Often when picking new tools and frameworks I see the warning "Will not work if the user has disabled Javascript". I then look at my Javascript powered slideshow, Javascript powered menu, and Javascript powered gallery. Funnily enough, I chose these to replace the original flash based versions since I worried about the number of people that would see them.
But is worrying about people disabling Javascript a legitimate worry? Should I try and design my site with JS being disabled in mind?
I'm surprised no one mentioned progressive enhancement. There's rarely a good reason to have functionality or content that requires JavaScript to work. Yes, JavaScript can make the user experience better, but it shouldn't be required to make the user experience possible.
So my answer is, you should build your website so everyone can access everything, even without JavaScript (assuming publicly available content. Site admins, intranets, etc, would be obvious exceptions as you can realistically dictate browser requirements in those situations).
EDIT
Oh, yeah. This is search engine friendly too. JavaScript is not.
That depends on the site, its purpose, and who the demographic is.
If you're designing a government website, a banking site, a corporate home page, etc, then you absolutely should make sure the site works without JavaScript. But if you're designing an entertainment/leisure site like Twitter or Facebook, then it's not so bad to require JavaScript. And if your site is a rich internet application that absolutely won't work without client-side scripting (e.g. Meebo), then it's absolutely reasonable to require JavaScript.
Most features on a typical website probably shouldn't require JavaScript though. It's fine to center the user experience around clients with JavaScript enabled, but if you're running a forum, you should make sure that users can still post/read messages without JavaScript, even if it means the interface is more basic.
Nicholas C. Zakas informs that around 1% of the actual visitor traffic to sites on the Yahoo network make JavaScript-disabled requests.
...the overwhelming majority of users has JavaScript-enabled browsers and can therefore take advantage of all of the enhanced functionality and dynamic interfaces developers and designers love to create. From a planning standpoint, it makes sense to spend more time on the experience that the largest numbers of users receive, knowing that your time investment is well worth it.
So find out the stats for your own site & build it so that it benefits the majority & still degrades gracefully for those who have JS disabled.
Worrying is not needed, no.
Having said that, making your website accessible and usable is a good idea. Usually heavy use of JavaScript and accessibility don't go hand in hand all that well.
So as long as your website is still usable with JavaScript off, you are in good shape. Even if the website does not necessarily appears as great as you would wish it to look like anymore.
Facebook will not work without JavaScript. What I have learned is that most people that don't have JavaScript are probably not going to subscribe to your product. We use JavaScript to enhance a user's experience, but don't make it a requirement.
So your site should work without JavaScript, but it doesn't have to look good. Chances are if someone is visiting your site without JS turned on they are a BOT, a Hacker, a 99 year old, or a weirdo on a dial up connection trying to speed things up.
If you offer a commercial product, SAS, or cloud app don't even worry about this small percentage of users. Its not worth the development time, and again they will probably never subscribe to your service.
If you are using JavaScript to store shopping cart items in an eCommerce store, then I would look at the architecture and make sure that you offer a non js functionality.
