My recommendation is to use the browserify-rails gem. For ES6, you would use a transform
such as this one: https://github.com/thlorenz/es6ify
Update some months later: I still recommend the same path but if you can get off of the asset pipeline, I highly recommend going to webpack or browserify directly. The asset pipeline has some benefits but in my experience, trying to make JavaScript a first-class citizen is painful with it and the benefits outweigh the drawbacks (for me).