[翻译]High Performance JavaScript(004)

XMLHttpRequest Script Injection  XHR脚本注入
    Another approach to nonblocking scripts is to retrieve the JavaScript code using an XMLHttpRequest (XHR) object and then inject the script into the page. This technique involves creating an XHR object, downloading the JavaScript file, then injecting the JavaScript code into the page using a dynamic <script> element. Here's a simple example:
    另一个以非阻塞方式获得脚本的方法是使用XMLHttpRequest(XHR)对象将脚本注入到页面中。此技术首先创建一个XHR对象，然后下载JavaScript文件，接着用一个动态<script>元素将JavaScript代码注入页面。下面是一个简单的例子：
var xhr = new XMLHttpRequest();
xhr.open("get", "file1.js", true);
xhr.onreadystatechange = function(){
  if (xhr.readyState == 4){
    if (xhr.status >= 200 && xhr.status < 300 || xhr.status == 304){
      var script = document.createElement_x("script");
      script.type = "text/javascript";
      script.text = xhr.responseText;
      document.body.appendChild(script);
    }
  }
};
xhr.send(null);
    This code sends a GET request for the file file1.js. The onreadystatechange event handler checks for a readyState of 4 and then verifies that the HTTP status code is valid (anything in the 200 range means a valid response, and 304 means a cached response). If a valid response has been received, then a new <script> element is created and its text property is assigned to the responseText received from the server. Doing so essentially creates a <script> element with inline code. Once the new <script> element is added to the document, the code is executed and is ready to use.
    此代码向服务器发送一个获取file1.js文件的GET请求。onreadystatechange事件处理函数检查readyState是不是4，然后检查HTTP状态码是不是有效（2XX表示有