es-toolkit/dist/fp/array/countBy.d.ts

A state-of-the-art, high-performance JavaScript utility library with a small bundle size and strong type annotations.

//#region src/fp/array/countBy.d.ts
/**
 * Creates a function that counts values by a derived key.
 *
 * The mapper receives each value, its index, and the full input array. The returned object
 * uses mapper results as keys and occurrence counts as values.
 *
 * @template T - The type of elements in the array.
 * @template K - The property-key type produced by the mapper.
 * @param mapper - Called with each value, index, and array to produce a key.
 * @returns A function that maps a readonly array to counts by key.
 *
 * @example
 * import { countBy, pipe } from 'es-toolkit/fp';
 *
 * pipe([1, 2, 3, 4, 5], countBy(value => (value % 2 === 0 ? 'even' : 'odd')));
 * // => { odd: 3, even: 2 }
 */
declare function countBy<T, K extends PropertyKey>(mapper: (item: T, index: number, array: readonly T[]) => K): (array: readonly T[]) => Record<K, number>;
//#endregion
export { countBy };