countBy (Functional Programming)

Creates a function that counts values by a key. Use it with pipe.

const result = pipe(array, countBy(mapper));

INFO

Prefer the original es-toolkit countBy in ordinary code. Use this fp variant when composing transformations with pipe.

Usage

countBy calls mapper for each value in the piped array and returns an object whose keys are mapper results and whose values are counts.

import { countBy, pipe } from 'es-toolkit/fp';

pipe(
  ['one', 'two', 'three'],
  countBy(word => word.length)
); // => { 3: 2, 5: 1 }

Parameters

• mapper ((item: T) => K): The function that returns the key used for counting.

Returns

((array: readonly T[]) => Record<K, number>): A function that maps a readonly T[] to counts by key.