Calling another observable with the response of another in Angular 4

Usually, when you have one Observable and you need to grab something from . it and return a different Observable, you can use switchMap:

ngOnInit() {    this._moviesDataService.getShowtimes()        .switchMap(res => {             const id = Object.keys(res[0].showtimes)[0]; // assuming you have one element in your array and you want the first id from showtimes            return this.getMovies(id); // assuming, you have a separate method that returns the movies        })        .subscribe(res => this.results = res)}

UPDATE

Since you need results of both Observable but also you need the results of the first to request the second here's an idea how you can do this:

ngOnInit() {    this._moviesDataService.getShowtimes()        .switchMap(res => {             const showtimes = res[0].showtimes;            const id = Object.keys(showtimes)[0];            return Observable.zip(                this.getMovies(id),                Observable.of(showtimes[id])            );        })        .subscribe(([movies, showtimes]) => {            this.results.movies = movies; // or assign it to some other property            this.results.showtimes = showtimes; // and use in the template        }

I think because you need to retrieve titles of all the movies, you have to chain the array of IDs in the first request's response, into a series of requests for movies' titles. something like this: (assuming you have a method like getMovieTitle that gets data of a movie based on it's id and it returns an observable)

this._moviesDataService.getShowtimes()        .switchMap(res => {             let resArray: any[] = res.map(                    item=>this._moviesDataService.getMovieTitle(                        Object.keys(item.showtimes)[0]            ))            return Observable.merge(...resArray);        })        .subscribe(res => /*you will get a new res for each movie title here*/)

what Observable.merge does, is it Turn multiple observables into a single observable. so you will get all the results in one subscription.

Note:don't forget to assign all of this to a subscription and unsubscribe it at ngOnDestroy of the component (to prevent memory leak)