Mutiple Http streams with RxJS

mutiple-http-streams-with-rxjs.ts

/**
* @function forkJoin multiple requests with ngrx selects
* we can't use forkJoin for ngrx selects because this method requires the Observables being 'complete'
* in case of ngrx store, all of their observables are usually based on a BehaviorSubject, and will never complete
* the only way we can do is using like this: forkJoin(user$.take(1), customCarList$.take(1)
*/
const result$ = forkJoin([
  this.store.select(selectOne).pipe(take(1)),
  this.store.select(selectTwo).pipe(take(1)),
  this.store.select(selectThree).pipe(take(1))
]);

result$.subscribe();

/**
* using combineLatest() with ngrx selects
*/
combineLatest(
  this.store.select(selectOne),
  this.store.select(selectTwo),
  this.store.select(selectThree)
)
.pipe(
  tap(([one, two, trhee]) => {
    // do something here
  })
 )
.subscribe();

/**
* switchMap() for requests that have to happen in a specific order.
*/

this.userService.getUserId()
.switchMap(userResult =>
  this.userService.getUserPermission(userResult.id)
  .switchMap(permissionsResult =>
    this.userService.getUserInfo(userResult.id)
      .map(infoResult => ({
        id: userResult.id,
        name: userResult.name,
        permissions: permissionsResult.permissions,
        info: infoResult.info
      }))
  )
)
.subscribe(v => console.log('switchmap:', v));

/**
* flatMap() combining Observables in series and parallel
*/

getBooksWithAuthor(): Observable<any[]> {
  return this.http.get('/api/books/')
    .map((res: any) => res.json())
    .flatMap((books: any[]) => {
      if (books.length > 0) {
        return Observable.forkJoin(
          books.map((book: any) => {
            return this.http.get('/api/authors/' + book.author_id)
              .map((res: any) => {
                let author: any = res.json();
                book.author = author;
                return book;
              });
          });
        );
      }
      return Observable.of([]);
    });
}