javascript - 请教个问题，这里的这个1怎么获取呀

<code>            var counter = xmlHttp.responseXML.getElementsByTagName("count")[0].firstChild.data;</code>

<code>            var counter = xmlHttp.responseXML.getElementsByTagName("count")[0].firstChild.data;</code>

<code>function startCallback(){
                if(xmlHttp.readyState == 4){
                    if(xmlHttp.status == 200){
                        setTimeout("pollServer()",5000);
                        refreshTime();
                    }
                }
            }
            
            function pollServer(){
                var url = "DynamicUpdateServlet.php?task=foo" + counter();
                createXMLHttpRequest();
                xmlHttp.open("GET",url,true);
                xmlHttp.onreadystatechange = pollCallback;
                xmlHttp.send(null);
            }
            
            function counter(){
                var res = "&counter=";
                var count = xmlHttp.responseXML.getElementsByTagName("count")[0].firstChild.data;
                res += count;
                return res;
            }
            
            </code>