gistfile1.js

// ----------------------------------------------------------
// A short snippet for detecting versions of IE in JavaScript
// without resorting to user-agent sniffing
// ----------------------------------------------------------
// If you're not in IE (or IE version is less than 5) then:
//     ie === undefined
// If you're in IE (>=5) then you can determine which version:
//     ie === 7; // IE7
// Thus, to detect IE:
//     if (ie) {}
// And to detect the version:
//     ie === 6 // IE6
//     ie > 7 // IE8, IE9 ...
//     ie < 9 // Anything less than IE9
// ----------------------------------------------------------

// UPDATE: Now using Live NodeList idea from @jdalton

var ie = (function(){

    var undef,
        v = 3,
        div = document.createElement('div'),
        all = div.getElementsByTagName('i');
    
    while (
        div.innerHTML = '<!--[if gt IE ' + (++v) + ']><i></i><![endif]-->',
        all[0]
    );
    
    return v > 4 ? v : undef;
    
}());

You can use bowser.

This is slick!

Unfortunately IE>=10 don't have conditional comments

The best option for IE / Edge these days is probably Layout Engine or something similar that uses JavaScript to detect browser features.

User agents can be forged or altered so I wouldn't rely on UA sniffing.

Maybe help:

var ie = 'NotIE'; //IE5-11
if( !!document.compatMode ) {
if( !!(Math && Math.acosh) ) ie = 'EDGE 12';
if( !!document.uniqueID){
if(!!window.ActiveXObject){ ie = 11; }
else if(!!document.all){
if(!!window.atob){ie = 10;}
else if(!!document.addEventListener) {ie = 9;}
else if(!!document.querySelector){ie = 8;}
else if(!!window.XMLHttpRequest){ie = 7;}
else if(!!document.compatMode){ie = 6;}
else ie = 5;
}
}
}