A short snippet for detecting versions of IE in JavaScript from James Padolsey

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

https://gist.github.com/527683