Unit testing a component with async service dependency in Angular

Create Project

Create new angular project

ng new testing-samples

Navigate to the project

cd testing-samples

Open VS code

code .

Simple Service

Create a simple service ng g s post

Service code

@Injectable({
  providedIn: 'root'
})
export class PostService {

  constructor(private httpClient: HttpClient) { }

  getPosts() {
    return this.httpClient.get("https://jsonplaceholder.typicode.com/posts")
  }
}

App component using service

export class AppComponent implements OnInit {
  posts: any = [];

  constructor(private postService: PostService) {}

  ngOnInit() {
    this.postService.getPosts().subscribe((data: any) => {
      this.posts = data;
    })
  }

Unit testing the component

Create spy object for service

postService = jasmine.createSpyObj("PostService", ["getPosts"]);

Tell angular to use it instead of original service

{provide: PostService, useValue: postService}

Setup Code

beforeEach(async () => {
    postService = jasmine.createSpyObj("PostService", ["getPosts"]);

    await TestBed.configureTestingModule({
      imports: [
        RouterTestingModule
      ],
      declarations: [
        AppComponent
      ],
      providers: [
        {provide: PostService, useValue: postService}
      ]
    }).compileComponents();
    fixture = TestBed.createComponent(AppComponent);
    component = fixture.componentInstance;
  });

Set the response when service method is called

postService.getPosts.and.returnValue(of(posts));

Unit test code

it('should set posts oninit - default synchronous way', () => {
    var posts = [{
      userId: 1,
      id: 1,
      title: "test",
      body: "test"
      }
    ];
    postService.getPosts.and.returnValue(of(posts));
    fixture.detectChanges(); //triggers onInit
    expect(postService.getPosts).withContext("service called").toHaveBeenCalled();
    expect(component.posts).withContext("set posts").toBe(posts);
  });

Unit testing asynchronous way

Using fakeAsync and tick

Use asyncData helper function to delay the response

function asyncData<T>(data: T) {
   return defer(() => Promise.resolve(data));
}

Calling tick() simulates the passage of time until all pending asynchronous activities finish.

it('should set posts oninit - asynchronous way using fakeasync and tick', fakeAsync(() => {
    var posts = [{
      userId: 1,
      id: 1,
      title: "test",
      body: "test"
      }
    ];
    postService.getPosts.and.returnValue(asyncData(posts));
    fixture.detectChanges();
    tick();
    expect(component.posts).toBe(posts);
  }));

Using waitForAsync and whenStable

it('should set posts oninit - asynchronous way using waitForAsync and whenStable', waitForAsync(() => {
    var posts = [{
      userId: 1,
      id: 1,
      title: "test",
      body: "test"
      }
    ];
    postService.getPosts.and.returnValue(asyncData(posts));
    fixture.detectChanges();
    fixture.whenStable().then(() => {
      expect(component.posts).toBe(posts);
    });
  }));

Using done function

it('should set posts oninit - using done', (done: DoneFn) => {
    var posts = [{
      userId: 1,
      id: 1,
      title: "test",
      body: "test"
      }
    ];
    let spy = postService.getPosts.and.returnValue(asyncData(posts));
    fixture.detectChanges();
    spy.calls.mostRecent().returnValue.subscribe(() => {
      expect(component.posts).toBe(posts);
      done();
    });
  });

Complete Code

describe('AppComponent', () => {
  let component: AppComponent;
  let fixture: ComponentFixture<AppComponent>;
  let postService: any;

  function asyncData<T>(data: T) {
    return defer(() => Promise.resolve(data));
  }

  beforeEach(async () => {
    postService = jasmine.createSpyObj("PostService", ["getPosts"]);

    await TestBed.configureTestingModule({
      imports: [
        RouterTestingModule
      ],
      declarations: [
        AppComponent
      ],
      providers: [
        {provide: PostService, useValue: postService}
      ]
    }).compileComponents();
    fixture = TestBed.createComponent(AppComponent);
    component = fixture.componentInstance;
  });

  it('should create the app', () => {
    expect(component).toBeTruthy();
  });

  it('should set posts oninit - default synchronous way', () => {
    var posts = [{
      userId: 1,
      id: 1,
      title: "test",
      body: "test"
      }
    ];
    postService.getPosts.and.returnValue(of(posts));
    fixture.detectChanges();
    expect(postService.getPosts).withContext("service called").toHaveBeenCalled();
    expect(component.posts).withContext("set posts").toBe(posts);
  });

  
  it('should set posts oninit - asynchronous way using fakeasync and tick', fakeAsync(() => {
    var posts = [{
      userId: 1,
      id: 1,
      title: "test",
      body: "test"
      }
    ];
    postService.getPosts.and.returnValue(asyncData(posts));
    fixture.detectChanges();
    tick();
    expect(component.posts).toBe(posts);
  }));

  it('should set posts oninit - asynchronous way using waitForAsync and whenStable', waitForAsync(() => {
    var posts = [{
      userId: 1,
      id: 1,
      title: "test",
      body: "test"
      }
    ];
    postService.getPosts.and.returnValue(asyncData(posts));
    fixture.detectChanges();
    fixture.whenStable().then(() => {
      expect(component.posts).toBe(posts);
    });
  }));

  it('should set posts oninit - using done', (done: DoneFn) => {
    var posts = [{
      userId: 1,
      id: 1,
      title: "test",
      body: "test"
      }
    ];
    let spy = postService.getPosts.and.returnValue(asyncData(posts));
    fixture.detectChanges();
    spy.calls.mostRecent().returnValue.subscribe(() => {
      expect(component.posts).toBe(posts);
      done();
    });
  });
});