Do i need to unsubscribe from a Promise that has been converted to an Observable with Observable.from()?

Question

I know you don't have to unsubscribe from a promise, but what if i use the from() rxjs operator to turn that promise into a observable and subscribe to it like so:

    from(image.decode()).pipe(catchError(error => of('failed to load image')))
    .subscribe(s => this.imageLoading = false)

I've looked in the official documentation for from() and on different tutorial sites but none of them mention anything about unsubscribing. Can anyone please clarify? Thank you

Answer 1

When you have from(promise) , I'd say you don't have to unsubscribe .

Here's how from is defined:

export function from<T>(input: ObservableInput<T>, scheduler?: SchedulerLike): Observable<T> {
  if (!scheduler) {
    if (input instanceof Observable) {
      return input;
    }
    
    // Reached when having `from(promise)`
    return new Observable<T>(subscribeTo(input));
  } else {
    return scheduled(input, scheduler);
  }
}

subscribeTo is defined as follows:

/* ... */
else if (isPromise(result)) {
  return subscribeToPromise(result);
}
/* ... */

And finally, subscribeToPromise :

export const subscribeToPromise = <T>(promise: PromiseLike<T>) => (subscriber: Subscriber<T>) => {
  promise.then(
    (value) => {
      if (!subscriber.closed) {
        subscriber.next(value);
        subscriber.complete();
      }
    },
    (err: any) => subscriber.error(err)
  )
  .then(null, hostReportError);
  return subscriber;
};

As you can see, after the first emitted value, it will send a complete notification, so there is no need to unsubscribe.

StackBlitz .