild", "_id": "def" }, 
            { "__typename": "Grandchild", "_id": "ghi" }, 
            { "__typename": "Grandchild", "_id": "jkl" }, 
            { "__typename": "Grandchild", "_id": "mno" }, 
            { "__typename": "Grandchild", "_id": "pqr" }
          ]
        }, 
        {
          "__typename": "Child",
          "_id": "456",
          "subitems": [
            { "__typename": "Grandchild", "_id": "aaa" }, 
            { "__typename": "Grandchild", "_id": "bbb" }, 
            { "__typename": "Grandchild", "_id": "ccc" }, 
            { "__typename": "Grandchild", "_id": "ddd" }
          ]
        }, 
        {
          "__typename": "Child",
          "_id": "789",
          "subitems": [
            { "__typename": "Grandchild", "_id": "eee" }
          ]
        }
      ]
    }
  ]
};

const res = data.queryResult.reduce((acc,parent) => {
  const { _id:parentId, items } = parent;
  items.forEach(({_id:itemId, subitems}) => {
    acc[`${parentId}-${itemId}`] = subitems;
  });
  return acc;
}, {});

console.log(res);