UJS drivers (rails.js / jquery-ujs.js) -- load them LAST.

Don't load your UJS driver (rails.js or jquery-ujs.js) before your application logic JavaScript.

Why? Because your UJS driver is going to hook into the submission events for ajax elements. If you need to run a validation (or anything like it) that has to be executed BEFORE form submission occurs, because it may cancel the submission event, you need to have that code hook into the submission events before your UJS driver does.

Therefore, load your UJS driver last.