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.

function countBy(set, mapper) {
    const result = new Map();
    for (const value of set) {
        const mappedKey = mapper(value, value, set);
        result.set(mappedKey, (result.get(mappedKey) ?? 0) + 1);
    }
    return result;
}

export { countBy };