How to unit test rxjs interval

Question

I'm trying to write a unit test with 100% code coverage (line) and I'm stuck with the following code snippet:

getValuesPeriodically(updateInterval: number) {
    interval(updateInterval)
      .subscribe(() =>
        this.getFilesFromService()
    );
  }

I don't know how to cover this part:

() =>
  this.getFilesFromService()

I tried implementing a unit test with fakeAsync and tick(), but the async gave me an error message:

it('timer test', fakeAsync(() => {

      fixture.detectChanges();
      expect(component.filesData.length).toBe(0);
      tick(1000);
      fixture.detectChanges();
      expect(component.filesData.length).toBeGreaterThan(0);

    }));

I get the following error:

TypeError: Cannot read property 'assertPresent' of null

I'm not sure if this is the right way to cover the missing part of my unit test anyways.

Can you help please?

In general when testing async Observables you need to pass an instance of `TestScheduler` to `interval` to avoid using real delays. https://github.com/ReactiveX/rxjs/blob/master/docs_app/content/guide/testing/marble-testing.md — martin, Jul 31 '19 at 10:34

score 1 · Answer 1 · answered Aug 07 '19 at 10:02

In general your code looks good to me. Your approach is super valid, I do it that way on my own. So I think it's more like a problem with your general setup. Maybe this can help you Testing | Cannot read property 'assertPresent' of undefined at resetFakeAsyncZone

How to unit test rxjs interval

1 Answers1