ajax简单应用

<script language="javascript">
        var xmlHttp;
        function CreateXMLHTTPRequest()
        {
           if(window.ActiveXObject)
           {
              xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
           }
           if(window.XMLHTTPRequest)
           {
              xmlHttp=new XMLHTTPRequest();
           }
        }
        function getMail()
        {
           CreateXMLHTTPRequest();
           var url="ajax/getMailCount.aspx";
           xmlHttp.onreadystatechange=handler;
           xmlHttp.open("post",url,true);
           xmlHttp.send(null);
        }
        
        function handler()
        {
           if(xmlHttp.readystate==4)
           {
             var Result=xmlHttp.responsetext;
                        document.getElementById("dd").value=Result;
setTimeout("getMail()", 1000);
           }