aureooms/js-array

export function group(key, iterable, out) {
    const len = iterable.length;

    if (len === 0) {
        return out;
    }

    let i = 0;
    let item = iterable[i];
    let next = key(item);

    do {
        const curr = next;
        const items = [item];
        const tuple = [curr, items];

        ++i;

        while (i < len) {
            item = iterable[i];
            next = key(item);

            if (next !== curr) {
                break;
            }

            items.push(item);
            ++i;
        }

        out.push(tuple);
    } while (i < len);

    return out;
}