countBy (Functional Programming)

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

typescript

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.

typescript

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

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

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

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