TokenInterceptor - Async HTTP Interceptors with Angular 4

token.interceptor.ts

import { Injectable } from '@angular/core';
import { HttpHandler, HttpInterceptor, HttpRequest } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import { AuthService } from './auth.service';

@Injectable()
export class TokenInterceptor implements HttpInterceptor {

    constructor (private auth: AuthService) {}
    
    intercept (request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    
        console.log('Hello request!', request);
        return next.handle(request);
        
    }

}