This article will explain how to mock a service call while trying to cover a method in a component.

Consider the following component,

export class HelloComponent  {


  constructor(private helloSvs: HelloService) {

  }

    serviceCall() {
        this.helloSvs.run().subscribe(val => {
            if(val) { console.log("Printing True"); }
            else { console.log("Printing False"); }
        });

    }

}

The service used there will be similar to this,

@Injectable({
  providedIn: 'root'
  )
export class HelloService {
  run() {
    // some operations and assume the method returns boolean value
    if (true) {
      return false;
    }
    return true;
  }
}

Then the test case for the above component,

describe('AppComponent', () => {
  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [ HelloComponent ],
    providers: [ HelloService  ] /** Add the service **/
    }).compileComponents();
  });

beforeEach(() => {
    fixture = TestBed.createComponent(HelloComponent );
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should mock the service with true as return', () => {
    /** Inject the service into fixture **/
    const service = fixture.debugElement.injector.get(HelloService);
    /** 
    Spy on the method present in the service and return the mock response 
    Here the mock calls will give a response as true.
    **/
    spyOn(service, 'run').and.returnValue(of(true));

/** While the call is made actually, the subscribe will get the response as true and corresponding lines will be executed **/
    component.serviceCall();

    .................
  });


});