function getXMLHttpRequest(){
    if(window.ActiveXObject){//用户是ie浏览器
http_request=new ActiveXObject("Microsoft.XMLHTTP");
}else{//其他的浏览器
    http_request=new XMLHttpRequest();
 }
return http_request;
}
    var httpRequest;
    function name(){
   httpRequest=getXMLHttpRequest();
   var url="链接地址";
   var date="username="+document.getElementById("username").value;
            httpRequest.open("post",url,true);
            httpRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
   httpRequest.onreadystatechange=chuli();
   httpRequest.send(date);
}
function chuli(){
   if(httpRequest.status==200){//html来返回数据 XML来返回数据
      var str=eval("(" +httpRequest.responseText+")");
      //alert(str.res);
      if(str.res == ''){
         document.getElementById("checkusername").innerHTML="用户名可用";
     document.getElementById("checkusername").style.color='#323232';
      }else{
     document.getElementById("checkusername").innerHTML="用户名存在<a href='user/tologin.html'>点击登录</a>";
         document.getElementById("checkusername").style.color='#323232';
         document.getElementById("username").value='';
      }
   }
}