es-toolkit/dist/set/countBy.mjs

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

//#region src/set/countBy.ts
/**
* Counts the occurrences of items in a Set based on a transformation function.
*
* This function takes a Set and a function that generates a key from each value.
* It returns a Map with the generated keys and their counts as values.
* The count is incremented for each element for which the transformation produces the same key.
*
* @template T - The type of elements in the Set.
* @template K - The type of keys produced by the transformation function.
* @param set - The Set to count occurrences from.
* @param mapper - The function to produce a key for counting.
* @returns A Map containing the mapped keys and their counts.
*
* @example
* const set = new Set([1, 2, 3, 4, 5]);
* const result = countBy(set, (value) => value % 2 === 0 ? 'even' : 'odd');
* // result will be Map(2) { 'odd' => 3, 'even' => 2 }
*
* @example
* const set = new Set(['apple', 'banana', 'cherry']);
* const result = countBy(set, (value) => value.length);
* // result will be Map(2) { 5 => 1, 6 => 2 }
*/
function countBy(set, mapper) {
	const result = /* @__PURE__ */ new Map();
	for (const value of set) {
		const mappedKey = mapper(value, value, set);
		result.set(mappedKey, (result.get(mappedKey) ?? 0) + 1);
	}
	return result;
}
//#endregion
export { countBy };