How to detect IE11?

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

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.

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.