How to detect IE 11 with javascript in Asp.net

You can explicitly detect IE11 with the following check (using feature detection):

if (Object.hasOwnProperty.call(window, "ActiveXObject") && !window.ActiveXObject) {
    // is IE11
}

Explanation: All versions of IE (except really old ones) have window.ActiveXObject property present. IE11, however hides this property from DOM and that property is now undefined. But the property itself is present within object, so checking for property presence returns true in all IE versions, but in IE11 it also returns false for second check.
And, finally, hasOwnProperty is called via Object because in IE8 (and I believe earlier) window is not an instanceof Object and does not have hasOwnProperty method.

Another method, using userAgent string:

var ua = window.navigator.userAgent;
var versionSplit = /[\/\.]/i;
var versionRe = /(Version)\/([\w.\/]+)/i; // match for browser version
var operaRe = /(Opera|OPR)[\/ ]([\w.\/]+)/i;
var ieRe = /(?:(MSIE) |(Trident)\/.+rv:)([\w.]+)/i; // must not contain 'Opera'
var match = ua.match(operaRe) || ua.match(ieRe);
if (!match) {
    return false;
}
if (Array.prototype.filter) {
    match = match.filter(function(item) {
        return (item != null);
    });
} else {
    // Hello, IE8!
    for (var j = 0; j < match.length; j++) {
        var matchGroup = match[j];
        if (matchGroup == null || matchGroup == '') {
            match.splice(j, 1);
            j--;
        }
    }
}
var name = match[1].replace('Trident', 'MSIE').replace('OPR', 'Opera');
var versionMatch = ua.match(versionRe) || match;
var version = versionMatch[2].split(versionSplit);

This will detect any version of IE if its userAgent string was not spoofed.

There very rare cases when you actually need to use browser detection as described above. In most cases feature detection approach is preferable.