How to check if jQuery is already in cache?

I'm guessing you're checking to see if jQuery was loaded from the Google Libraries API?

Your browser will cache jQuery whether it's from the CDN or from your LAN. If it's already in the cache from a previous retrieval from the CDN, it'll load faster than from your LAN. If it's not already in your cache from either a previous visit to your site, or another site using the same CDN, it'll only need to load once: subsequent visits will load from the cache.

Splitting the URL for jQuery between the CDN and your LAN will just cause two copies to get cached. Let the browser cache do what it was meant to do. :)

This should do it:

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.js"></script><script>!window.jQuery && document.write(unescape('%3Cscript src="js/libs/jquery-1.4.4.js"%3E%3C/script%3E'))</script>

You need to adapt the path for the fallback, which should be located at your domain.

to my knowledge, that's not really possible.2 things jump to mind however that can help you:

1. Use Etags: Identify your resources on the webserver side. This way, your browser does that work for you: identify if a resource is already loaded, regardless from the domain it was loaded from
2. If you really want to know which domain jQuery was loaded from, you can do dummy AJAX requests. You can only do AJAX requests from the same domain your library was loaded from (except ofcourse when doing a JSONP request). So if your jQuery was loaded from your LAN resource, you will get trplies to AJAX requests to your LAN webserver, and that rules out the CDN.

Hope it helps,

Bart