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.

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;
}

export { countBy };