.toPromise() and lastValueFrom() in rxjs

user_3168618 0
Viewed 7155

I have this observable

createMyRecord(): Observable<myRecord> {
        return of(TEMPLATE_DB).pipe(
            mergeMap((template) => doTask(template)),
            mergeMap(() => EMPTY)
        );
    }

I call it with

await createMyRecord().toPromise();

.toPromise() is deprecated, so I would like to change the call with lastValueFrom():

await lastValueFrom(createMyRecord());

but I receive the following error:

EmptyError
no elements in sequence

UPDATE: for now, resolved with:

 await lastValueFrom(createMyRecord()).catch((err) => {
            if (err instanceof EmptyError) {
                log.info(OK");
            }
        });

but is there a better solution?

promise rxjs javascript
2 Answers

Is there a better solution?

Yes and no.

In your case mergeMap(_ => EMPTY) will ensure that your observable completes without emitting a value. Promises resolve to a value or they error. So the only thing to do here that meets the spec is to throw an error.

A work-around

You can sidestep this by emitting something. For example, here I emit null after the source completes:

createMyRecord(): Observable<myRecord> {
  return of(TEMPLATE_DB).pipe(
    mergeMap((template) => doTask(template)),
    mergeMap(() => EMPTY),
    s => concat(s, of(null as myRecord))
  );
}

Now your promise will resolve with a null once the observable completes successfully.

Something idiomatic

Rather than changing your code, you can change how you call it. This way you don't need to worry about how Observables and Promises interact.

Instead of await lastValueFrom(createMyRecord()); write:

createMyRecord().subscribe();
user_135009860

lastValueFrom now takes a configuration parameter as its second parameter, and you can specify a default value that will be emitted if the observable is empty:

await rxjs.lastValueFrom(observableThing, {defaultValue: "oh no - empty!"})
user_730460
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