javascript - Why is browser sniffing not a recommended practice?

Question

You hear it all over the place: using javascript to sniff the user agent string to detect browser versions is a Very Bad Thing. The latest version of jQuery has now deprecated its $.browser object in place of $.support. But what should I do if there's a bug or problem which is only affecting IE and not the other browsers, and I'm not sure why?

In my case, some jQuery code makes a tooltip appear and disappear with an animation on mouseover and mouseout. In Internet Explorer, it looks awful, and jittery, with the tooltip div changing to a really large size before hiding, and if you run your mouse over a heap of items with the tip it really kills the browser. I have no idea what particular feature IE doesn't "support" that I should be testing against, so it's much easier to just sniff for IE and use a different method. What could/should I do instead?

Answer 1

Because just sniffing the user agent (which is what jquery does to populate the $.browser object) doesn't tell you the whole truth.

The user agent string can be easily changed in many browsers, so if you for example disable some features that don't work in IE from everybody who seems to be using IE, you might accidentally disable those features from some future browsers or users who just, for some reason (like for example to get around limitations based on browser sniffing), pretend to be using IE.

This might not seem too big of a problem, but it is still bad practice.

And yes, I am a IE sniffer too. I use

$.browser.msie && document.all

just to be sure.