One of the cons of using any of the good and popular third-party JS libraries is the file size of the .js files associated with them. A lot has been done by the library developers to address this issue - providing minified versions, segregating the script files based on what they do and using loading-on-demand, among others. There's more good news - Yahoo is now offering hosting of their YUI library files.
How is it good for you?
- less on the bandwidth bill - you don't need to worry about hosting these files yourself
- high availability - well Yahoo is behind this, so you can rest assured that these files are delivered to your surfer
- small downloads - Yahoo hosts the minified versions and gzips them
- files already cached!
The last one is the topic I had in mind with the this blog's title. Since Yahoo will be using the same locations for the libraries you need and since Yahoo is the most popular site, chances are your visitors have already checked their Y! mail or their Y! finance page and searched or done anything on the Yahoo network of sites. This means they have already requested and (hopefully) cached these .js files. And as proved before, lowering the number of HTTP requests is top 1 performance optimization you can ever do.
Even better is that you can include only libraries you need (less transfer) or you can include several in a batch (less requests).
If you only need Event and DOM, include only yahoo-dom-event.js (8K)
If you need only Event, DOM, AJAX, include yahoo-dom-event.js and connection-min.js (8K + 4K)
If you also need more like Drag and Drop and animation and AJAX, why not include all utility classes at once, utilities.js. After all this file is 22K, probably less than two images that will appear on your page.
Note: all filesizes above are when files are gzipped, which is how Yahoo serves them.
So bottom line, your visitors hit your site and, lo and behold, they have all the JS already cached, resulting in your page loading as fast as a rocket Sweet.