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https://github.com/tc39/proposal-array-grouping

A proposal to make grouping of array items easier
https://github.com/tc39/proposal-array-grouping

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A proposal to make grouping of array items easier

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# proposal-array-grouping



Note: this proposal is now [at stage 4](https://github.com/tc39/proposals/commit/e11ea25d41c8ab28b6632b5d355359dcb528ee6f). See the spec PR here: https://github.com/tc39/ecma262/pull/3176



A proposal to make grouping of items in an array (and iterables) easier.



```js

const array = [1, 2, 3, 4, 5];



// `Object.groupBy` groups items by arbitrary key.

// In this case, we're grouping by even/odd keys

Object.groupBy(array, (num, index) => {

  return num % 2 === 0 ? 'even': 'odd';

});

// =>  { odd: [1, 3, 5], even: [2, 4] }



// `Map.groupBy` returns items in a Map, and is useful for grouping

// using an object key.

const odd  = { odd: true };

const even = { even: true };

Map.groupBy(array, (num, index) => {

  return num % 2 === 0 ? even: odd;

});

// =>  Map { {odd: true}: [1, 3, 5], {even: true}: [2, 4] }

```



## Champions



- Justin Ridgewell ([@jridgewell](https://github.com/jridgewell))

- Jordan Harband ([@ljharb](https://github.com/ljharb))



## Status



Current [Stage](https://tc39.es/process-document/): 4



## Motivation



Array grouping is an extremely common operation, best exemplified by

SQL's `GROUP BY` clause and MapReduce programming (which is better

thought of map-group-reduce). The ability to combine like data into

groups allows developers to compute higher order datasets, like the

average age of a cohort or daily LCP values for a webpage.



Two methods are offered, `Object.groupBy` and `Map.groupBy`. The first

returns a null-prototype object, which allows ergonomic destructuring

and prevents accidental collisions with global Object properties. The

second returns a regular `Map` instance, which allows grouping on

complex key types (imagine a compound key or [tuple]).



## Why static methods?



We've [found][sugar-bug] a web compatibility issue with the name

`Array.prototype.groupBy`. The [Sugar][sugar] library until v1.4.0

conditionally monkey-patches `Array.prototype` with an incompatible

method. By providing a native `groupBy`, these versions of Sugar would

fail to install their implementation, and any sites that depend on their

behavior would break. We've found some 660 origins that use these

versions of the Sugar library.



We then attempted the name `Array.prototype.group`, but this ran into

[code that uses an array as an arbitrary hashmap][arraymap-bug]. Because

these bugs are exceptionally difficult to detect (it requires devs to

detect and know how to report the bug to us), the committee didn't want

to attempt another prototype method name. Instead we chose to use static

method, which we believe is unorthodox enough to not risk a web

compatibility issue. This also gives us a nice way to support Records

and Tuples in the future.



## Polyfill



- A polyfill is available in the [core-js] library. You can find it in

  the [ECMAScript proposals section][core-js-section].



## Related



- Lodash

  - [`_.groupBy`][lodash] ([850k downloads/week][lodash-npm])



[tuple]: https://github.com/tc39/proposal-record-tuple

[core-js]: https://github.com/zloirock/core-js

[core-js-section]: https://github.com/zloirock/core-js#array-grouping

[lodash]: https://lodash.com/docs/4.17.15#groupBy

[lodash-npm]: https://www.npmjs.com/package/lodash.groupby

[sugar]: https://sugarjs.com/

[sugar-bug]: https://github.com/tc39/proposal-array-grouping/issues/37

[arraymap-bug]: https://github.com/tc39/proposal-array-grouping/issues/44