When conducting Angular testing using Karma and Jasmine, a method that makes use of HttpClient.get may result in undefined values in the service spec. However, it successfully

Issue with Angular service method retrieving data from database during testing

In myservice.ts, I have a method named getEvents() that creates an array, fetches data from the database using http.get, fills the array, and returns it. However, when I try to call this method in myservice.spec.ts, it returns undefined.

myservice.service.ts:

import { Injectable } from '@angular/core';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { Observable } from 'rxjs';
import { MyComponent } from './mycomponent.component.ts';

@Injectable({
  providedIn: 'root'
})
export class MyService {
    getEvents(): string[] {
        const events = [];
        this.http.get<MyClass[]>('http://localhost:3001/getEvents')
                 .subscribe(subscribedEvents => {
                    for (const event of subscribedEvents ) {
                        events.push(event);
                    }
                 });
        return events;
    }
  constructor(private http: HttpClient) { }
}

myservice.service.spec.ts:

import { async, TestBed } from '@angular/core/testing';
import { HttpClientModule } from '@angular/common/http';
import { MyService } from './myservice.service';

describe('MyService', () => {
    beforeEach(async(() => {
        TestBed.configureTestingModule({
          imports: [ HttpClientModule ],
          providers: [ MyService ]
        })
        .compileComponents();
      }));
  it('getEvents', () => {
    const service: MyService = TestBed.get(MyService);
    expect(service.getEvents()[0]).toEqual('FirstEvent');
  });
});

The getEvents() function sends a request to the backend server, retrieves data successfully (verified by the component and Postman), populates an array with the data received through .subscribe(), and then returns the filled array. While this works fine in the component, it seems to be returning undefined in the service spec.

I apologize for the generic naming of variables; I prefer not to share my original code.

Error Message:

Expected undefined to equal 'FirstEvent'.

Answer №1

Are you looking to conduct a specific test in this scenario? Are you interested in executing an actual http call or simulating the response?

Additionally, your service needs to provide an observable since the method getEvents will initially return events = [];, meaning an empty array that will only populate once the observable is subscribed to.

Therefore, MyService::getEvents should yield an observable which can then be subscribed to within your component.

To properly test this functionality, it's crucial to mock/spy on MyService::getEvents in order to avoid triggering an authentic http call.

Answer №2

In the context of testing, we refrain from using actual backend services to receive responses; instead, we opt for HttpTestingController to mimic the behavior. A typical test scenario for a sample service might look something like this:

 it('should retrieve events', async(inject([MyService, HttpTestingController],
    (service: MyService, httpMock: HttpTestingController) => {
      // Set up
      let finalResponse;
      const event = { id : '1234' };
      // Execution
      service.getEvents().subscribe(result => {
        // Verification
        finalResponse = result;
      });
      const httpRequest = httpMock.expectOne((req: HttpRequest<any>) => {
        return req.method === 'GET'
      }, `retrieving event`);
      // Verification
      httpRequest.flush({ id: '1234' });
      expect(finalResponse).toEqual(event);
    })));

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