I'm not sure if this is baked into Rails 5, but you can get es6 support (via Babel) with the sprockets-es6 gem:
https://github.com/TannerRogalsky/sprockets-es6
Rails is still a second-class JS citizen due to the asset pipeline being opinionated and controlling compared to the node way of doing things, but it gets you closer to 'the new hotness'.