select.ts

export function select<T, Props, K>(selector: (state: T, props?: Props) => any) {
  return function selectOperator(source$: Observable<T>): Observable<K> {
    let mapped$: Observable<any>;

    mapped$ = source$.pipe(
      map(source => selector(source))
    );

    return mapped$.pipe(distinctUntilChanged());
  };
}