无刷新检测用户名

功能为：当用户输入用户名，用户名的文本框失去焦点后就检测用户名是否重复，不用再点击一个按钮才能检测前台代码：
以下是引用片段：
这个是我在网上找的   我不知道怎吗用   哪位高手给注释一下    详细的
谢谢了



<script type="text/JavaScript" language="javascript">
        var XMLHttp = null;
        function CheckUser()
        {
            try
            {
                xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
            }
            catch(err1)
            {
                try
                {
                    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
                }
                catch(err2)
                {
                    xmlHttp = new XMLHttpRequest();
                }
            }
            var Text1 = document.getElementById("Text1");
            var url = "chkuser.aspx?User=" + Text1.value;
            xmlHttp.open("GET",url,true);
            xmlHttp.onreadystatechange = OnHandler;
            xmlHttp.send(null);
        }
        function OnHandler()
        {
            if(xmlHttp.readyState == 4)
            {
                var isValid = xmlHttp.responseText;
                var exists = document.getElementById("exsits");
                exists.innerHTML = isValid.substring(0,4);
            }
        }
</script>
<input id="Text1" type="text"  /><label d="exsits"></label><br />
　　然后建一个chkuser.aspx，其后台代码改为：
以下是引用片段：
protected void Page_Load(object sender, EventArgs e)
    {
        string user = this.Request.QueryString["User"];
        if (UserExist(user))
        {
            Response.Write("存在");
        }
        else
        {
            Response.Write("不存在");
        }
    }
    private bool UserExist(string username)
    {
        ClassLib.Employee emp = new ClassLib.Employee();
        return emp.EmpNameExist(username);
    }