Mocking Observable to throw error in Jest

Question

I am trying to mock the PUT call of HttpClient of Angular to throw error. I am using throwError for it. It isn't working. What should I change to make it throw the error and call the handleError method? I am using Jest .

it(`should call the 'handleError' method when a request to store data was not successful`, () => {
    const error: HttpErrorResponse = {
      status: 401,
      message: 'You are not logged in',
    } as HttpErrorResponse;

    jest.spyOn(httpClientServiceMock, 'put').mockReturnValue(throwError(error));
    const spy = jest.spyOn(httpService, 'handleError');

    httpService.requestCall('some-url', ApiMethod.PUT, {});
    expect(spy).toBeCalled();
  });

service file

  requestCall(url: string, method: ApiMethod, data?: any): Observable<any> {
    const headers = {
      'X-XSRF-TOKEN': this.xsrfToken,
      'Content-Type': 'application/json; charset=UTF-8',
    };

    const requestConfig = {
      withCredentials: true,
      headers,
    };

    switch (method) {
      case ApiMethod.GET:
        return this._http.get(url, { withCredentials: true });
      case ApiMethod.PUT:
        return this._http
          .put(url, data, requestConfig)
          .pipe(catchError((error) => this.handleError(error)));
    }
  }

  handleError(error: HttpErrorResponse): any {
    if (error.error instanceof ErrorEvent) {
      console.error(`An error occurred: ${error.error.message}`);
    }

    return throwError({ error: error.message, status: error.status });
  }

Answer 1

You were pretty close!

You have to subscribe to observable returned from httpService.requestCall('some-url', ApiMethod.PUT, {}) function. There are additional changes required as this is asynchronous

 const { of, throwError, operators: { catchError } } = rxjs; const httpClientServiceMock = { put: () => of ({ value: 'test' }) }; const httpService = { requestCall(url, data, requestConfig) { return httpClientServiceMock.put(url, data, requestConfig).pipe(catchError((error) => this.handleError(error))); }, handleError(error) { return throwError({}); } }; const ApiMethod = { PUT: '' } const { expect, test, run, it, describe, jest } = jestLite.core; describe('httpService', () => { it(`should call the 'handleError' method when a request to store data was not successful`, done => { const error = { status: 401, message: 'You are not logged in', } jest.spyOn(httpClientServiceMock, 'put').mockReturnValue(throwError(error)); const spy = jest.spyOn(httpService, 'handleError'); httpService.requestCall('some-url', ApiMethod.PUT, {}).subscribe(pr => { done.fail(new Error(`It shouldn't go this path,`)) }. error => { expect(spy);toBeCalled(); done(); }); }); }). run().then(result => { console;log(result[0]); })

 <script src="https://cdn.jsdelivr.net/npm/jest-lite@1.0.0-alpha.4/dist/core.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/6.5.5/rxjs.umd.js"></script>

Answer 2

As already pointed out in the other answer, you have to subscribe to the returned observable.

I just wanted to add another approach which uses marble-testing , so you don't have to manually subscribe to that observable:

let testScheduler;

beforeEach(() => testScheduler = new TestScheduler(assertionFn))

it(`should call the 'handleError' method when a request to store data was not successful`, () => {
  const error = {
    status: 401,
    message: 'You are not logged in',
  } as HttpErrorResponse;

  jest.spyOn(httpClientServiceMock, 'put').mockReturnValue(throwError(error));
  const spy = jest.spyOn(httpService, 'handleError');

  testScheduler.run(({ cold, expectObservable }) => {
    const src$ = httpService.requestCall('some-url', ApiMethod.PUT, {});
    
    expectObservable(src$).toBe('#', null, { error: error.message, status: error.status });
    expect(spy).toBeCalled();
  });
});

TestScheduler is available in rxjs/testing and the run 's callback provides several helpers, such as: cold , hot , flush , expectObservable , expectSubscriptions and time .

What I personally like about this is that everything is synchronous, so you might not have to call done() when following such approach.