jRails - jQuery on Rails

For me it's all down to philosophy. Prototype adds a bunch of global variables and extends loads of built-in ojbects. That's fine, unless you ever want to include code that has a different idea about how any of those extensionss should work (like mooTools, which I also avoid on philosophical grounds). jQuery adds just one global variable (called jQuery) along with a revertible alias (the $ symbol). That pretty much guarantees jQuery will work flawlessly with any other code.

The base jQuery is very light, perhaps half the size of Prototype if I recall correctly.

The added bloat comes from the effects and UI libraries. The UI library is probably superfluous so that should cut the size significantly.

I haven't seen where Rails natively supports using different Javascript files in production than in development -- I've always done it myself. Then again, jQuery hasn't been attached to Rails before, so that's understandable too.

Without that being set up, either you run on the minified version of jQuery (a developer-hostile move), or the big old version with all the comments, which is inconvenient for the user. For a 0.1 release, they made the right call putting in the full versions of the Javascript so you have a chance of seeing what's happening if something goes wrong.