How to clear the session when the user closes a window

<script type="text/javascript">
function window.onbeforeunload()
{
    var xmlHttp;
    try
    {
        // Firefox, Opera 8.0+, Safari
        xmlHttp=new XMLHttpRequest();
    }
    catch (e)
    {
        // Internet Explorer
    try
    {
        xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
    }
    catch (e)
    {
        try
        {
            xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
        }
        catch (e)
        {
            alert("Your browser does not support AJAX!");
            return false;
        }
    }
    if(event.clientX>document.body.clientWidth&&event.clientY<0||event.altKey)
    {
        xmlhttp.open("GET","exit.aspx",false);
        xmlhttp.send();
    }
 }
</script>