JavaScript support, who does, who doesn't?

JavaScript or lack of is a thorny issue, but if we take a quick look at some of the user groups that this effects:

1: User agents that don't have JavaScript enabled. As a lot of user tracking software relies on JavaScript to work so it's hard to tell how many users actually don't have JS, but in the beginning of 2009 it could be about 6%. Made up of user-agents that don't support JS, user-agents that have it turned off as part of company policy and user-agents routed though a proxy (e.g. public proxies normally don't support JavaScript)

2: User-agent that you haven't tested your JavaScript on. Just because a user-agent supports JavaScript doesn't mean your script will work (ever done this: if (!document.getElementById){return} ? ), even the four (webkit/moz/opera/ie) main desktop browsers diverge radically in some areas, meaning that there is a good chance that you scripts may not work on anything you haven't tested them on.

3: User-agents where your script is meaningless. e.g. if the user is using a small screen (or zoom software) and they click on a DHTML link that moves some page content from out of view left (eg. margin-left:-9999px) to out of view right (e.g. margin-left:800px;). In these cases JavaScript is no help to the user.

4: Assistive technologies: Most assistive technologies like screen-readers run on the top of your normal browser so although the browser understands JavaScript the assistive technology may not. They may not see things you have written in with JavaScript rendering your JavaScript content inaccessible.

There are probably other user groups effected, will add when they come to mind...

So our user groups fall in to a number of categories...

1. users with browsers that don't support JavaScript at all, or have JavaScript turned off
2. users with browsers can't run your scripts (they error or don't support DOM)
3. users accessing your site though anal proxy, firewall etc...
4. users for whom your JavaScript is redundant (not really any help)
5. users for whom your JavaScript is actively blocking access to content