AJAX简单实例

<script>
     var xmlHttp = false;
     function checkUserName(){
         //获取用户名
         var tbUserName = document.getElementById('username');
         if (tbUserName.value == "")
             return;
         //创建XMLHttpRequest对象
         try{
             xmlHttp = new ActiveXObject("Msxm12.XMLHTTP");
         }catch(e){
             try{
                 xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
             }
             catch(e2){
                 xmlHttp = false;
             }
         }
         if (!xmlHttp && typeof XMLHttpRequest != 'undefined'){
             xmlHttp = new XMLHttpRequest();
         }
         //创建服务器端CGI的URL
         var url = "UserinfoInsert?username=" + tbUserName.value;
         //连接服务器
         xmlHttp.open("GET",url,true);
         xmlHttp.onreadystatechange = callBack_checkUserName;
         xmlHttp.send(null);        
     }
     //获取服务器端的响应
     function callBack_checkUserName(){
         if (xmlHttp.readyState == 4){
    if(xmlHttp.responseText=="1"){
     alert("存在");
    }else{
     alert("不存在");
    }
         }
     }
     </script>