RxJS - Combine multiple observables and emit any value

Question

I know how to combine observables and fetch the result when all observables emit at least one value (combineLatest, forkJoin, etc.).

But how can I emit a value when one of many observables emits a value?

For example:

const adModelChangedObs$ = this.editAdService.adModelChanged; // Angular subject one
const refreshPreviewClickedObs$ = this.editAdService.refreshPreviewClick // Angular subject two

merge([adModelChangedObs$, refrshPreviewClickedObs$]).subscribe(() => {
     console.log('One of the two observables emitted a value');
});

The subscribe handler is not being executed, although I tried it with many RxJS operators for far.

Does this answer your question? RxJS combineLatest without waiting for source observables to emit? — Harun Yilmaz
– Harun Yilmaz, Commented Feb 11, 2022 at 13:42
Can you make sure by subscribing separately to the above adModel and refreshPreview that they are emitting? — HassanMoin
– HassanMoin, Commented Feb 11, 2022 at 13:48
You can use combineLatest. combineLatest starts emitting after one of the observable emits. forkJoin waits all to emit at least once. — Fatih Ersoy
– Fatih Ersoy, Commented Feb 11, 2022 at 14:35
merge should work, but you have to give it observables, not an array. — Mrk Sef
– Mrk Sef, Commented Feb 11, 2022 at 14:37
Sorry but none of your advices did the trick. I have to use "startsWith" to initialize the observables. Otherwise, it does not work. — enne87
– enne87, Commented Feb 12, 2022 at 17:15

Amer · Accepted Answer · 2022-02-14 06:51:24Z

7

As mentioned in the comments, the merge function should help in your case, however, you have to pass the observables as args, not as an array.

You can try it like the following:

// import { merge } from 'rxjs';

const adModelChangedObs$ = this.editAdService.adModelChanged; // Angular subject one
const refreshPreviewClickedObs$ = this.editAdService.refreshPreviewClick; // Angular subject two

merge(adModelChangedObs$, refrshPreviewClickedObs$).subscribe(() => {
  console.log('One of the two observables emitted a value');
});

However, if that's not working, then you need to make sure that your editAdService observables emit the value correctly.

edited Feb 14, 2022 at 6:51

answered Feb 14, 2022 at 6:41

Amer

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1 Comment

enne87 Over a year ago

I'll re-check it. Thank you very much!

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