How to mock Angular 4.3 httpClient an error response in testing

I have a below interceptor auth-interceptor.service.ts

import {Injectable, Injector} from '@angular/core';
import {HttpErrorResponse, HttpEvent, HttpHandler, HttpInterceptor, HttpRequest} from '@angular/common/http';
import {Observable} from 'rxjs/Observable';
import {Cookie} from './cookie.service';
import {Router} from '@angular/router';
import {UserService} from './user.service';
import {ToasterService} from '../toaster/toaster.service';
import 'rxjs/add/operator/catch';
import 'rxjs/add/observable/throw';

@Injectable()
export class AuthInterceptor implements HttpInterceptor {
    constructor(private injector: Injector) {}

    private handleError(err: HttpErrorResponse): Observable<any> {
        let errorMsg;
        if (err.error instanceof Error) {
            // A client-side or network error occurred. Handle it accordingly.
            errorMsg = `An error occurred: ${err.error.message}`;
        } else {
            // The backend returned an unsuccessful response code.
            // The response body may contain clues as to what went wrong,
            errorMsg = `Backend returned code ${err.status}, body was: ${err.error}`;
        }
        if (err.status === 401 || err.status === 403) {
            this.injector.get(UserService).purgeAuth();
            this.injector.get(ToasterService).showError(`Unauthorized`, errorMsg);
            this.injector.get(Router).navigateByUrl(`/login`);
        }
        console.error(errorMsg);
        return Observable.throw(errorMsg);
    }

    intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        // Clone the request to add the new header.
        const authReq = req.clone({headers: req.headers.set(Cookie.tokenKey, Cookie.getToken())});
        // Pass on the cloned request instead of the original request.
        return next.handle(authReq).catch(err => this.handleError(err));
    }
}

Now I am trying to mock the http.get to throw the error, so that method handleError consoles the error message.

Below is my approach to the test case auth-interceptor.service.specs.ts

import {async, inject, TestBed} from '@angular/core/testing';

import {AuthInterceptor} from './auth-interceptor.service';
import {ApiService} from './api.service';
import {HttpClientTestingModule, HttpTestingController} from '@angular/common/http/testing';
import {environment} from '../../../environments/environment';

describe(`AuthInterceptor`, () => {
    const somePath = `/somePath`;

    beforeEach(() => {
        TestBed.configureTestingModule({
            imports: [HttpClientTestingModule],
            providers: [AuthInterceptor, ApiService]
        });
    });

    it(`should be created`, inject([AuthInterceptor], (service: AuthInterceptor) => {
        expect(service).toBeTruthy();
    }));


    it(`should log an error to the console on error on get()`, async(inject([ApiService, HttpTestingController],
        (apiService: ApiService, httpMock: HttpTestingController) => {
            spyOn(console, 'error');
            apiService.get(somePath).subscribe((res) => {
                console.log(`in success:`, res);
            }, (error) => {
                console.log(`in error:`, error);
            });

            const req = httpMock.expectOne(`${environment.apiUri}${somePath}`);
            req.flush({
                type: 'ERROR',
                status: 404,
                body: JSON.stringify({color: `blue`})
            });
            expect(console.error).toHaveBeenCalled();
        }))
    );
});

When flushing the response, I am not sure how to flush a error response, so that the method handleError will be called in my interceptor and that eventually calls console.error. Documentation doesn't have any example to my situation. Any help or suggestion is appreciated.

The expectOne method in HttpTestingController class returns a TestRequest object. This TestRequest class has a flush method which can be used to deliver

both successful and unsuccessful responses.

We can resolve the request by returning a body along with some additional response headers (if any). Relevant info can be found here.

Now, coming back to the point how you can do this. You can customize the below code snippet as per your use case.

http = TestBed.get(HttpTestingController);
let response: any;
let errResponse: any;
const mockErrorResponse = { status: 400, statusText: 'Bad Request' };
const data = 'Invalid request parameters';
apiService.get(somePath).subscribe(res => response = res, err => errResponse = err);
http.expectOne('url/being/monitored').flush(data, mockErrorResponse);
expect(errResponse).toBe(data);

NOTE: At the time of writing this comment, statusText is required in mockErrorResponse. Related info can be found here.

P.S.: The error method of TestRequest class can be used to simulate network error in our test case, as it expects an instance of Error. The following code snippet shows that.

http.expectOne(someUrl).error(new ErrorEvent('network error'));