Map.groupBy minimal ponyfill

mapGroupBy.ts

/* This is equivalent to Map.groupBy; replace this function with Map.groupBy in Node 21+ */
function mapGroupBy<T, K>(array: T[], iteratee: (value: T, index: number, array: T[]) => K) {
  return array.reduce((result, value, index, array) => {
    const key = iteratee(value, index, array);
    const group = result.get(key);
    if (group) {
      group.push(value);
    } else {
      result.set(key, [value]);
    }
    return result;
  }, new Map<K, T[]>());
}