ajax手写代码实现

    <script type = "text/javascript">
    var xmlhttp = null;
    try
    {
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    catch(e)
    {
        try
        {
            xmlhttp = new XMLHttpRequest();
        }
        catch(e)
        {
            alert("xmlhttp创建失败");
        }
    }
    
    function state_Change()
    {
        if(xmlhttp.readyState == 4)
        {
            if(xmlhttp.status == 200)
            {
                var mes = xmlhttp.responseText;
                document.getElementById("t1").innerText=mes;
            }
        }
    }
    
    function GetMes()
    {
        //异步
        xmlhttp.open("POST" ,"Default.aspx",true);
        xmlhttp.onreadystatechange=state_Change;
        xmlhttp.send(null);

        
        //同步
        xmlhttp.open("POST","Default.aspx",false);
        xmlhttp.send(null);
        var mes = xmlhttp.responseText;
        document.getElementById("t1").innerText=mes;
        
        setTimeout(GetMes,1000);
    }
    
    GetMes();
    </script>