For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt, and this page is available as Markdown at /reference/fp/countBy.md.
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    countBy (Functional Programming) es-toolkit

    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) => Krequired

    The function that returns the key used for counting.

    Returns

    A function that maps a readonly T[] to counts by key.

    (array: readonly T[]) => Record<K, number>
    Source: es-toolkit

    Re-exported verbatim from es-toolkit. Implementation, edge cases, and performance behavior are owned upstream. This page mirrors the documentation at the pinned version; the linked source is authoritative.