Angular 5 Http Interceptors error when injecting service

Question

I am receiving the following strange dependency injection behavior when using custom HttpInterceptors in angular 5+.

The following simplified code works fine:

Answer 1

you need to add Injector into constructor and inject AuthService via injector

export class AuthInterceptor implements HttpInterceptor {
            constructor(private inj: Injector) {}

            intercept(req: HttpRequest, next: HttpHandler): Observable> {
                const auth = this.inj.get(AuthService);
                const token = this.auth.getToken();
                return next.handle(req);
            }
        }

don't forget import

import {Injector} from '@angular/core';