// holds an instance of XMLHttpRequest
var xmlHttp = createXmlHttpRequestObject();

// creates an XMLHttpRequest instance
function createXmlHttpRequestObject() 
{
  // will store the reference to the XMLHttpRequest object
  var xmlHttp;
  // this should work for all browsers except IE6 and older
  try
  {
    // try to create XMLHttpRequest object
    xmlHttp = new XMLHttpRequest();
  }
  catch(e)
  {
    // assume IE6 or older
    var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
                                    "MSXML2.XMLHTTP.5.0",
                                    "MSXML2.XMLHTTP.4.0",
                                    "MSXML2.XMLHTTP.3.0",
                                    "MSXML2.XMLHTTP",
                                    "Microsoft.XMLHTTP");
    // try every prog id until one works
    for (var i=0; i element on the page
  myp = document.getElementById("mypElement");
  // display the status of the request 
  if (xmlHttp.readyState == 1)
  {
    myp.innerHTML += "Request status: 1 (loading) 
";
  }
  else if (xmlHttp.readyState == 2)
  {
    myp.innerHTML += "Request status: 2 (loaded) 
";
  }
  else if (xmlHttp.readyState == 3)
  {
    myp.innerHTML += "Request status: 3 (interactive) 
";
  }
  // when readyState is 4, we also read the server response
  else if (xmlHttp.readyState == 4) 
  {
    // continue only if HTTP status is "OK"
    if (xmlHttp.status == 200) 
    {
      try
      {
        // read the message from the server
        response = xmlHttp.responseText;
        // display the message 
        myp.innerHTML += 
                      "Request status: 4 (complete). Server said: 
";
        myp.innerHTML += response;
      }
      catch(e)
      {
        // display error message
        alert("Error reading the response: " + e.toString());
      }
    } 
    else
    {
      // display status message
      alert("There was a problem retrieving the data:\n" + 
            xmlHttp.statusText);
    }
  }
}

为什么用handleRequestStateChange 而不是用handleRequestStateChange()；
我改成handlerequeststatechange()只能load第一个状态。
xmlHttp.onreadystatechange = handleRequestStateChange;这里状态发生改变之后，应该是调用handleRequestStateChange()这个方法吧？而这里只写了handleRequestStateChange。所以非常的不解。。

最近刚开始学js。。望大神指点下