Spy on the result of an Observable Subscription with Jasmine

Question

I am Jasmine unit testing an angular component, which uses Observables. My component has this lifecycle hook that I am testing:

ngOnInit() {
  this.dataService.getCellOEE(this.cell).subscribe(value => this.updateChart(value));
}

I have a test that ensures that getCellOEE has been called, but now I want to check that updateChart is called when the observable resolves with a new value. This is what I have so far:

let fakeCellService = {
  getCellOEE: function (value): Observable<Array<IOee>> {
    return Observable.of([{ time: moment(), val: 67 }, { time: moment(), val: 78 }]);
  }
};

describe('Oee24Component', () => {
  let component: Oee24Component;
  let service: CellService;
  let injector: Injector;
  let fixture: ComponentFixture<Oee24Component>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [Oee24Component],
      providers: [{ provide: CellService, useValue: fakeCellService }]
    })
      .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(Oee24Component);
    component = fixture.componentInstance;
    injector = getTestBed();
    service = injector.get(CellService)
    fixture.detectChanges();
    spyOn(service, 'getCellOEE').and.returnValue({ subscribe: () => { } });
    spyOn(component, 'updateChart');
  });

  it('should get cell oee on init', () => {
    component.ngOnInit();
    expect(service.getCellOEE).toHaveBeenCalled();
  });

  it('should update chart on new data', () => {
    component.ngOnInit();
    expect(component.updateChart).toHaveBeenCalled();
  });
});

However, I get the error:

chrome 56.0.2924 (Windows 10 0.0.0) Oee24Component should update chart on new data FAILED

Expected spy updateChart to have been called.

Presumably this is a timing issue because the observable hasn't necessarily resolved when the test checks? If that is the case, how do I set this up correctly?

Update:

Here is my component:

@Component({
  selector: 'app-oee24',
  templateUrl: './oee24.component.html',
  styleUrls: ['./oee24.component.css']
})
export class Oee24Component implements OnInit {
  public barChartData: any[] = [{ data: [], label: 'OEE' }];

  constructor(public dataService: CellService) { }

  ngOnInit() {
    this.dataService.getCellOEE(this.cell).subscribe(value => this.updateChart(value));
  }

  updateChart(data: Array<IOee>) {
    this.barChartData[0].data = data.map(val => val.val);
  }
}
 

Answer 1

Did you ever come up with a solution? What about using the jasmine-marbles package and the complete event?

it('should update chart on new data', () => {
    const obs$ = cold('--a-|');
    spyOn(service, 'getCellOEE').and.returnValue(obs$);
    component.ngOnInit(); 
    obs$.subscribe({
        complete: () => {
            expect(component.updateChart).toHaveBeenCalledWith('a');
        }
    });
});

Answer 2

Not sure if this is the best way to do it, but I've seen it working on a project I'm working on. The approach is basically get a reference to the callback function provided to the subscribe method, and call it manually to simulate the observer emitting a value:

it('should update chart on new data', () => {
    component.ngOnInit();

    // this is your mocked observable
    const obsObject = service.getCellOEE.calls.mostRecent().returnValue;

    // expect(obsObject.subscribe).toHaveBeenCalled() should pass

    // get the subscribe callback function you provided in your component code
    const subscribeCb = obsObject.subscribe.calls.mostRecent().args[0];

    // now manually call that callback, you can provide an argument here to mock the "value" returned by the service
    subscribeCb(); 

    expect(component.updateChart).toHaveBeenCalled();
  });

Answer 3

Instead of

spyOn(service, 'getCellOEE').and.returnValue({ subscribe: () => { } });

You could try

spyOn(service, 'getCellOEE').and.returnValue( {subscribe: (callback) => callback()});

Answer 4

fixture.detectChanges triggers ngOnInit . so, no need to call ngOnInit manually if fixture.detectChanges was executed.

It's a bad practice to check if the method was called . Instead, it is more reliable to check the expected results of the code execution.

There is no need in the line spyOn(service, 'getCellOEE').and.returnValue({ subscribe: () => { } }); because fakeCellService already mocks properly the service.

The tested code is async, so we need to wait till it would be executed. await fixture.whenStable(); does exactly this.

So, the resulting test:

const fakeData = [{ time: moment(), val: 67 }, { time: moment(), val: 78 }];
const expectedChartData = [67, 78];
const fakeCellService = {
  getCellOEE: function (value): Observable<Array<IOee>> {
    return Observable.of(fakeData);
  }
};

describe('Oee24Component', () => {
  let component: Oee24Component;
  let service: CellService;
  let injector: Injector;
  let fixture: ComponentFixture<Oee24Component>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [Oee24Component],
      providers: [{ provide: CellService, useValue: fakeCellService }]
    })
      .compileComponents();
  }));

  beforeEach(async () => {
    fixture = TestBed.createComponent(Oee24Component);
    component = fixture.componentInstance;
    fixture.detectChanges();
    await fixture.whenStable();
  });

  it('maps and saves value from the CellService.getCellOEE to barChartData[0].data when initialized', () => {
    expect(component.barChartData[0].data).toEqual(expectedChartData);
  });
});