How to unit test Angular Interceptor

Question

I have been looking for ways to test Angular Interceptor codes for error handling but I can't seem to figure out how.

@Injectable()
export class ErrorInterceptor implements HttpInterceptor {
  constructor(private authenticationService: AuthenticationService, private router: Router) { }

  intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    return next.handle(request).pipe(
      catchError(err => {

        let errorMessage = '';
        if (err instanceof HttpErrorResponse) { // server side error

          switch (err.status) {
            case 401:
              // auto logout if 401 response returned from api  
              errorMessage = "Ongeldige inloggegevens"
              break;
            case 500:   //server error
              errorMessage = `Error Status: ${err.status}\nMessage: ${err.message}`;
              this.router.navigate([`/code500`]);
              break;
          }

          return throwError(errorMessage);
        }
      })
    )
  }
}

Answer 1

Try something like this, hope it helps.

import { HttpErrorResponse } from '@angular/common/http';
import { ErrorInterceptor } from './ErrorInterceptor';
import { of } from 'rxjs';
import { take } from 'rxjs/operators';
import { HttpErrorResponse } from '@angular/common/http';
import { AuthenticationService } from '....';

describe('ErrorInterceptor', () => {
  let interceptor: ErrorInterceptor;
  let mockAuthenticationService: AuthenticationService;
  beforeEach(() => {
    mockAuthenticationService = {
      logout: jasmine.createSpy('logout')
    };
    mockRouter = {
      navigate: jasmine.createSpy('navigate')
    };
    interceptor = new ErrorInterceptor(mockAuthenticationService as any, mockRouter as any);
  });


  describe('intercept', () => {
    beforeEach(() => {
      const payload = {
        status: 401
      },
        response = new HttpErrorResponse(payload),
        next: any = {
          handle: jasmine.createSpy('handle').and.callFake(() => of(response))
        };

      interceptor.intercept(response as any, next).pipe(take(1))
        .subscribe();
    });


    describe('when status is 401', () => {
      it('should logout', () => {

        expect(mockAuthenticationService.logout).toHaveBeenCalled();
        expect(mockRouter.navigate).toHaveBeenCalledWith([`\logout`]);
      });

    });
     ...
});