Benefits vs. Pitfalls of hosting jQuery locally

I always use the CDN (Content Delivery Network) from Google. But just in case it's offline:

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script>!window.jQuery && document.write('<script src="jquery-1.4.2.min.js"><\/script>')</script>

Grab Google CDN's jQuery and fallback to local if necessary

Edit: If you don't need to support IE6 and your site has partial https usage you can remove the http as well:

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>

Google CDN:

• caching, good for performance, more users likely to have it already, and it downloads in parallel
• if ever, heaver forbid cdn goes down. you're screwed.
• if a new version breaks your existing plugins or site, you'll know about it possibly too late

Locally:

• development without being connected to the net is possible
• can still get some performance benefits by gzipping, in addition to minifying

Others have covered the benefits. Pitfalls:

• If you only include content from your own server, that's one server that needs to be running—and not blocked by firewalls etc—to make your site work. Pull script from a third party and now that's two servers that need to be running and unblocked to make your site work.

• Any site you pull <script> from can completely control the user's experience on your site. If Google were feeling evil they could put something in their copy of jQuery to log your keypresses, steal personal information from the page you're on to tie into their web tracking database, make you post “I love Google!” comments to every form, and so on.

Google probably aren't actually going to do that, but it's a factor that's out of your control, and certainly something to worry about with other script-hosting services. There have been incidents before where stats scripts have been compromised with malware loaders.

Before including any script from a third party—even on one single page of your site—you must 100% trust them with all user-accessible functionality visible on that hostname (including web-facing admin functions).

The main benefit of having them on a CDN is that the files can be downloaded in parallel to files downloaded from your own website. This reduces latency on every page. So, the flip side of this is a pitfall of hosting locally - increased latency. The main reason for that is that browsers are limited in the number of connections that they can make at the same time to the same web domain. In IE6 this was defaulted to 2 concurrent connections to the same domain - shared between all open windows of IE!! In IE8+ it improved, defaulting to 6, which is inline with FF/Chrome, but still, if you have a lot of images and you are not using sprites, you will experience heavy latency.

Using a CDN, I would always set the library version explicitly rather than getting the latest one. This reduces the risk of new versions breaking your code. Not very likely with jQuery, but possible.

The other main benefit of using a CDN is reduced traffic on your site. If you pay per GB or you are on a virtual server with limited resources, you might find that overall site performance increases and hosting costs come down when you farm off some of your content to a public CDN.

Make sure you also read the other answer to this question by @Xaver. This is a very good trick