es-toolkit/dist/map/countBy.mjs

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

//#region src/map/countBy.ts
/**
* Counts the occurrences of items in a Map based on a transformation function.
*
* This function takes a Map and a function that generates a key from each value-key pair.
* It returns an object with the generated keys as properties and their counts as values.
* The count is incremented for each entry for which the transformation produces the same key.
*
* @template K - The type of the Map's keys.
* @template V - The type of the Map's values.
* @template K2 - The type of keys produced by the transformation function.
* @param map - The Map to count occurrences from.
* @param mapper - The function to produce a key for counting.
* @returns An object containing the mapped keys and their counts.
*
* @example
* const map = new Map([
*   ['a', 1],
*   ['b', 2],
*   ['c', 1]
* ]);
* const result = countBy(map, (value) => value);
* // result will be { 1: 2, 2: 1 }
*
* @example
* const map = new Map([
*   ['alice', 20],
*   ['bob', 30],
*   ['carol', 20]
* ]);
* const result = countBy(map, (value, key) => key[0]);
* // result will be { a: 1, b: 1, c: 1 }
*/
function countBy(map, mapper) {
	const result = /* @__PURE__ */ new Map();
	for (const [key, value] of map) {
		const mappedKey = mapper(value, key, map);
		result.set(mappedKey, (result.get(mappedKey) ?? 0) + 1);
	}
	return result;
}
//#endregion
export { countBy };