countBy

Groups the entries of a Map by a derived key and counts how many fall into each group.

Available since v3.0.3

Import

import { countBy } from '@helpers4/map';
// or, from the all-in-one package (same code, one install):
import { countBy } from 'helpers4/map';

Signature

countBy<K, V, R>(map: ReadonlyMap<K, V>, fn: function): Map<R, number>

Parameters

Parameter	Type	Description
map	ReadonlyMap<K, V>	The Map to summarize
fn	function	Function deriving the grouping key from (value, key)

Returns

Map<R, number> — A Map from derived key to the number of entries in that group

Examples

Count entries by parity

Groups values by a derived key and counts how many fall into each group.

countBy(new Map([['a', 1], ['b', 2], ['c', 3], ['d', 4]]), v => v % 2 === 0 ? 'even' : 'odd')
// => Map(2) { 'odd' => 2, 'even' => 2 }

Name conflict

A helper named countBy also exists in @helpers4/array, @helpers4/set. If you need both in the same file, rename at import with as:

import { countBy as countBy4map } from '@helpers4/map';
import { countBy as countBy4array } from '@helpers4/array';
import { countBy as countBy4set } from '@helpers4/set';

See Name Conflicts for the full resolution guide.

Source

View source on GitHub