RxJS - switchMap not emitting value if the input observable is empty array

Question

I have the setup where I query firebase for list of user favourite posts.

Basically, at first I query for user likes and then for each like get the corresponding post - all in one observable sequence.

The problem arises when user dislikes the only left post. In that case (when the likes array becomes empty) nothing is fired from the observable and view is not updated (there is always at least one post present).

On the one hand, this behaviour seems logical and understandable, but on the other hand, I'm not sure how to make final Observable emit even if the input to the switchMap was empty. Maybe should change the operator.

getUserFavourites(userId = ""):Observable<Post[]>
{
  if (!this.userFavourites$) {
    this.userFavourites$ = this.af.database.list('/ranks_by_user/' + userId, {
        query: {
          limitToFirst: 50
        }
      }) //Emits value here (even empty array)
      .switchMap((likes: any[]) => Observable.combineLatest(
        likes.map(like => this.af.database.object("/posts/" + like.$key).first())
      )) //Does not emit new value here if likes array was empty
      .map(p => {
        return p.map(cit => Post.unpack(p));
      }).publishReplay(1).refCount()
  }
  return this.userFavourites$;
}

I think that when the likes array it's null should throw a error and that it's stoping the chain you could try to add a catch — Victor Godoy
– Victor Godoy, Commented Jan 18, 2017 at 15:54
Thanks for the idea, mate. It will probably work. Actually I will give it a try :) — Daniil Andreyevich Baunov
– Daniil Andreyevich Baunov, Commented Jan 18, 2017 at 16:03

Daniil Andreyevich Baunov · Accepted Answer · 2017-01-18 16:17:14Z

7

Solved the problem by adding a condition inside switchMap:

Original - https://github.com/ReactiveX/rxjs/issues/1910

getUserFavourites(userId = ""):Observable<Post[]>
{
  if (!this.userFavourites$) {
    this.userFavourites$ = this.af.database.list('/ranks_by_user/' + userId, {
        query: {
          limitToFirst: 50
        }
      }) //Emits value here (even empty array)
      .switchMap((likes: any[]) => {
      return likes.length === 0 ?
        Observable.of(likes) :
        Observable.combineLatest(
          likes.map(like => this.af.database.object("/citations/" + like.$key))
      )
    }) //Emits either combined observables array or empty array
      .map(p => {
        return p.map(cit => Post.unpack(p));
      }).publishReplay(1).refCount()
  }
  return this.userFavourites$;
}

answered Jan 18, 2017 at 16:17

Daniil Andreyevich Baunov

2,8833 gold badges28 silver badges50 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

the-catalin Over a year ago

this was a life saver for me!

Jon Onstott Over a year ago

To summarize, forkJoin([]) in a switchMap would stop the RxJS stream from continuing its processing. Returning of([]) from the switchMap allows processing to continue.

Zhentian Wan · Accepted Answer · 2017-05-30 06:47:25Z

0

.switchMap((likes) => likes.length > 0 ?
   Observable.combineLatest(
    likes.map(like => this.af.database.object("/posts/" + like.$key).first():
   Observable.empty() // if emit empty() then .map() will not run 
 )
 .map(...)

answered May 30, 2017 at 6:47

Zhentian Wan

91 bronze badge

2 Comments

Colwin Over a year ago

Can you please add a little more explanation around your answer?

Zhentian Wan Over a year ago

For siwtchMap().map() chain. If you return empty Observable (Observable.empty()) inside switchMap, then .map() chain won't be triggered.

Collectives™ on Stack Overflow

RxJS - switchMap not emitting value if the input observable is empty array

2 Answers 2

2 Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related