Angular Cookbook ng-jest-services-stubs tests for CounterService

counter.component.spec.ts

// replace the tests below
it('should call the CounterService.getFromStorage method on component init', () => {
  jest.spyOn(CounterServiceMock, 'getFromStorage');
  component.ngOnInit();
  expect(component.counterService.getFromStorage).toHaveBeenCalled();
});

it('should retrieve the last saved value from CounterService on component init', () => {
  jest.spyOn(CounterServiceMock, 'getFromStorage').mockReturnValue(12);
  component.counterService.saveToStorage(12);
  component.ngOnInit();
  expect(component.counter).toBe(12);
});

it('should save the new counterValue to CounterService on increment, decrement and reset', () => {
  jest.spyOn(localStorage, 'setItem');
  component.counter = 0;
  component.increment();
  expect(CounterServiceMock.saveToStorage).toHaveBeenCalledWith(1);
  component.counter = 20;
  component.decrement();
  expect(CounterServiceMock.saveToStorage).toHaveBeenCalledWith(19);
  component.reset();
  expect(CounterServiceMock.saveToStorage).toHaveBeenCalledWith(0);
});