Say I have a web app that uses jQuery.  Is it better practice to host the necessary javascript files on my own servers along with my website files, or to reference them on jQuery's CDN (example: http://code.jquery.com/jquery-1.7.1.min.js)?

I can see pros for both sides:


If it's on my servers, that's one less external dependency; if jQuery went down or changed their hosting structure or something like that, then my app breaks.  But I feel like that won't happen often; there must be lots of small-time sites doing this, and the jQuery team will want to avoid breaking them.
If it's on my servers, that's one less external reference that someone could call a security issue
If it's referenced externally, then I don't have to worry about the bandwidth to serve the files (though I know it's not that much).
If it's referenced externally and I'm deploying this web site to lots of servers that need to have their own copies of all the files, then it's one less file I have to remember to copy/update.


You should do both:
Start with hosting from a CDN such as Google's because it will likely have a higher up-time than your own site and will be configured for the fastest response time. Additionally, anyone who has visited a page that links to the CDN will use their cached copy of the file, so they won't even have to re-download a copy, making the initial loading even faster.
Then add a fallback reference to your own server in case the CDN happens to be down (not likely, but safe is safe). Fallbacks are relatively easy to understand, but need to be customized to suit the script being used:
<script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.18/jquery-ui.min.js"></script>
<script>
    if (!window.jQuery) document.write('<script src="/path/to/jquery-ver.sion.min.js"><\/script>');
</script>

Make sure you don't write </script> anywhere within a <script> element, as it will close the HTML element and cause the script to fail. The simple fix is to use a backslash as an escape: <\/script>.

One more reason to do both:
If you pick a popular CDN it's highly unlikely that it'll ever have any down-time, however in the far far future (~18 months from now given Moore's law) when the hosting format changes, or the address is adjusted, or the network is placed behind a paywall, or anything else, it's possible that your link will no longer work as-is. If you use a fallback, then it'll give you a bit of time to adjust to any new format for hosting before having to go back through every website you've ever created and change the CDN links.

another reason to do both:
Recently I've been hit with a string of internet outages. I was able to keep working locally on projects where I'd linked local copies of script resources, and I quickly found that there were a number of projects that needed to have local copies linked.
