If your use case is that you have a form (e.g., a login form) and your server-side script needs to know if the user has JavaScript enabled, you can do something like this:
<form onsubmit="this.js_enabled.value=1;return true;">
<input type="hidden" name="js_enabled" value="0">
<input type="submit" value="go">
</form>
This will change the value of js_enabled to 1 before submitting the form. If your server-side script gets a 0, no JS. If it gets a 1, JS!