type KeySelector<T> = (item: T) => string;

export function groupBy<T>(array: Iterable<T>, keySelector: KeySelector<T>): Record<string, T[]> {
  return Array.from(array).reduce(
    (acc: Record<string, T[]>, item: T) => {
      const key = keySelector(item);
      if (key in acc) {
        // found key, push new item into existing array
        acc[key].push(item);
      } else {
        // did not find key, create new array
        acc[key] = [item];
      }
      return acc;
    },
    {} // start with empty object
  );
}