Using RxJs would you unsubscribe when using from(myPromise)?

Question

I use the async pipe in Angular, or takeUntil(...) on components using:

export class BaseComponent implements OnDestroy {
  ngUnsubscribe = new Subject<void>();

  ngOnDestroy(): void {
    this.ngUnsubscribe.next();
    this.ngUnsubscribe.complete();
  }

}

But, I've always wondered what I need to do when using from(myPromise). Promises only provide a single result, but I've converted it to an observable so it feels like if I make an equivalent method that provides an observable instead that I should be using take(1) or unsubscribing depending on use since I don't always subscribe directly and instead use it in an exhaustMap(() => this.hasTokenExpired$().

  public async hasTokenExpired(): Promise<boolean> {
    const token = await this.getToken();

    if (token) {
      return this.jwtHelper.isTokenExpired(token);
    }

    return true;
  }

  // Observable equivalent method:
  public hasTokenExpired$(): Observable<boolean> {
    return from(this.hasTokenExpired()); // Could pipe `take(1)` permanently
  }

Should I being using take(1) on from(myPromise)? It's been on my mind for awhile in case this causes a memory leak, but I can't find an article confirming that I should.

Also, if there's a better way to make these kinds of observable equivalent methods I'm always open to suggestions.

Answer 1

You never need to use take in this case, as an Observable created from a Promise will only ever either:

• Emit 1 value and complete if the Promise resolves.
• Error if the Promise rejects.

This answer may be helpful to read too: Does toPromise() unsubscribe from the Observable?