How to cover Jasmine unit test for RxJS subject in angular

Question

I am very new Jasmine unit test cases. My scenario may be easy but I am not sure how to cover test case for ngInit for below class. Can someone help me,

export class Component1 implements OnInit {
    details$: Observable<any>; 
    name: string; 
    error: string 

    constructor(private service1: Service1, private service2: Service2) { }

    ngOnInit() {
       this.service1.service1Subject.subscribe( info => {
            if(info['url']){
                this.details$ = this.service2.get(info['url'])
                this.details$.subscribe(
                 (info) => { this.name = info['name']}; 
                 (error) => { this.erro = error['error']}; 
                ); 
            }  
       }); 
    }
}

Test case :

describe('Component1', () => {
  let component: Component1;
  let fixture: ComponentFixture<Component1>;

  beforeEach(async(() => {
   TestBed.configureTestingModule({
     declarations: [Component1],
     imports: [
       HttpClientTestingModule, CommonModule
     ],
     providers: [Service1, Service2]
   })
     .compileComponents();
   }));

   beforeEach(() => {
    fixture = TestBed.createComponent(Component1);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should call Get Data', () => {
      const service2: Service2 = TestBed.get(Service2);
      const spy = jest.spyOn(service2, 'get').mockImplementation(() => {
          return {
             info :  [...],
              name : ''  
          }
      });
      component.ngOnInit();
      expect(spy).toHaveBeenCalled();
  });
});

The problem here is, I am not sure how to mock service1, RxJS subject. Please someone help me.

Answer 1

Mock the subject first and then add it to mocked service.

 describe('Component1', () => {
    
     let component: Component1;
     let fixture: ComponentFixture<Component1>;
     let mockSubject = new Subject<boolean>();
     mockSubject.next(true);
     let mockService = jasmine.createSpyObj(Service, ['getTheme']);
      
     beforeEach(async () => {
        await TestBed.configureTestingModule({
          declarations: [Component1],
          providers: [{ provide: Service, useValue: mockService }],
        }).compileComponents();
    
        mockService.getTheme.and.returnValue(false);

        mockService.theme$ = mockSubject; <---- assign mocked value
        
        fixture = TestBed.createComponent(Component1);
        component = fixture.componentInstance;
        fixture.detectChanges();
      });
    })

Answer 2

I see you're using Jest, but here's how I've set up component tests that use a service that expose a Subject.

1. Create mocks for all your services
2. Provide them in your test module
3. Mock implementations to control the flow of data

4. Perform your assertions

   describe('Component1', () => { 
       let component: Component1;
       let fixture: ComponentFixture<Component1>;
   
       //create mock service objects
       let service1Mock = jasmine.createSpyObj('service1', ['toString']);
       let service2Mock = jasmine.createSpyObj('service2', ['get']);
   
       beforeEach(async(() => {
           TestBed.configureTestingModule({
               declarations: [Component1],
               imports: [
                   HttpClientTestingModule,
                   CommonModule
               ],
               providers: [
                   //Set up the dependency injector, but use the mocks as the implementation
                   { provide: Service1, useValue: service1Mock },
                   { provide: Service2, useValue: service2Mock }
               ]
           }).compileComponents();
       }));
   
       beforeEach(() => {
           //add an observable to your service
           //this will also help reset the observable between each test
           service1Mock.service1Subject = new Subject<any>(); 
       });
   
       beforeEach(() => {
           fixture = TestBed.createComponent(Component1);
           component = fixture.componentInstance;
           fixture.detectChanges();
       });
   
       it('should get the data', () => {
           //configure the mock implementation of "service2.get" to successfully return data
           //You can alternatively use "throw({error: 'some_error'})" to test your "error" case
           service2Mock.get.and.returnValue(of({name: 'some_name'}));
   
           //tell the mock to emit some data!
           service1Mock.service1Subject.next( {url: 'some_url'} );
   
           //Your component subcriptions should handle the event, perform whatever test needs to do
       });
   });
   

I know that Jasmine was planning on making it possible to create spy objects with attributes, but I haven't actually used it myself.

As an aside, if you're not using details$ in your template, you can eliminate the variable entirely.

ngOnInit(){
    this.service1.service1Subject.subscribe( info => {
        if(info['url']){
            this.service2.get(info['url']).subscribe(
                (info) => { this.name = info['name']}; 
                (error) => { this.error = error['error']}; 
            ); 
        }  
    });
}