Ajax返回DataSet时

 function get_city_Result_CallBack(response)
            {
                if (response.value !=null)
                {                   
                    //debugger;
                    document.all("DropDownList2").length=0;       
                var ds = response.value;
                    if(ds !=null&&typeof(ds) =="object"&& ds.Tables !=null)
                    {                   
                        for(var i=0; i<ds.Tables[0].Rows.length; i++)
                        {
                          var name=ds.Tables[0].Rows[i].city;
                          var id=ds.Tables[0].Rows[i].cityID;
                          document.all("DropDownList2").options.add(new Option(name,id));                         
                        }
                    }
                }               
                return
            }