Give User Confirmation to close browser in javascript

<script type="text/javascript">
<!--
var g_isPostBack = false;

window.onbeforeunload = function ()
{
 if ( g_isPostBack == true )
  return;

 var closeMessage =
  'You are exiting this page.\n' +
  'If you have made changes without saving, your changes will be lost.\n' +
  'Are you sure that you want to exit?';

 if ( window.event )
 {
  // IE only...

  window.event.returnValue = closeMessage;
 }
 else
 {
  // Other browsers...

  return closeMessage;
 }

 g_isPostBack = false;
}

// -->
</script>