Mock a single method within an Angular service

Within my Angular service, I have a method that listens for state changes and returns an observable. However, other methods within the same service handle transformation logic:

ngOnInit() {
    this.isLoading = true;
    this.myService.stateListener().subscribe((res: any) => {
        this.data = this.myService.transformModel(res);
        this.isLoading = false;
        this.cd.detectChanges();
    }, (err) => {
        this.errMessage = 'Data could not be loaded';
        this.isLoading = false;
    });
}

During testing, I mock the state listener to provide specific testable data as follows:

const mockService = jasmine.createSpyObj('myService', ['stateListener']);
mockService.stateListener.and.returnValue(of({
    number: 107,
    mock: someMock
}));

The issue arises when mocking the service because the 'transformerModel' method is no longer accessible (on the mocked service). As a result, the component's bound data becomes 'undefined.'

Is there a way to solely mock the returned observable (stateListener) without affecting the transformer method in the same service?

Answer №1

const fakeData = {}; // Placeholder response for Observable or Model
const dataInjector = fixture.debugElement.injector.get(MyService);
spyOn(dataInjector, 'stateListener').and.returnValue(
      Observable.of(fakeData)
    );

To simulate and return the observable, you can utilize spyOn, returnValue, and Observable.of.

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