我已经写好了一些

xmlHttp.js
/**
* @version 1.0
* @时间  2008-06-26
* @作者  林松斌
* @QQ    410728115
* @Email linsongbin@126.com
* @博客 http://linsongbin.cnblogs.com/
*/
Ajax = function() {
this._url = null;
this._method = 'GET';
this._xmlhttp = null;
this.createXMLRequest();
}
Ajax.prototype.setUrl = function(url) {
    this._url = url;
}
Ajax.prototype.setMethod = function(method) {
    this._method = method;
}
  
Ajax.prototype.createXMLRequest = function() {
    try {
        this._xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (ex1) {
        try {
            this._xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        } catch (ex2) {
            this._xmlhttp = null;
        }
    }
if (!this._xmlhttp) {
  if (typeof XMLHttpRequest != "undefined") {
   this._xmlhttp = new XMLHttpRequest();
  }
}
}

Ajax.prototype.send = function(url, method, data) {
    this._url = url;
    this._methdo = method;
    var self = this;
    if (this._method.toLowerCase() == "get") {
        this._xmlhttp.open("GET", this._url, true);
        this._xmlhttp.send(null);
    } else if(this._method.toLowerCase() == "post") {
        this._xmlhttp.open("POST", this._url, true);
        this._xmlhttp.setRequestHeader('Content-Type', 'application/ x-www-form-ur lencoded');
        this._xmlhttp.send(data);
    }
this._xmlhttp.onreadystatechange = function(){self.process.call(self)};  
}
//这里需要从写
Ajax.prototype.process = function() {
switch (this._xmlhttp.readyState) {
  case 1:
   //this.onLoading();
   break;
  case 2:
   //this.onLoaded();
   break;
  case 3:
   //this.onInteractive();
   break;
  case 4:
      alert(this._xmlhttp.responseText);
}
}
/**
* 提供简易操作接口
*/
sendGET = function(url) {
    var ajax = new Ajax();
    ajax.send(url, 'post', null);
}

接着就要对 callback 函数进行重构
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