Keith's Example using jQuery:
jQuery(function($) {
$('body').addClass('hasJS');
});

CSS
.hasJS .module {
display: none;
}

Why add a class if JavaScript is enabled?

Although the majority of desktop users have JavaScript enabled, mobile is one of the most used platforms to surf the web. Some mobile browsers don't support JavaScript. Keep in mind that search engines don't run JavaScript, so NEVER add important content using JavaScript.

My current project at work requires the content to be rendered for both enabled and disabled JavaScript browsers. I did a bit of research and found that most websites/solutions add the hasJS class to the body after the DOM is loaded. My first thought was, if the class gets added after the DOM is loaded, a shift on the page will occur providing a bad user experience. Yahoo was a site I researched and they have, IMHO, the cleverest solution of them all. They add the class to the HTML element instead of the BODY element. The first script block on the page to be loaded adds the hasJS class without dependency on libraries. Note: Yahoo uses another class name "jsenabled".

You might be asking why add the class to the HTML element? I think it makes more sense because the BODY is the beginning of the content. This means that everything after the body is "visible" to the user and blocking the content with a script tag isn't a good optimization practice. Here is the code to add the hasJS class to the body:

hasJS Script Block
<script type="text/javascript">
  document.getElementsByTagName('html')[0].className = 'hasJS';
</script>

NOTE: If you already have more classes on the HTML element, use document.getElementsByTagName('html')[0].className += ' hasJS'; instead.