用Ajax返回的DataTable，在javascript中如何获得DataTable的列数和列名？

C#:　　DataSet ds = new DataSet("表名");
　　da.Fill(ds);
　　XmlTextWriter writer = new XmlTextWriter(Response.OutputStream, Response.ContentEncoding);
writer.Formatting=Formatting.Indented;
writer.Indentation=4;
writer.IndentChar=' ';
ds.WriteXml(writer);

js:
var oHttpReq = new ActiveXObject("MSXML2.XMLHTTP");
var oDoc = new ActiveXObject("MSXML2.DOMDocument");
oHttpReq.open("POST", "eatabout_recive.aspx?state="+str[0], false);
oHttpReq.send("");
result = oHttpReq.responseText;
oDoc.loadXML(result);
items = oDoc.selectNodes("//表名/Table");
var cc=items[0].selectSingleNode("字段").text








function get_area_Result_CallBack(response)
            {
                if (response.value != null)
                {                   
                    document.all("DropDownList3").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].area;
　　　　　　                var id=ds.Tables[0].Rows[i].areaID;
　　　　　　                document.all("DropDownList3").options.add(new Option(name,id));
　　　　                }               
                    }
                }
                return
            }