=> void, onCompleted: () => void, ): Subscription; map( (T) => V): Observable<V>; flatMap((T) => Observable<V>): Observable<V>; scan( (V, T) => V, V): Observable<V>; filter( (T) => boolean): Observable<T>; withLatestFrom(Observable<V>, ...): Observable<[T, V, ...]>; combineLatest( Observable<V>, ...): Observable<[T, V, ...]>; … any many more! }