@Medium article FlushMicrotasks the Service and the Component

auth.ts

export class AuthService {
  getUser(): Promise<User> {
    // probably make a xhr but we'll just
    return Promise.resolve(User.Empty);
  } 
}

component.ts

export class HelloComponent {
  // ...omitted for brevity

  login() {
    this.auth.getUser().then(u => {
      if(u === User.Empty) {
        this.router.navigate('/login')
      }
    })
  }
}

spec.ts

it('should redirect to login if getUser returns the empty user', () => {
    // given
    // setup the empty user
    auth.getUser.and.returnValue(Promise.resolve(User.Empty));
    // when
    comp.login();

    // then
    expect(router.navigate).toHaveBeenCalledWith('/login');
  });