Mocking a single function

[azaeng04]

Hi I am trying to mock a single function in a service instead of the entire service and their functions. One function I want to mock and the other I want to use the actual implementation of. Also both functions reside in the same service.

How do I accomplish?

[getsaf]

If you want to keep the entire service except for one function I would generally do this by using dontMock and injecting the service directly into the test, then applying a single spy:

  const {inject} = await shallow.dontMock(MyService).render();
  const realServiceInstance = inject(MyService);
  jest.spyOn(realServiceInstance, 'someAsyncFunction').mockResolvedValue(123);

Here it is in a full example:

it('can get real instances of a service', async () => {
  const {inject, fixture} = await shallow
    .dontMock(MyService)
    .render({
      // Optional arguments if the service is used in an onInit, you can delay onInit
      // by skipping detectChanges and whenStable checks on render
      detectChanges: false,
      whenStable: false
    });

  const realServiceInstance = inject(MyService);
  jest.spyOn(realServiceInstance, 'someAsyncFunction').mockResolvedValue(123);

  // This fires onInit after you've mocked your service.
  fixture.detectChanges();
  await fixture.whenStable();
  //...
});

[azaeng04]

I just tried that and I seem to get an error thrown:

beforeEach( async () => {
      shallowComponent = new Shallow(LoginComponent, LoginModule)
      .replaceModule(RouterModule, RouterTestingModule)
      .replaceModule(HttpClientModule, HttpClientTestingModule)
      .dontMock(ThemeService)
      .mock(VossApiService, { getLogin: () => of(loginData) })
      .mock(Store, {})
      .mock(ConfirmationService, {})
      .mock(AuthenticationService, { clearSessionMonitor: () => null });
    });

    it('shows the input', async () => {
      const { inject, find } = await shallowComponent
      .render({
        detectChanges: false,
        whenStable: false
      });
      themeService = inject(ThemeService);
      spyOn(themeService, 'getThemeByName')

NullInjectorError: R3InjectorError(DynamicTestModule)[ThemeService -> ThemeService]:   NullInjectorError: No provider for ThemeService! 

[getsaf]

My best guess here is that your Login module does not provide the ThemeService. You can provide it manually in this test with .provide or .provideMock. If this is a top-level service that you need for all your tests, you can use Shallow.alwaysProvide in your global test setup so you only have to do it once.

[azaeng04]

Thanks so much you are right! That helped.