Javascript Grouping by object property, when property is an array

Question

I currently have an array of items that looks a bit like this: I want to group the items by category lookup, with the slight problem that category lookup is potentially an array, such that Parent Item 2 would be listed twice (once in My Cat) and once in something else) I tried using https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/groupBy but it doesn't seem to be able to handle this?

     [
           {
                "tool_id": "4-19de-454673d9-9ef5-4545",
                "somekey" : "Parent Item 2"
                "categoryLookup": [
                    {
                        "category_name": "New Item",
                    }
                ]
            },
            {
                "tool_id": "be7ea707-19de-43d9-9ef1-d4a3ff79f77a",
                "somekey" : "Parent Item"
                "categoryLookup": [
                    {
                        "category_name": "My Cat",
                    },
                    {
                        "category_name": "Something Else",
                    }
                ]
            }
     ]

The final result would look something like:

    [
       {
           New Item: [
              {...contains 4-19de-454673d9-9ef5-4545 }
           ],
           My Cat: [
              {...contains be7ea707-19de-43d9-9ef1-d4a3ff79f77a}
           ],
           Something Else: [
              {... contain be7ea707-19de-43d9-9ef1-d4a3ff79f77a} 
           ]
        }
    ]

.groupBy() only allows one key per element, so no, it would not work. — Pointy
– Pointy, Commented Apr 29, 2022 at 13:08
What would the expected result look like if more than item belongs to a certain category? — Mario Varchmin
– Mario Varchmin, Commented Apr 29, 2022 at 13:11
And how is this wrong? You'd want to find the Parent Item when looking up either category, right? What output are you trying to get? — DustInComp
– DustInComp, Commented Apr 29, 2022 at 13:11
Could you just loop over the original array and construct the final result? Also I guess the same category_name could be in multiple objects and thus giving you more than one object in some of the final arrays... — Johan Faerch
– Johan Faerch, Commented Apr 29, 2022 at 13:17

Amir MB · Accepted Answer · 2022-04-29 13:42:40Z

3

You can iterate over the original array and create the final one:

var data = [{
    "tool_id": "4-19de-454673d9-9ef5-4545",
    "somekey": "Parent Item 2",
    "categoryLookup": [{
      "category_name": "New Item",
    }]
  },
  {
    "tool_id": "be7ea707-19de-43d9-9ef1-d4a3ff79f77a",
    "somekey": "Parent Item",
    "categoryLookup": [{
        "category_name": "My Cat",
      },
      {
        "category_name": "Something Else",
      }
    ]
  }
];

function groupByCategory(data) {
  const res = {};

  data.forEach(item => {
    item.categoryLookup.forEach(category => {
      const name = category.category_name;
      res[name] ??= [];
      res[name].push({
        item: item.tool_id //or all the properties you want
      });
    });
  });
  return res;
}

console.log( groupByCategory(data) );

edited Apr 29, 2022 at 13:42

answered Apr 29, 2022 at 13:15

Amir MB

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4 Comments

RenaudC5 Over a year ago

Might be res[name] ?? []; instead @Lee Taylor

Amir MB Over a year ago

@LeeTaylor please go to the edit history and you can see that YOU changed it to ? ?, it was correct before your edit!!

Lee Taylor Over a year ago

@AmirMB Apologies. It seems that clicking the "Tidy" button in the snippet editor does that. Sorry again.

Amir MB Over a year ago

@LeeTaylor no worries.

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