Angular unit test ngOnInit subscription

Question

I am starting out with unit testing in Angular 9 with Jasmine.

I am testing a simple component which implements ngOnInit :

export class HomeComponent implements OnInit {

      constructor(private router: Router
        , private authenticationService: AuthenticationService) { }

        ngOnInit(): void {
        this.authenticationService.checkIsAuthenticatedObservable()
        .subscribe(
          (isAuthenicated: boolean) => {
            if (isAuthenicated === true) {
              this.router.navigate(['/observation-feed']);
            }
          });
        }
      }

I am hitting the error with executing the ngOnInIt lifecycle hook:

TypeError: Cannot read property 'subscribe' of undefined
    at <Jasmine>
    at HomeComponent.ngOnInit (http://localhost:9876/_karma_webpack_/main.js:8140:13)

My test spec is setup like this:

describe('HomeComponent', () => {
  let component: HomeComponent;
  let fixture: ComponentFixture<HomeComponent>;
  let router: Router;
  let mockAuthenticationService;

  beforeEach(async(() => {
    mockAuthenticationService = jasmine.createSpyObj(['checkIsAuthenticatedObservable']);

    TestBed.configureTestingModule({
      imports: [
        RouterTestingModule.withRoutes([
          // { path: 'login', component: DummyLoginLayoutComponent },
        ])
      ],
      declarations: [ HomeComponent ],
      providers: [
        { provide: AuthenticationService, useValue: mockAuthenticationService }
      ]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(HomeComponent);
    router = TestBed.get(Router);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should create', () => {
    mockAuthenticationService.checkIsAuthenticatedObservable.and.returnValue(of(false));
    fixture.detectChanges();
    // component.ngOnInit();

    expect(component).toBeTruthy();
  });
});

I have attempted various combinations of setting up the mock object, and calling fixture.detectChanges(); and component.ngOnInit(); at different points in the intialization. None of what I have tried has worked. What's going wrong here?

Answer 1

When you are calling fixture.detectChanges in the beforeEach section, Angular runs lifecycle hooks and ngOnInit is called. This is why you're getting error - you are mocking checkIsAuthenticatedObservable in the test, after the first fixture.detectChanges . Move your mock to the beforeEach section, before fixture.detectChanges and it will work correctly. Also, with Angular 9, you should use TestBed.inject instead of TestBed.get which is now deprecated.

beforeEach(() => {
    fixture = TestBed.createComponent(HomeComponent);
    router = TestBed.inject(Router);
    component = fixture.componentInstance;
    mockAuthenticationService.checkIsAuthenticatedObservable.and.returnValue(of(false));
    fixture.detectChanges();
  });

  it('should create', () => {
    fixture.detectChanges();
    expect(component).toBeTruthy();
  });