Unit testing with private service injected using jasmine angular2

I have a problem trying to unit test an angular service. I want to verify that this service is properly calling another service that is injected into it.

Lets say I have this ServiceToTest that injects ServiceInjected:

ServiceToTest .service.ts

@Injectable()
export class ServiceToTest  {
    constructor(private _si: ServiceInjected) {}
    public init() {
      this._si.configure();
    }

}

ServiceInjected.service.ts

@Injectable()
export class ServiceInjected {
    constructor() {}
    public configure() {
    /*Some actions*/
    }

}

With these services, now I write my unit test:

const serviceInjectedStub = {
  configure(): void {}
}


describe('ServiceToTest service Test', () => {
  beforeEach(() => {
    TestBed.configureTestingModule({
      providers: [ServiceToTest ,
        { provide: ServiceInjected, useValue: serviceInjectedStub }]
    });
  });
  
  it('should be initialize the service injected', inject([ServiceToTest],
    (tService: ServiceToTest) => {
      spyOn(serviceInjectedStub, 'configure');
      tService.init();
      expect(serviceInjectedStub.configure).toHaveBeenCalled();
    }));

I expected my test to be positive, however I receive the following error:

Expected spy configure to have been called.

On the other hand, it works OK if I set the injected service public in this way:

private _si: ServiceInjected by public si: ServiceInjected

You don't spy on the service tied to your TestBed. Get the service from your Testbed

beforeEach(() => {
  TestBed.configureTestingModule({
    providers: [ServiceToTest ,
      { provide: ServiceInjected, useValue: serviceInjectedStub }]
  });
  injectedService = TestBed.get(ServiceInjected);
});

And test on it

spyOn(injectedService, 'configure').and.returnValue(/* return same data type here */);
// ...
expect(injectedService.configure).toHaveBeenCalled();