无法在Angular 5的http拦截器中获取响应头

Question

登录后，我能够从本地存储中获取令牌。之后，我需要从标头响应中获取令牌，因为我需要在较早的令牌过期之前再次存储新令牌。

下面是我的拦截文件：

    import { environment } from './../../environments/environment';
    import { Injectable, Inject } from '@angular/core';
    import {
        HttpRequest,
        HttpHandler,
        HttpEvent,
        HttpInterceptor,
        HttpErrorResponse,
        HttpResponse,
        HttpHeaderResponse,
        HttpClientModule
    } from '@angular/common/http';
    import { Router } from '@angular/router';
    import { Observable } from 'rxjs/Observable';
    import 'rxjs/add/operator/do';
    import 'rxjs/add/operator/catch';
    import 'rxjs/Observable';
    import 'rxjs/add/observable/throw';
    import 'rxjs/add/observable/empty';

    @Injectable()
    export class AppHttpInterceptor implements HttpInterceptor {

        constructor(private router: Router) { }

        intercept(req: HttpRequest<any>, next: HttpHandler): 
        Observable<HttpEvent<any>> {

            const token = this.getToken();  //first token after signin

            const JWT = `Bearer ${token}`;
            req = req.clone({
                setHeaders: {
                    Authorization: JWT
                }
            });

            req = req.clone({ url: this.prepareUrl(req.url) });

            return next.handle(req)
                .do((event: HttpEvent<any>) => {
                    if (event instanceof HttpResponse) {
                        console.log(event);  //here I am checking for a new 
                        token in event but unable to get that.
                    }
                })
                .catch((error, caught) => {
                    //intercept the response error and displace it to the console
                    console.log(error);

                    if (error instanceof HttpErrorResponse) {
                        if (error.status === 401) {
                            // redirect to the login route
                            this.router.navigate(['/login']);
                        }
                        else if (error.status === 0) {
                            // Show the error message
                            return Observable.empty<HttpEvent<any>>();
                        }
                    }

                    //return the error to the method that called it
                    return Observable.throw(error);


                }) as any;
        }


        private isAbsoluteUrl(url: string): boolean {
            const absoluteHttpPattern = /^http?:\/\//i;
            const absoluteHttpsPattern = /^https?:\/\//i;
            return absoluteHttpPattern.test(url) || absoluteHttpsPattern.test(url);
        }

        private prepareUrl(url: string): string {
            url = this.isAbsoluteUrl(url) ? url : environment.api + url;
            return url.replace(/([^:]\/)\/+/g, '$1');
        }

        getToken(): string {
            return localStorage.getItem('accessToken');
        }
    }

以下是我收到的日志响应：

HttpResponsebody: Array[2]headers: HttpHeaderslazyInit: ()lazyUpdate: nullnormalizedNames: Map__proto__: Objectok: truestatus: 200statusText: "OK"type: 4url: null__proto__: HttpResponseBase

当我打开开发人员工具时，我能够看到响应头，但在我的拦截文件中却没有。

Cache-Control:no-cache, no-store, max-age=0, must-revalidate
Content-Type:application/json;charset=UTF-8
Date:Tue, 11 Dec 2018 13:16:54 GMT
Expires:0
Pragma:no-cache
t:eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJhZG1pbkBuZXRhcHAuY29tIiwic2NvcGVzIjpbeyJhdXRob3JpdHkiOiIxIn1dLCJpc3MiOiJodHRwOi8vY3JvLm5ldGFwcC5jb20iLCJpYXQ (I need to get this "t" value in my intercept file)
Transfer-Encoding:chunked
X-Content-Type-Options:nosniff
X-Frame-Options:DENY
X-XSS-Protection:1; mode=block