所以，我使用的是普通的JavaScript（无jQuery的），发送文件到服务器。服务器脚本PHP的返回状态code 200在最后，而是JavaScript是越来越readyState的== 2。

So I'm using plain javascript (no jquery), to send a file to the server. Server script PHP returns status code 200 at the end, but instead javascript is getting readyState == 2.

PHP的code发送回状态code 200：

The PHP code sends back status code 200:

header('X-PHP-Response-Code: 200', true, 200);
exit;

的JavaScript是这样做的：

The javascript is doing:

request.onreadystatechange = function() {
        if (request.readyState == 4) {
            var message;
            switch(request.status) {
                case '200':
                     message = "Data uploaded successfully.";
                break;

                case '406':
                    message = "Incorrect file format.  Please try again.";
                break;

                case '410':
                    message = "Unexpected error.  Please contact support.";
                break;

                default:
                break;
            }
            status_message_container.innerHTML = message;
            submit_button.disabled = false;
        }
        else {
            alert( "Unexpected error:  " + this.statusText + ".\nPlease try again");
        }
    };

    request.send(formData);

即使知道HTTP 200状态code回来正确的（我得到'OK'）的前端。该JS脚本是看到 readyState的== 2 （即else块总是命中）

Even know the HTTP 200 status code comes back correctly (I get 'OK') on frontend. The JS script is seeing readyState==2 (i.e. else block always hit)

我的理解是，200的服务器状态code应该给的readyState == 4 ??

My understanding is that a server status code of 200 should give readyState == 4??

推荐答案

首先， onreadystate 不只是触发一次。它的发射多次，你需要能够处理。这是C $ CS，你需要处理$：

Firstly, onreadystate isn't just fired once. It's fired multiple times, you need to be able to handle that. These are the codes you need to handle:

0 UNSENT - 的的open（）没有被调用尚未的 1打开 - 发送（）没有被调用尚未的 2 HEADERS_RECEIVED - 发送（）被调用，头和状态都可以的 3 LOADING下载; - 的responseText持有​​的部分数据的 4 - 的的操作完成的

0 UNSENT - open()has not been called yet 1 OPENED - send()has not been called yet 2 HEADERS_RECEIVED - send() has been called, and headers and status are available 3 LOADING Downloading; - responseText holds partial data 4 - The operation is complete

您code是打别人块 readyState的== 2 （接收的头），并假设这是一个错误的状态时，它不是。

Your code is hitting the else block on readyState == 2 (headers received) and assuming that is an error status when it's not.

您错误校验应该是 request.readyState == 4 检查里面。这样，请求完成但也可能是一个错误：

You error check should be inside the request.readyState == 4 check. That way, the request is complete but there could also have been an error:

if (request.readyState == 4) {
    switch(request.status) {
        case '200':
            message = "Data uploaded successfully.";
        break;
        // Error handling here
        default: alert( "Unexpected error:  " + this.statusText + ".\nPlease try again"); break;
    }
}

https://developer.mozilla.org/ EN-US /文档/网页/ API / XMLHtt prequest