Native Ajax implementation code

//开始初始化XMLHttpRequest对象      if(window.XMLHttpRequest)       {   //Mozilla 浏览器          ajax = new XMLHttpRequest();          if (ajax.overrideMimeType)           {   //设置MiME类别              ajax.overrideMimeType("text/xml");          }      }      else if (window.ActiveXObject)       {   // IE浏览器          try           {              ajax = new ActiveXObject("Msxml2.XMLHTTP");          }           catch (e)           {              try               {                  ajax = new ActiveXObject("Microsoft.XMLHTTP");              }               catch (e) {}          }      }      if (!ajax)       {   // 异常，创建对象实例失败          window.alert("不能创建XMLHttpRequest对象实例.");          return false;      }                        //通过Post方式打开连接      ajax.open("GET", url, true);       //定义传输的文件HTTP头信息      //ajax.setRequestHeader("Content-Type","application/x-www-form-urlencoded");          ajax.setRequestHeader("If-Modified-Since","0");      //发送POST数据      ajax.send(null);       //获取执行状态      ajax.onreadystatechange = function()       {           //如果执行状态成功，那么就把返回信息写到指定的层里          if (ajax.readyState == 4 && ajax.status == 200)           {               cdown.innerHTML = ajax.responseText;           }       }  

 

Native Ajax implementation code