function  HandleonreadyState()
       {
         
          if(xmlHttp.readyState==4)
          {
            
            if(xmlHttp.status==200)//进行数据的获取，进行前台的编辑
            {
              
              var node = document.getElementById("ddlCity");
              node.options.length = 0;
              node.options.add(new Option("--请选择--","0"));
              alert(' ');
              
              var xmldom = new ActiveXObject("Microsoft.XMLDOM");
              alert(' ');
              xmlhttp.responseXML.createProcessingInstruction("xml"," version=\"1.0\" encoding=\"gb2312\"");
              alert(' ');
              xmldom = xmlhttp.responseXML.documentElement;
              
              var length = xmldom.getElementsByTagName("CityID").length;
              alert(length);
              
            }
          }
       }
后台的数据返回的没有问题，但是却总是执行不下去，为什么？