How do I use ES6 (ES2015) in a ruby on rails app?

Answering for myself 3 years later...

Rails > 5.1 has webpacker and native support for webpack builds and various front end frameworks if desired (react, vue, etc)

Replace this in your production.rb file

config.assets.js_compressor = :uglifier

with this

config.assets.js_compressor = Uglifier.new(harmony: true)

For dev, Chrome compiles es6.

https://github.com/lautis/uglifier

https://www.rubydoc.info/gems/uglifier

At present unfortunately there isn't really a 'standard' way of doing this - a lot of it depends on the requirements of your app and if you are able to upgrade Sprockets and/or Rails.

Option 1: Stay on Rails 4.2 and Sprockets 3, then use this gem to add ES6 support and gradually migrate to ES6 modules: https://github.com/rmacklin/sprockets-bumble_d

Option 2: Upgrade to Sprockets 4 (still in beta), then use either https://github.com/fnando/babel-schmooze-sprockets or https://github.com/babel/ruby-babel-transpiler to add babel for ES6 support. Both have pretty solid documentation.

Option 3: Use webpack either instead of or alongside sprockets. A google search will reveal some approaches for this. Rails 5.1 (still in beta) will introduce native webpack (and thus babel) support via the webpacker gem. Admittedly this may be the more difficult option for an existing application. There is a good article about it here: https://medium.com/statuscode/introducing-webpacker-7136d66cddfb#.cb4sixyah