How To Clear The Session?

<script type="text/javascript"> function window.onbeforeunload() {      var XMLHttpRe;      try      {           //For "Firefox", "Opera 8 Or Higher" & "Safari" Browsers           XMLHttpRe=new XMLHttpRequest();      }      catch (e)      {           // Internet Explorer Browser           try          {                XMLHttpRe=new ActiveXObject("Msxml2.XMLHTTP");          }          catch (e)          {                try                {                      XMLHttpRe=new ActiveXObject("Microsoft.XMLHTTP");                }                catch (e)                {                      alert("Your Browser Does Not Support!");                      return false;                }           }           if(event.clientX > document.body.clientWidth && event.clientY < 0 || event.altKey)           {                XMLHttpRe.open("GET","exit.aspx",false);                XMLHttpRe.send();           }      } } </script>