TokenInterceptor - Async HTTP Interceptors with Angular 4

token.interceptor.ts

import { Injectable } from '@angular/core';
import { HttpErrorResponse, HttpHandler, HttpInterceptor, HttpRequest } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/mergeMap';

import { AuthService } from './auth.service';

@Injectable()
export class TokenInterceptor implements HttpInterceptor {

    constructor (private auth: AuthService) {}
    
    intercept (request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        return this.auth.getUser()
            .mergeMap((user: User) => {

                if (user) {
                    // clone and modify the request
                    request = request.clone({
                        setHeaders: {
                            Authorization: `Bearer ${user.access_token}`
                        }
                    });
                }

                return next.handle(request)
                    // new
                    .do(() => {
                        // success
                    }, (err: any) => {
                        // error
                        if (err instanceof HttpErrorResponse) {
                            if (err.status === 401) {
                                // redirect user to login
                                this.auth.login();
                            }
                        }
                    });
            });
    }
}