Best way to test a function that returns an observable operator

Question

Best way to test a function that returns an observable with tap operator.

operations(numbers): Observable{
  const sumValue = numbers[0]+numbers[1]
  return sumService.getOperations(sumValue).pipe(tap(randomOperationValue => {this.value =    randomOperationValue}
))
}

I do not see how to test the value inside tap. I tried to mock getOperations() but without success.

Answer 1

A quick something should get you started

describe('AppComponent', () => {
  let svc: OperationsService;
  let fixture: ComponentFixture<AppComponent>;
  let component: AppComponent;

  beforeEach(async () => {
    TestBed.configureTestingModule({
      declarations: [AppComponent],
      providers: [{
        provide: OperationsService,
        useValue: {
          getOperations: () => of(1000),
        }
      }]
    });

    svc = TestBed.inject(OperationsService);
    fixture = TestBed.createComponent(AppComponent);
    component = fixture.componentInstance;
  });

  it('should ...', fakeAsync(() => {
    const spy = jest.spyOn(svc, 'getOperations');
    const obs$ = component.operations([100, 200]);

    obs$.subscribe();
    tick();

    expect(spy).toHaveBeenCalledWith(300);
    expect(component.value).toBe(1000); // this is the fixed value we mock ^
  }));
});