var OSName="Unknown OS"; 
if (navigator.appVersion.indexOf("Win")!=-1) 
    OSName="Windows"; 
else if (navigator.appVersion.indexOf("Mac")!=-1) 
    OSName="MacOS"; 
else if (navigator.appVersion.indexOf("X11")!=-1) 
    OSName="UNIX"; 
else if (navigator.appVersion.indexOf("Linux")!=-1) 
    OSName="Linux"; 

console.log('Your OS is: '+OSName);

参考链接：

https://stackoverflow.com/questions/14501185/detect-windows-vs-mac-os