How can I `await` on an Rx Observable?

Question

I'd like to be able to await on an observable, eg

const source = Rx.Observable.create(/* ... */)
//...
await source;

A naive attempt results in the await resolving immediately and not blocking execution

Edit: The pseudocode for my full intended use case is:

if (condition) {
  await observable;
}
// a bunch of other code

I understand that I can move the other code into another separate function and pass it into the subscribe callback, but I'm hoping to be able to avoid that.

Answer 1

You have to pass a promise to await . Convert the observable's next event to a promise and await that.

if (condition) {
  await observable.first().toPromise();
}

Edit note: This answer originally used .take(1) but was changed to use .first() which avoids the issue of the Promise never resolving if the stream ends before a value comes through.

As of RxJS v8, toPromise will be removed. Instead, the above can be replaced with await firstValueFrom(observable)

Answer 2

It likely has to be

await observable.first().toPromise();

As it was noted in comments before, there is substantial difference between take(1) and first() operators when there is empty completed observable.

Observable.empty().first().toPromise() will result in rejection with EmptyError that can be handled accordingly, because there really was no value.

And Observable.empty().take(1).toPromise() will result in resolution with undefined value.

Answer 3

Use the new firstValueFrom() or lastValueFrom() instead of toPromise() , which as pointed out here, is deprecated starting in RxJS 7, and will be removed in RxJS 8.

import { firstValueFrom} from 'rxjs';
import { lastValueFrom } from 'rxjs';

this.myProp = await firstValueFrom(myObservable$);
this.myProp = await lastValueFrom(myObservable$);

This is available in RxJS 7+

See: https://indepth.dev/rxjs-heads-up-topromise-is-being-deprecated/

Answer 4

You will need to await a promise, so you will want to use toPromise() . See this for more details on toPromise() .

Answer 5

If toPromise is deprecated for you, you can use .pipe(take(1)).toPromise but as you can see here it's not deprecated.

So please juste use toPromise (RxJs 6) as said:

//return basic observable
const sample = val => Rx.Observable.of(val).delay(5000);
//convert basic observable to promise
const example = sample('First Example')
  .toPromise()
  //output: 'First Example'
  .then(result => {
    console.log('From Promise:', result);
  });

async/await example:

//return basic observable
const sample = val => Rx.Observable.of(val).delay(5000);
//convert basic observable to promise
const example = await sample('First Example').toPromise()
// output: 'First Example'
console.log('From Promise:', result);

Read more here .

And please remove this wrong claim saying toPromise is deprecated.

Answer 6

Using toPromise() is not recommended as it is getting depreciated in RxJs 7 onwards. You can use two new operators present in RxJs 7 lastValueFrom() and firstValueFrom() . More details can be found here

const result = await lastValueFrom(myObservable$);

Implementations in Beta version are available here:

• firstValueFrom
• lastValueFrom

Answer 7

I am using RxJS V 6.4.0, hence I should use deprecated one in V 7.xx toPromise() . Inspired by other answers, here is what I did with toPromise()

import { first, ... } from 'rxjs/operators';

...

if (condition) {
  await observable$.pipe(first()).toPromise();
}

...

Note how I used last() inside a pipe() . Because on mine observable.first() does not works just like mentioned by macil

Hope this helps others who using RxJS V 6.xx as I do :).

Thanks.