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;
    
}());

The expression in the while statement was done proper though for clarity I'd use a do-while.
Interesting :{D, fail on IE10, the last code you sent looks odd to me mainly because agent sniffing, I find the conditional comment interesting, I and requires just don't like the condition compilation stuff for IE10 detection. Because James uses a pre-increment to increment v:

(+/-) from the grade listed.

(+/-) from the grade listed.

(+/-) from the grade listed.

(+/-) from the grade listed.

I wanted this script to include IE10+11 as well, and my tests works fine by just returning document.documentMode instead of undefined, since it will be undefined for non-IE browsers anyway (I also prefer do...while...):

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

So I tried the code above, also davidhellsing's version, and I tested this on ie8, and it always says it's version 7, when it's 8. So I don't think this works on ie8.

Also, just tried it in ie10 and ie gets declared as undefined.

@ttfkam Best Solution for IE10+

For any future spelunkers, the @davidhellsing version just above works fine on IE8 through IE11

If I'm testing IE11, but it's forced into ie8 via 'IE=EmulateIE8', this solution from @davidhellsing still show the IE version as 8, not as 11. Any thoughts? I want to determine if it's really IE10/11.

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;
}
}
}