How to detect IE version using JavaScript

function getInternetExplorerVersion()
// Returns the version of Windows Internet Explorer or a -1
// (indicating the use of another browser).
{
   var rv = -1; // Return value assumes failure.
   if (navigator.appName == 'Microsoft Internet Explorer')
   {
      var ua = navigator.userAgent;
      var re  = new RegExp("MSIE ([0-9]{1,}[\.0-9]{0,})");
      if (re.exec(ua) != null)
         rv = parseFloat( RegExp.$1 );
   }
   return rv;
}
function checkIEVersion()
{
   var msg = "You're not using Windows Internet Explorer.";
   var ver = getInternetExplorerVersion();
   if ( ver> -1 )
   {
      if ( ver>= 8.0 )
         msg = "You're using Windows Internet Explorer 8.";
      else if ( ver == 7.0 )
    	  msg = "You're using Windows Internet Explorer 7.";
      else if ( ver == 6.0 )
    	  msg = "You're using Windows Internet Explorer 6.";
      else
    	  msg = "You should upgrade your copy of Windows Internet Explorer";
    }
   alert( msg );
}