XMLHttpRequest 方法的顺序很重要吗？

Question

这段代码工作正常：

function callFromFlex(url, method, payload) {
   console.log("call from Flex: " + method + " " + url + " " + payload);
   var xhttp = new XMLHttpRequest();
   xhttp.open(method, url, true);
   xhttp.setRequestHeader("Content-Type", "application/json");
   xhttp.onreadystatechange = function() {
    console.log(xhttp.readyState);
       if (xhttp.readyState == 4) { 
        console.log("trying to call flash...");
           // Callback to Flash here
           ...  
       }
   };

   xhttp.send(payload);
}

但这不会 - 永远不会调用 onreadystatechange：

function callFromFlex(url, method, payload) {
    console.log("call from Flex: " + method + " " + url + " " + payload);
    var xhttp = new XMLHttpRequest();

    xhttp.setRequestHeader("Content-Type", "application/json");
    xhttp.onreadystatechange = function() {
        console.log(xhttp.readyState);
        if (xhttp.readyState == 4) {    
            console.log("trying to call flash...");
            // Callback to Flash here;
            ... 
        }
    };
    xhttp.open(method, url, true);
    xhttp.send(payload);
}

我刚刚将 xhttp.open(method, url, true) 移动到另一个位置，但从未调用过 xhttp.onreadystatechange。检查过 Firefox 45.0.2 和 IE 11，我相信它与 Flash 播放器无关。订单应该不会影响这一切吧？

Answer 1

方法顺序对于 XMLHttpRequest 来说绝对重要。 description of open 开头为：

Initializes a request. This method is to be used from JavaScript code; to initialize a request from native code, use openRequest() instead.

在调用 open 之前，请求未完全初始化（此处分配不是初始化）并且不能保证其他方法正常工作。

来自 the examples in the WhatWG spec, onreadystatechange ought to work, but I can't imagine setRequestHeader will. In fact, calling setRequestHeader before open should throw an InvalidStateError, it seems 中的一些：

If the state is not OPENED, throw an "InvalidStateError" exception.

XMLHttpRequest 方法的顺序很重要吗？

XMLHttpRequest methods order is important?

javascript

flash

xmlhttprequest

call

onreadystatechange