mapGroupByA polyfill for the Map.groupBy() static method. function mapGroupBy<K, T>(items: Iterable<T>, keySelector: (item: T, index: number) => K): Map<K, T[]>;§mapGroupBy<K, T>(items: Iterable<T>, keySelector: (item: T, index: number) => K): Map<K, T[]>[src]§Type Parameters§K[src]§T[src]§Parameters§items: Iterable<T>[src]§keySelector: (item: T, index: number) => K[src]§Return Type§Map<K, T[]>[src]