Ext.isIE return false in IE 11

IE 11 is not officially supported prior to ExtJS 4.2.2 (see property isIE11). Microsoft removed the MSIE part from IE11's user agent (refer here), which is the reason why it does not work with ExtJS 4.2.1 and below.

From the source code, for ExtJS 4.2.1:

isIE = !isOpera && check(/msie/),
isIE7 = isIE && ((check(/msie 7/) && docMode != 8 && docMode != 9 && docMode != 10) || docMode == 7),
isIE8 = isIE && ((check(/msie 8/) && docMode != 7 && docMode != 9 && docMode != 10) || docMode == 8),
isIE9 = isIE && ((check(/msie 9/) && docMode != 7 && docMode != 8 && docMode != 10) || docMode == 9),
isIE10 = isIE && ((check(/msie 10/) && docMode != 7 && docMode != 8 && docMode != 9) || docMode == 10),
isIE6 = isIE && check(/msie 6/),

and for ExtJS 4.2.2:

isIE = !isOpera && (check(/msie/) || check(/trident/)),
isIE7 = isIE && ((check(/msie 7/) && docMode != 8 && docMode != 9 && docMode != 10) || docMode == 7),
isIE8 = isIE && ((check(/msie 8/) && docMode != 7 && docMode != 9 && docMode != 10) || docMode == 8),
isIE9 = isIE && ((check(/msie 9/) && docMode != 7 && docMode != 8 && docMode != 10) || docMode == 9),
isIE10 = isIE && ((check(/msie 10/) && docMode != 7 && docMode != 8 && docMode != 9) || docMode == 10),
isIE11 = isIE && ((check(/trident\/7\.0/) && docMode != 7 && docMode != 8 && docMode != 9 && docMode != 10) || docMode == 11),
isIE6 = isIE && check(/msie 6/),

It's very simple:

navigator.userAgent.toLowerCase().lastIndexOf('rv:11') > -1

will return true if it's a IE11 browser.