How to detect my browser version and operating system using JavaScript?

Detecting browser's details:

var nVer = navigator.appVersion;
var nAgt = navigator.userAgent;
var browserName  = navigator.appName;
var fullVersion  = ''+parseFloat(navigator.appVersion); 
var majorVersion = parseInt(navigator.appVersion,10);
var nameOffset,verOffset,ix;

// In Opera, the true version is after "Opera" or after "Version"
if ((verOffset=nAgt.indexOf("Opera"))!=-1) {
 browserName = "Opera";
 fullVersion = nAgt.substring(verOffset+6);
 if ((verOffset=nAgt.indexOf("Version"))!=-1) 
   fullVersion = nAgt.substring(verOffset+8);
}
// In MSIE, the true version is after "MSIE" in userAgent
else if ((verOffset=nAgt.indexOf("MSIE"))!=-1) {
 browserName = "Microsoft Internet Explorer";
 fullVersion = nAgt.substring(verOffset+5);
}
// In Chrome, the true version is after "Chrome" 
else if ((verOffset=nAgt.indexOf("Chrome"))!=-1) {
 browserName = "Chrome";
 fullVersion = nAgt.substring(verOffset+7);
}
// In Safari, the true version is after "Safari" or after "Version" 
else if ((verOffset=nAgt.indexOf("Safari"))!=-1) {
 browserName = "Safari";
 fullVersion = nAgt.substring(verOffset+7);
 if ((verOffset=nAgt.indexOf("Version"))!=-1) 
   fullVersion = nAgt.substring(verOffset+8);
}
// In Firefox, the true version is after "Firefox" 
else if ((verOffset=nAgt.indexOf("Firefox"))!=-1) {
 browserName = "Firefox";
 fullVersion = nAgt.substring(verOffset+8);
}
// In most other browsers, "name/version" is at the end of userAgent 
else if ( (nameOffset=nAgt.lastIndexOf(' ')+1) < 
          (verOffset=nAgt.lastIndexOf('/')) ) 
{
 browserName = nAgt.substring(nameOffset,verOffset);
 fullVersion = nAgt.substring(verOffset+1);
 if (browserName.toLowerCase()==browserName.toUpperCase()) {
  browserName = navigator.appName;
 }
}
// trim the fullVersion string at semicolon/space if present
if ((ix=fullVersion.indexOf(";"))!=-1)
   fullVersion=fullVersion.substring(0,ix);
if ((ix=fullVersion.indexOf(" "))!=-1)
   fullVersion=fullVersion.substring(0,ix);

majorVersion = parseInt(''+fullVersion,10);
if (isNaN(majorVersion)) {
 fullVersion  = ''+parseFloat(navigator.appVersion); 
 majorVersion = parseInt(navigator.appVersion,10);
}

document.write(''
 +'Browser name  = '+browserName+'<br>'
 +'Full version  = '+fullVersion+'<br>'
 +'Major version = '+majorVersion+'<br>'
 +'navigator.appName = '+navigator.appName+'<br>'
 +'navigator.userAgent = '+navigator.userAgent+'<br>'
)

Source JavaScript: browser name.
See JSFiddle to detect Browser Details.

Detecting OS:

// This script sets OSName variable as follows:
// "Windows"    for all versions of Windows
// "MacOS"      for all versions of Macintosh OS
// "Linux"      for all versions of Linux
// "UNIX"       for all other UNIX flavors 
// "Unknown OS" indicates failure to detect the OS

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

document.write('Your OS: '+OSName);

source JavaScript: OS detection.
See JSFiddle to detect OS Details.

    var nVer = navigator.appVersion;
    var nAgt = navigator.userAgent;
    var browserName  = navigator.appName;
    var fullVersion  = ''+parseFloat(navigator.appVersion); 
    var majorVersion = parseInt(navigator.appVersion,10);
    var nameOffset,verOffset,ix;
    
    // In Opera, the true version is after "Opera" or after "Version"
    if ((verOffset=nAgt.indexOf("Opera"))!=-1) {
     browserName = "Opera";
     fullVersion = nAgt.substring(verOffset+6);
     if ((verOffset=nAgt.indexOf("Version"))!=-1) 
       fullVersion = nAgt.substring(verOffset+8);
    }
    // In MSIE, the true version is after "MSIE" in userAgent
    else if ((verOffset=nAgt.indexOf("MSIE"))!=-1) {
     browserName = "Microsoft Internet Explorer";
     fullVersion = nAgt.substring(verOffset+5);
    }
    // In Chrome, the true version is after "Chrome" 
    else if ((verOffset=nAgt.indexOf("Chrome"))!=-1) {
     browserName = "Chrome";
     fullVersion = nAgt.substring(verOffset+7);
    }
    // In Safari, the true version is after "Safari" or after "Version" 
    else if ((verOffset=nAgt.indexOf("Safari"))!=-1) {
     browserName = "Safari";
     fullVersion = nAgt.substring(verOffset+7);
     if ((verOffset=nAgt.indexOf("Version"))!=-1) 
       fullVersion = nAgt.substring(verOffset+8);
    }
    // In Firefox, the true version is after "Firefox" 
    else if ((verOffset=nAgt.indexOf("Firefox"))!=-1) {
     browserName = "Firefox";
     fullVersion = nAgt.substring(verOffset+8);
    }
    // In most other browsers, "name/version" is at the end of userAgent 
    else if ( (nameOffset=nAgt.lastIndexOf(' ')+1) < 
              (verOffset=nAgt.lastIndexOf('/')) ) 
    {
     browserName = nAgt.substring(nameOffset,verOffset);
     fullVersion = nAgt.substring(verOffset+1);
     if (browserName.toLowerCase()==browserName.toUpperCase()) {
      browserName = navigator.appName;
     }
    }
    // trim the fullVersion string at semicolon/space if present
    if ((ix=fullVersion.indexOf(";"))!=-1)
       fullVersion=fullVersion.substring(0,ix);
    if ((ix=fullVersion.indexOf(" "))!=-1)
       fullVersion=fullVersion.substring(0,ix);
    
    majorVersion = parseInt(''+fullVersion,10);
    if (isNaN(majorVersion)) {
     fullVersion  = ''+parseFloat(navigator.appVersion); 
     majorVersion = parseInt(navigator.appVersion,10);
    }
    
    document.write(''
     +'Browser name  = '+browserName+'<br>'
     +'Full version  = '+fullVersion+'<br>'
     +'Major version = '+majorVersion+'<br>'
     +'navigator.appName = '+navigator.appName+'<br>'
     +'navigator.userAgent = '+navigator.userAgent+'<br>'
    )

    // This script sets OSName variable as follows:
    // "Windows"    for all versions of Windows
    // "MacOS"      for all versions of Macintosh OS
    // "Linux"      for all versions of Linux
    // "UNIX"       for all other UNIX flavors 
    // "Unknown OS" indicates failure to detect the OS
    
    var OSName="Unknown OS";
    if (navigator.appVersion.indexOf("Win")!=-1) OSName="Windows";
    if (navigator.appVersion.indexOf("Mac")!=-1) OSName="MacOS";
    if (navigator.appVersion.indexOf("X11")!=-1) OSName="UNIX";
    if (navigator.appVersion.indexOf("Linux")!=-1) OSName="Linux";
    
    document.write('Your OS: '+OSName);

Update

I had some good experiences with Platform.js (demo here), but still, caution is adviced:

Original Post

I'd like to refer you to the author of WhichBrowser: Everybody lies.

Basically, no browser is being honest. No matter if you use Chrome or IE, they both will tell you that they are "Mozilla Netscape" with Gecko and Safari support. Try it yourself on any of the fiddles flying around in this thread:

hims056's fiddle

Hariharan's fiddle

or any other... Try it with Chrome (which might still succeed), then try it with a recent version of IE, and you will cry. Of course, there are heuristics, to get it all right, but it will be tedious to grasp all the edge cases, and they will very likely not work anymore in a year's time.

Take your code, for example:

<div id="example"></div>
<script type="text/javascript">
txt = "<p>Browser CodeName: " + navigator.appCodeName + "</p>";
txt+= "<p>Browser Name: " + navigator.appName + "</p>";
txt+= "<p>Browser Version: " + navigator.appVersion + "</p>";
txt+= "<p>Cookies Enabled: " + navigator.cookieEnabled + "</p>";
txt+= "<p>Platform: " + navigator.platform + "</p>";
txt+= "<p>User-agent header: " + navigator.userAgent + "</p>";
document.getElementById("example").innerHTML=txt;
</script>

Chrome says:

Browser CodeName: Mozilla

Browser Name: Netscape

Browser Version: 5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.115 Safari/537.36

Cookies Enabled: true

Platform: Win32

User-agent header: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.115 Safari/537.36

IE says:

Browser CodeName: Mozilla

Browser Name: Netscape

Browser Version: 5.0 (Windows NT 6.1; WOW64; Trident/7.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; InfoPath.3; rv:11.0) like Gecko

Cookies Enabled: true

Platform: Win32

User-agent header: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; InfoPath.3; rv:11.0) like Gecko

At least Chrome still has a string that contains "Chrome" with the exact version number. But, for IE you must extrapolate from the things it supports to actually figure it out (who else would boast that they support .NET or Media Center :P), and then match it against the rv: at the very end to get the version number. Of course, even such sophisticated heuristics might very likely fail as soon as IE 12 (or whatever they want to call it) comes out.