Ajax在网页中的简单应用

苏朋老师的例子。该例子实现到www.163.com上验证一个用户是否注册。
首先注册并得到xmlhttp对象。然后提交并得到返回值。

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %>

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>无标题页</title>
        <script language="javascript" type="text/jscript">
        //检测浏览器,创建xmlhttp-request对象
        function getXMLRequester()
        {
            var xmlhttp_request = false;
            try
            {
                if(window.ActiveXObject)
                {
                    for(var i = 5;i;i--)
                    {
                        try
                        {
                            if(i==2)
                            {
                                xmlhttp_request = new ActiveXObject("Microsoft.XMLHTTP");
                            }
                            else
                            {
                                xmlhttp_request = new ActiveXObject("Msxml2.XMLHTTP" +i+"0");
                                xmlhttp_request.setRequestHeader("Content-Type","text-xml");
                                xmlhttp_request.setRequestHeader("Content_Type","gb2312");
                            }
                            break;
                        }
                        catch(e)
                        {
                            xmlhttp_request=false;
                        }
                    }
                }
                else if(window.XMLHttpRequest)
                {
                    xmlhttp_request = new XMLHttpRequest();
                    if(xmlhttp_request.overrideMineType)
                    {
                        xmlhttp_request.overrideMineType("text/xml");
                    }
                }
            }
            catch(e)
            {
                xmlhttp_request = false;
            }
            return xmlhttp_request;
        }

        function IDRequest(n)
        {
            url = n + document.getElementById("txtUserID").value;//定义网址参数

            xmlhttp_request = getXMLRequester();
            xmlhttp_request.onreadystatechange = doContents;//调用doContent函数;
            xmlhttp_request.open("Get",url,true);

            xmlhttp_request.send(null);
        }

        function doContents()
        {
            if(xmlhttp_request.readyState == 4)//收到完整的服务器响应
            {
                if(xmlhttp_request.status == 200)//Http服务器响应ok
                    document.getElementById("message").innerHTML = xmlhttp_request.responseText;//服务器响应的值
                else
                    alert(http_request.status);
            }
        }

        </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <input type="text" id="txtUserID" style="font-size:12px;" />
        <input type="button" id="btnOk" value="校验" style="font-size:12px" onclick ="IDRequest('http://reg.163.com/register/checkssn.jsp?username=')" />
        <div id="message"></div>
    </div>
    </form>
</body>
</html>