function JREVersion () {
  //for(i=0;i<navigator.mimeTypes.length;i++)
        //alert(navigator.mimeTypes.length);
  var appletMimeType = navigator.mimeTypes['application/x-java-applet'];
  if (!appletMimeType || !appletMimeType.enabledPlugin)
    return null;
  var re = /JRE (\d\.\d(\.\d))/
  var match = re.exec(appletMimeType.enabledPlugin.description);
  if (!match)
    return null
  else {
    var jreVersion = match[1];
    return jreVersion;
  }
}
// example:
alert(JREVersion())