Unit testing the intercept function in an Angular HTTP interceptor

Question

I've written an Angular 9 HTTP Interceptor, and I would like to unit test the intercept function in relative isolation. From what I've seen on StackOverflow already, people are suggesting to make fake HTTP requests or return a basic request, which isn't working for me as I use the .pipe method (TypeError: next.handle(...).pipe is not a function)

Here's my intercept function, what would you suggest to unit test this function please?

intercept(request: HttpRequest < unknown >, next: HttpHandler): Observable < HttpEvent < unknown >> {
  this.loadingService.show();
  return next.handle(request).pipe(
    catchError((error: HttpErrorResponse) => {
      if (error.status !== 401) {
        this.snackBar.open("There was an error when making your request! " + error.message, 'Okay, I will refresh the page', { duration: 10000 });
      }
      return throwError(error);
    }),
    finalize(
      () => {
        this.loadingService.hide()
      }
    )
  );
}

Answer 1

I was able to solve my issue by adding the following in the providers section of

TestBed.configureTestingModule

By providing the HTTP_INTERCEPTORS the HttpClientTestingModule will use the interceptor I have made:

{
  provide: HTTP_INTERCEPTORS,
  useClass: LoadingInterceptor,
  multi: true
},