='undefined'){xmlHttp=newXMLHttpRequest();}//检测用户名是否存在functioncallServerCheckUname(){//通过GET方法请求,将请求封装在URL中varuname=document.getElementById.value;xmlHttp.open("POST",'CheckUname?u_name='+uname);//设置服务器响应的处理方法xmlHttp.onreadystatechange=updatePageCheckUname