Failed to get location header of ajax request?

Question

I'm trying to access location header of the following ajax response. But it gives me null. My javascript code is,

    var xmlhttp = new XMLHttpRequest();
    xmlhttp.open("POST", "http://localhost:8080/openam/oauth2/authorize", true);
    xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
    xmlhttp.setRequestHeader("Cache-Control", "no-cache");
    xmlhttp.withCredentials = true;

    xmlhttp.onreadystatechange = function() {//Call a function when the state changes.
            if(this.readyState == this.HEADERS_RECEIVED) {
                alert(xmlhttp.getResponseHeader("Location"));
         }
    }

    xmlhttp.send(params);

Following are my corrosponding options and post requests,

Option Request

OPTIONS /openam/oauth2/authorize HTTP/1.1
Host: localhost:8080
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Access-Control-Request-Method: POST
Access-Control-Request-Headers: cache-control
Origin: http://127.0.0.1:4000
Connection: keep-alive

Option Response

HTTP/1.1 200 
Access-Control-Allow-Origin: http://127.0.0.1:4000
access-control-allow-credentials: true
Access-Control-Max-Age: 10
Access-Control-Allow-Methods: POST

Access-Control-Allow-Headers: iplanetdirectorypro,cookie,origin,x-requested-with,access-control-request-headers,x-openam-password,content-type,location,x-openam-username,access-control-request-method,cache-control,accept

Content-Length: 0
Date: Mon, 20 Feb 2017 17:57:34 GMT

Post Request

POST /openam/oauth2/authorize HTTP/1.1  
Host: localhost:8080 
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Cache-Control: no-cache 
Referer: http://127.0.0.1:4000/login.htm 
Content-Length: 139
Origin: http://127.0.0.1:4000
Cookie: amlbcookie=01; 

Post response

HTTP/1.1 302 
Access-Control-Allow-Origin: http://127.0.0.1:4000
access-control-allow-credentials: true
access-control-expose-headers: Access-Control-Allow-Origin,Access-Control-Allow-Credentials,Set-Cookie,Date,Location
Cache-Control: no-store
Date: Mon, 20 Feb 2017 17:57:34 GMT
Accept-Ranges: bytes
Location: http://localhost:8080?code=669b8303-d8cd-4e01-90ad-7cb89007fc65&scope=address%20openid%20profile%20email

Access-Control-Allow-Headers & Access-Control-Expose-Headers are available with proper Location value. But why can't i access Location header?

Answer 1

您无法获得 Location 标头，因为浏览器会透明地跟随它，因此 XHR 最终处理的响应是下一个（不会有 Location 标头）。