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A proposal to make grouping of items in an array easier.

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

// group groups items by arbitrary key.
// In this case, we're grouping by even/odd keys, index, array) => {
  return num % 2 === 0 ? 'even': 'odd';

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

// groupToMap returns items in a Map, and is useful for grouping using
// an object key.
const odd  = { odd: true };
const even = { even: true };
array.groupToMap((num, index, array) => {
  return num % 2 === 0 ? even: odd;

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



Current Stage: 3


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, group and groupToMap. 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 group and not groupBy?

We've found a web compatibility issue with the name groupBy. The 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.

In a similar situation, we renamed the Array.prototype.flatten proposal to flat. Note that we did not rename it to flattened (though it was considered). We've taken the same approach, renaming groupBy to group.




A proposal to make grouping of array items easier