Angular 2 - test for change in route params

Question

I have a component in angular 2 which responds to changes in the route parameters (the component doesn't reload from scratch because we're not moving out of the main route. Here's the component code:

export class MyComponent{
    ngOnInit() {
        this._routeInfo.params.forEach((params: Params) => {
            if (params['area']){
                this._pageToShow =params['area'];
            }
        });
    }
}

This works a treat and _pageToShow is set appropriate on navigation.

I'm trying to test the behaviour on a change to the route (so a second trigger of the observable but it's refusing to work for me.) Here's my attempt:

it('sets PageToShow to new area if params.area is changed', fakeAsync(() => {
    let routes : Params[] = [{ 'area': "Terry" }];
    TestBed.overrideComponent(MyComponent, {
        set: {
            providers: [{ provide: ActivatedRoute,
                useValue: { 'params': Observable.from(routes)}}]
        }
    });

    let fixture = TestBed.createComponent(MyComponent);
    let comp = fixture.componentInstance;
    let route: ActivatedRoute = fixture.debugElement.injector.get(ActivatedRoute);
    comp.ngOnInit();

    expect(comp.PageToShow).toBe("Terry");
    routes.splice(2,0,{ 'area': "Billy" });

    fixture.detectChanges();
    expect(comp.PageToShow).toBe("Billy");
}));

But this throws a TypeError: Cannot read property 'subscribe' of undefined exception when I run it. If I run it without the fixture.detectChanges(); line it fails as the second expectation fails.

Answer 1

Firstly, you should use a Subject instead of an Observable. The observable only gets subscribed to once. So it will only emit the first set of params. With a Subject, you can keep emitting items, and the single subscription will keep getting them.

let params: Subject<Params>;

beforeEach(() => {
  params = new Subject<Params>();
  TestBed.configureTestingModule({
    providers: [
      { provide: ActivatedRoute, useValue: { params: params }}
    ]
  })
})

Then in your test just emit new values with params.next(newValue).

Secondly, you need to make sure to call tick(). This is how fakeAsync works. You control asynchronous task resolution. Since the observable as asychrounous, the moment we sent the event, it will not get to the subscriber synchronously. So we need to force synchronous behavior with tick()

Here is a complete test (Subject is imported from 'rxjs/Subject')

@Component({
  selector: 'test',
  template: `
  `
})
export class TestComponent implements OnInit {

  _pageToShow: string;

  constructor(private _route: ActivatedRoute) {
  }

  ngOnInit() {
    this._route.params.forEach((params: Params) => {
      if (params['area']) {
        this._pageToShow = params['area'];
      }
    });
  }
}

describe('TestComponent', () => {
  let fixture: ComponentFixture<TestComponent>;
  let component: TestComponent;
  let params: Subject<Params>;

  beforeEach(() => {
    params = new Subject<Params>();
    TestBed.configureTestingModule({
      declarations: [ TestComponent ],
      providers: [
        { provide: ActivatedRoute, useValue: { params: params } }
      ]
    });
    fixture = TestBed.createComponent(TestComponent);
    component = fixture.componentInstance;
  });

  it('should change on route param change', fakeAsync(() => {
    // this calls ngOnInit and we subscribe
    fixture.detectChanges();

    params.next({ 'area': 'Terry' });

    // tick to make sure the async observable resolves
    tick();

    expect(component._pageToShow).toBe('Terry');

    params.next({ 'area': 'Billy' });
    tick();

    expect(component._pageToShow).toBe('Billy');
  }));
});