This Rails 5 application shows the bare minimum requirements to get ES6 compilation running under the Rails asset pipeline as described in my blog post Modern Javascript and Rails.
To run this example yourself:
bundle install
npm install
rails s
Visit localhost:3000/teaspoon and click on default
If all is well, you'll get a green test suite. You can inspect the resulting hello_world.js to see that it was properly transpiled into ES5.