为什么用handleRequestStateChange 而不是用handleRequestStateChange()；

Question

// 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<XmlHttpVersions.length && !xmlHttp; i++) 
    {
      try 
      { 
        // try to create XMLHttpRequest object
        xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
      } 
      catch (e) {}
    }
  }
  // return the created object or display an error message
  if (!xmlHttp)
    alert("Error creating the XMLHttpRequest object.");
  else 
    return xmlHttp;
}


// called to read a file from the server
function process()
{
  // only continue if xmlHttp isn't void
  if (xmlHttp)
  {
    // try to connect to the server
    try
    {
      // initiate reading the async.txt file from the server
      xmlHttp.open("GET", "async.txt", true);
      xmlHttp.onreadystatechange = handleRequestStateChange;
      xmlHttp.send(null);
    }
    // display the error in case of failure
    catch (e)
 
    {
      alert("Can't connect to server:\n" + e.toString());
    }
  }
}

// function that handles the HTTP response
function handleRequestStateChange() 
{
  // obtain a reference to the <div> element on the page
  myDiv = document.getElementById("myDivElement");
  // display the status of the request 
  if (xmlHttp.readyState == 1)
  {
    myDiv.innerHTML += "Request status: 1 (loading) <br/>";
  }
  else if (xmlHttp.readyState == 2)
  {
    myDiv.innerHTML += "Request status: 2 (loaded) <br/>";
  }
  else if (xmlHttp.readyState == 3)
  {
    myDiv.innerHTML += "Request status: 3 (interactive) <br/>";
  }
  // 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 
        myDiv.innerHTML += 
                      "Request status: 4 (complete). Server said: <br/>";
        myDiv.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

Answer 1

假设有

func(){
    return 0;
}

其中func指向的是这个函数，其值就是这个函数；func()是调用func这个函数，值是这个函数返回结果0。
xmlHttp.onreadystatechange这个是指定当状态发生变化时需要调用的函数是什么，所以应该赋函数，也就是func