Array grouping

class Object {
  static groupBy(items: Iterable, callbackfn: (value: any, index: number) => key): { [key]: Array<mixed> };
}

class Map {
  static groupBy(items: Iterable, callbackfn: (value: any, index: number) => key): Map<key, Array<mixed>>;
}

CommonJS entry points

core-js/proposals/array-grouping-v2