es-toolkit/docs/reference/array/countBy.md

countBy

Count the occurrences of each item in an array based on a mapper function.

Signature

function countBy<T, K extends PropertyKey>(arr: T[], mapper: (item: T) => K): Record<K, number>;

Parameters

• arr (T[]): The input array to count occurrences.
• mapper ((item: T) => K): The transformation function that maps each item to a key.

Returns

(Record<K, number>) An object containing the count of each item based on the transformation function.

Examples

import { countBy } from 'es-toolkit/array';

const array = [1, 2, 3, 4, 5, 6];
const result = countBy(array, x => (x % 2 === 0 ? 'even' : 'odd'));

console.log(result);
// Output: { 'odd': 3, 'even': 3 }