es-toolkit/dist/array/countBy.mjs

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

//#region src/array/countBy.ts
/**
* Count the occurrences of each item in an array
* based on a transformation function.
*
* This function takes an array and a transformation function
* that converts each item in the array to a key. It then
* counts the occurrences of each transformed item and returns
* an object with the transformed items as keys and the counts
* as values.
*
* @template T - The type of the items in the input array.
* @template K - The type of keys.
* @param arr - The input array to count occurrences.
* @param mapper - The transformation function that maps each item, its index, and the array to a key.
* @returns An object containing the transformed items as keys and the
* counts as values.
*
* @example
* const array = ['a', 'b', 'c', 'a', 'b', 'a'];
* const result = countBy(array, x => x);
* // result will be { a: 3, b: 2, c: 1 }
*
* @example
* const array = [1, 2, 3, 4, 5];
* const result = countBy(array, item => item % 2 === 0 ? 'even' : 'odd');
* // result will be { odd: 3, even: 2 }
*
* @example
* // Using index parameter
* const array = ['a', 'b', 'c', 'd'];
* const result = countBy(array, (item, index) => index < 2 ? 'first' : 'rest');
* // result will be { first: 2, rest: 2 }
*/
function countBy(arr, mapper) {
	const result = {};
	for (let i = 0; i < arr.length; i++) {
		const item = arr[i];
		const key = mapper(item, i, arr);
		result[key] = (result[key] ?? 0) + 1;
	}
	return result;
}
//#endregion
export { countBy };