How to group an array of objects by key?

Question

How do you group an array of objects by an object key to create a new array of objects based on the grouping? For example, I have an array of car objects:

const array = [
  {red: [ {height: 50} ]},
  {green: [ {height: 20} ]},
  {blue: [ {height: 30} ]},
  {blue: [ {height: 40} ]},
  {red: [ {height: 10} ]},
  {green: [ {height: 60} ]}
]

I want to create a new array of objects.(key is color)

const result = [
  {red: [{height: 50}, {height: 10}]},
  {green: [{height: 20}, {height: 60}]},
  {blue: [{height: 30}, {height: 40}]}
]

I tried to use lodash.groupBy, however I don't know how to solve this problem at all.

The first one is exactly doing what OP was asking suing lodash — mplungjan, Jan 14 '19 at 08:12
@mplungjan all dupes seem to be grouping by key value, not key name. — Keith, Jan 14 '19 at 08:13
Hey, It's not duplicate question. Look at my question carefully. — sonicmario, Jan 14 '19 at 08:15
Im on mobile atm, but the trick would be to use `Object.keys(item)[0]` to get keyname for grouping purposes. — Keith, Jan 14 '19 at 08:18
https://stackoverflow.com/a/40774906/295783 - seems very close to the accepted answer — mplungjan, Jan 14 '19 at 08:26

Drew Reese · Accepted Answer · 2019-01-14T08:22:35.580

6

Using array reduce you can iterate data and calculate result object.

const array = [
  { 'red': [ { height: 50 } ] },
  { 'green': [ { height: 20 } ] },
  { 'blue': [ { height: 30 } ] },
  { 'blue': [ { height: 40 } ] },
  { 'red': [ { height: 10 } ] },
  { 'green': [ { height: 60 } ] }
];

const res = array.reduce((acc, element) => {
  // Extract key and height value array
  const [key, heightValue] = Object.entries(element)[0];
  // Get or create if non-exist, and push height value from array, index 0
  (acc[key] || (acc[key] = [])).push(heightValue[0]);
  return acc;
}, {});

console.log(res);

edited Jan 14 '19 at 08:22

answered Jan 14 '19 at 08:19

Drew Reese

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Looks pretty much like one of the answers in the dupe I suggested https://stackoverflow.com/a/40774906/295783 - more elegant but very close – mplungjan Jan 14 '19 at 08:24

Ori Drori · Answer 2 · 2019-01-14T09:02:08.330

You can use lodash's _.mergeWith() to combine the objects with the same key, and then use _.map() to convert it back to an array:

const array = [{"red":[{"height":50}]},{"green":[{"height":20}]},{"blue":[{"height":30}]},{"blue":[{"height":40}]},{"red":[{"height":10}]},{"green":[{"height":60}]}]

const fn = _.flow([
  arr => _.mergeWith({}, ...arr, (o, s) => _.isArray(o) ? o.concat(s) : s),
  objs => _.map(objs, (v, k) => ({ [k]: v }))
])

const result = fn(array)

console.log(result)

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.min.js"></script>

How to group an array of objects by key?

2 Answers2

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