Intercepting Http Requests-- Using And Testing Angular's HttpClient

The HttpClientModule, which debuted in Angular 4.3, is an easy to use API. It automatically expects json as its default response type, builds in the ability to intercept both requests and responses, and makes testing a breeze.

I recently worked on converting calls to use HttpClientModule from HttpModule. While working on this project I had a hard time finding good resources and examples, especially examples of tests. So I decided to write a simple app that uses HttpClientModule along with corresponding unit tests as an example with a little more oomph than what’s available on Angular’s documentation. It’s not perfect as I’m still learning myself, but I hope you’ll find it helpful.

Update

6/10/2018- The code examples in this post and the code in the GitHub repo now uses Angular v6.

Assumptions

This tutorial assumes you have some familiarity with Angular, angular-cli, Material2 libraries, Jasmine, calling REST apis,... The scope of this tutorial is only using the HttpClientModule and the associated unit tests for making http calls.

Background

Using angular-cli, I generated a new application and used Material 2 starter app as a base for the design and for the cute icons. angular-cli was used to generate all the scaffolding for the services, components, and classes. I used JSONPlaceholder API to GET and POST user information. Complete app and code can be found in my GitHub repo.

Creating A User Service

The User Service wraps functionality to create and retrieve user information. This is where we will use HttpClient. In the method getUsers() notice that we no longer have to map the response to json format. We can also declare the type for the response data. I’m using an incredibly unhelpful Array<any> but that’s just to show the syntax.

  public getUsers(): Observable<User[]> {
    return this.http.get<Array<any>>(this.apiEndpoint, {
      headers: new HttpHeaders().set('Accept', 'application/json')
    }).pipe(
      map(this.mapUsers),
      catchError(error => {
        return observableThrowError('An error occurred');
      }),
    );
  }

Don’t forget to add HttpClientModule to app.module.ts imports.

Testing The User Service

Angular made a lot of improvements to testing http calls via the HttpClientTestingModule and HttpClientTestingController. Simply import the HttpClientTestingModule into the TestBed and inject both the HttpClient and HttpTestingController into the tests.

describe('UserService', () => {
  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [UserService]
    });
  });

  afterEach(inject([HttpTestingController], (httpMock: HttpTestingController) => {
    httpMock.verify();
  }));

In the test, you mock out a http response and then simulate the http call by flushing TestRequest. You can assert on the http method, expected response, expected headers, and the number of times the http call is made. You’ll want an afterEach method to verify that no more requests remain to be consumed. Here’s a happy path example to verify my response filtering and mapping works.

it('returns users with an id <= 5', inject([HttpClient, HttpTestingController], (http: HttpClient, httpMock: HttpTestingController) => {
      const mockResponse = [
        {
          id: 5,
          name: 'Test5',
          company: {bs: 'blah'}
        },
        {
          id: 6,
          name: 'Test6',
          company: {bs: 'blah'}
        }
      ];

      const userService = getTestBed().get(UserService);
      userService.getUsers().subscribe(
        actualUsers => {
          expect(actualUsers.length).toBe(1);
          expect(actualUsers[0].id).toEqual(5);
        }
      );

      const req = httpMock.expectOne(userService.apiEndpoint);
      expect(req.request.method).toEqual('GET');

      req.flush(mockResponse);
      httpMock.verify();
    }));

Testing the unhappy path isn’t hard at all.

it('should throw with an error message when API returns an error',
      inject([HttpClient, HttpTestingController], (http: HttpClient, httpMock: HttpTestingController) => {
        const userService = getTestBed().get(UserService);
        userService.getUsers().subscribe({
          error(actualError) {
            expect(of(actualError)).toBeTruthy();
            expect(actualError).not.toBeNull();
            expect(actualError).not.toBeUndefined();
          }
        });

        const req = httpMock.expectOne(userService.apiEndpoint);
        expect(req.request.method).toEqual('GET');

        req.flush({ errorMessage: 'Uh oh!' }, { status: 500, statusText: 'Server Error' });
        httpMock.verify();
      }));

Intercepting Calls

In a real app, we need to authenticate our requests. So we can create our own interceptor by implementing Angular’s HttpInterceptor interface. In this example, I add an Authorization header to each of my calls. I created a fake authentication service to spoof token retrieval and inject the authentication service to better mimic real life and form my token.

export class AuthInterceptor implements HttpInterceptor {
  constructor(private authService: AuthService) {}

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
  const authReq = req.clone({setHeaders: {Authorization: `${this.authService.tokenType} ${this.authService.tokenValue}`}});
    return next.handle(authReq);
  }
}

In order to use the interceptor, we have to provide it to the app.module.ts.

providers: [UserService, AuthService,
    {
      provide: HTTP_INTERCEPTORS,
      useClass: AuthInterceptor,
      multi: true
    }],

Now all calls made using HttpClient will include the Authorization header.

Testing Interceptors

Just like before, you need to prep the Testbed. But this time we’re also going to provide the interceptor. I’m also providing a mock to the authentication service.

beforeEach(() => {
        TestBed.configureTestingModule({
            imports: [HttpClientTestingModule],
            providers: [
                {
                    provide: AuthService,
                    useValue: mockAuthService
                },
                {
                    provide: HTTP_INTERCEPTORS,
                    useClass: AuthInterceptor,
                    multi: true
                }]
        });
    });

Testing the interceptor is pretty much the same as testing the User Service. Except in this case we’ll customize the verification to check for the header.

describe('making http calls', () => {
        it('adds Authorization header', inject([HttpClient, HttpTestingController], (http: HttpClient, httpMock: HttpTestingController) => {

            http.get('/data').subscribe(
                response => {
                    expect(response).toBeTruthy();
                }
            );

            const req = httpMock.expectOne(r =>
                r.headers.has('Authorization') &&
                r.headers.get('Authorization') === `${mockAuthService.tokenType} ${mockAuthService.tokenValue}`);
            expect(req.request.method).toEqual('GET');

            req.flush({ hello: 'world' });
            httpMock.verify();
        }));
    });

That’s all I have for now. I hope you found this useful. All my code can be found in my GitHub repo. Please feel free to check it out and I’m open to any feedback you have.

Alisa @alisaduncan