How to detect IE11?

When I want to detect IE I use this code:

function getInternetExplorerVersion()
{
  var rv = -1;
  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 checkVersion()
{
  var msg = "You're not using Internet Explorer.";
  var ver = getInternetExplorerVersion();

  if ( ver > -1 )
  {
    msg = "You are using IE " + ver;
  }
  alert( msg );
}

But IE11 is returning "You're not using Internet Explorer". How can I detect it?

IE11 no longer reports as MSIE, according to this list of changes it's intentional to avoid mis-detection.

What you can do if you really want to know it's IE is to detect the Trident/ string in the user agent if navigator.appName returns Netscape, something like (the untested);

function getInternetExplorerVersion()
{
  var rv = -1;
  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 );
  }
  else if (navigator.appName == 'Netscape')
  {
    var ua = navigator.userAgent;
    var re  = new RegExp("Trident/.*rv:([0-9]{1,}[\\.0-9]{0,})");
    if (re.exec(ua) != null)
      rv = parseFloat( RegExp.$1 );
  }
  return rv;
}

console.log('IE version:', getInternetExplorerVersion());

Note that IE11 (afaik) still is in preview, and the user agent may change before release.

Use !(window.ActiveXObject) && "ActiveXObject" in window to detect IE11 explicitly.

To detect any IE (pre-Edge, "Trident") version, use "ActiveXObject" in window instead.

Use MSInputMethodContext as part of a feature detection check. For example:

//Appends true for IE11, false otherwise
window.location.hash = !!window.MSInputMethodContext && !!document.documentMode;

References

• MSInputMethodContext Object
• Input Method Editor API
• What I’d like to see in IE12: Internet Explorer 12 Wishlist of Bug Fixes and New Features
• TypeScript/lib.dom.d.ts at master · Microsoft/TypeScript IE DOM APIs

I've read your answers and made a mix. It seems to work with Windows XP(IE7/IE8) and Windows 7 (IE9/IE10/IE11).

function ie_ver(){  
    var iev=0;
    var ieold = (/MSIE (\d+\.\d+);/.test(navigator.userAgent));
    var trident = !!navigator.userAgent.match(/Trident\/7.0/);
    var rv=navigator.userAgent.indexOf("rv:11.0");

    if (ieold) iev=new Number(RegExp.$1);
    if (navigator.appVersion.indexOf("MSIE 10") != -1) iev=10;
    if (trident&&rv!=-1) iev=11;

    return iev;         
}

Of course if I return 0, means no IE.

Get IE Version from the User-Agent

var ie = 0;
try { ie = navigator.userAgent.match( /(MSIE |Trident.*rv[ :])([0-9]+)/ )[ 2 ]; }
catch(e){}

How it works: The user-agent string for all IE versions includes a portion "MSIE space version" or "Trident other-text rv space-or-colon version". Knowing this, we grab the version number from a String.match() regular expression. A try-catch block is used to shorten the code, otherwise we'd need to test the array bounds for non-IE browsers.

Note: The user-agent can be spoofed or omitted, sometimes unintentionally if the user has set their browser to a "compatibility mode". Though this doesn't seem like much of an issue in practice.

Get IE Version without the User-Agent

var d = document, w = window;
var ie = ( !!w.MSInputMethodContext ? 11 : !d.all ? 99 : w.atob ? 10 : 
d.addEventListener ? 9 : d.querySelector ? 8 : w.XMLHttpRequest ? 7 : 
d.compatMode ? 6 : w.attachEvent ? 5 : 1 );

How it works: Each version of IE adds support for additional features not found in previous versions. So we can test for the features in a top-down manner. A ternary sequence is used here for brevity, though if-then and switch statements would work just as well. The variable ie is set to an integer 5-11, or 1 for older, or 99 for newer/non-IE. You can set it to 0 if you just want to test for IE 1-11 exactly.

Note: Object detection may break if your code is run on a page with third-party scripts that add polyfills for things like document.addEventListener. In such situations the user-agent is the best option.

Detect if the Browser is Modern

If you're only interested in whether or not a browser supports most HTML 5 and CSS 3 standards, you can reasonably assume that IE 8 and lower remain the primary problem apps. Testing for window.getComputedStyle will give you a fairly good mix of modern browsers, as well (IE 9, FF 4, Chrome 11, Safari 5, Opera 11.5). IE 9 greatly improves on standards support, but native CSS animation requires IE 10.

var isModernBrowser = ( !document.all || ( document.all && document.addEventListener ) );