Unit testing a BehaviorSubject in a service

I’m working on a toy project to learn unit testing in Angular (replaced Karma with Jest). I’ve seen similar questions but all of them handle subscriptions differently, so I wasn’t able to get the exact answer to work for me.

TL;DR: I’m trying to test a BehaviorSubject, but rather than the value I’m expecting, it’s sending it’s initial value to me. I understand the ordering of the subscription in the test matters, but I couldn’t figure the proper ordering yet.

I have a BehaviorSubject in a service:

export class SomeService {
  // ...
  remainingSeconds: number;

  private _seconds: Subject<number> = new BehaviorSubject(RegularTimerSeconds.WORK_TIME);
  public readonly seconds$: Observable<number> = this._seconds.asObservable();

  setTimer(seconds: number) {
    this.remainingSeconds = seconds;
    this._seconds.next(this.remainingSeconds);
  }

  // ...
}

and a listener on a component, which subscribes to the Observable using the async pipe in the template

export class SomeComponent implements OnInit, OnDestroy {
  // ...
  remainingSeconds$ = this.timer.seconds$.pipe(tap(seconds => console.log(`remainingSeconds$: ${seconds}`)));

  constructor(public timer: TimerService) {}
}

Subscription happens in the template as follows

<ng-container *ngIf="{
  remainingSeconds: remainingSeconds$ | async,
  currentState: currentState$ | async,
  informationText: informationText$ | async,
  doneCounter: doneCounter$ | async
} as data">
  <!-- ... -->
  <div class="regular-timer-container" *ngIf="data.remainingSeconds !== undefined">
    <p>Remaining seconds: {{ data.remainingSeconds }}</p>  
  </div>
</ng-container>

I would like to test that when the setTimer function in the service is called, the remainingSeconds$ observable in the component also gets the correct value. Especially, if the “remainingSeconds” value in the service is undefined, I’d like to make sure that the remainingSeconds$ observable gets the updated undefined value, and thus the appropriate UI element is hidden.

The problem I’m having with the test (for which the code is below) is that when I call service.setTimer(undefined) from the component, I’m getting the initial value of the BehaviorSubject rather than the undefined value I wanted (which makes the test useless).

it(`shouldn't render the timer component if the seconds value is unset`, async(() => {
  component.remainingSeconds$
    .subscribe(
      seconds => {
        expect(seconds).toEqual(undefined);

        fixture.detectChanges();

        const timerContainerDom: DebugElement = fixture.debugElement;
        const regularTimerComponent = timerContainerDom.query(By.directive(AppComponent));

        expect(regularTimerComponent).toBeFalsy();
      }
    );

  timerService.setTimer(undefined);
}));

I’m adding a stackblitz link, but sadly I couldn’t run npm test on there. Hopefully it’ll give some basic idea as to what I’m trying to accomplish.

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