How to Detect Mac OS X Version in JavaScript

I have detected OS versions for windows (code below) and I want it to work for Mac too:

// OS detection
var _os_ = (function(){
var userAgent = navigator.userAgent.toLowerCase();
return {
    isWin2K: /windows nt 5.0/.test(userAgent),
    isXP: /windows nt 5.1/.test(userAgent),
    isVista: /windows nt 6.0/.test(userAgent),
    isWin7: /windows nt 6.1/.test(userAgent),
};
}());

// get OS shorthand names
var OS;
if(_os_.isWin2K){
OS = "Windows 2000";
}
if(_os_.isXP){
OS = "Windows XP";
}
if(_os_.isVista){
OS = "Windows Vista";
}
if(_os_.isWin7){
OS = "Windows 7";
}

alert(OS);

I'm wondering if it's possible to do this SAME thing for Mac OS X. Like,

 ...
 return {
     isMac10.5: /mac osx 10.5/.test(userAgent),
     isMac10.6: /mac osx 10.6/.test(userAgent),
     isMac10.7: /mac osx 10.7/.test(userAgent),
     isMac10.8: /mac osx 10.8/.test(userAgent),
 };

 ....
 if(_os_.isMac10.5){
 OS = "Mac OS X Leopard";
 }

 etc...

return {
     isMac105: /Mac OS X 10_5/.test(userAgent),
     isMac106: /Mac OS X 10_6/.test(userAgent),
     isMac107: /Mac OS X 10_7/.test(userAgent),
     isMac108: /Mac OS X 10_8/.test(userAgent),
 };

useragent for mac e.g.

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_1) AppleWebKit/536.25 (KHTML, like Gecko) Version/6.0 Safari/536.25

Macintosh; U; Intel Mac OS X 10_5_8; ru) AppleWebKit/533.18.1 (KHTML, like Gecko) Version/5.0.2 Safari/533.18.5