Opal fun - Opal::Server version: config.ru that made the current version of opal work for me. Put your files (whatever.js.opal or whatever.js.rb) in the assets directory. Change s.main to the name of the appropriate js.opal file. rackup. Enjoy. Some parts may be redundant. Investigating. Non-server version: convert.rb

config.ru

require 'opal'

run Opal::Server.new { |s|
 s.main = 'doi'

 e = s.sprockets
 e.append_path 'assets'
}

convert.rb

#!/usr/bin/ruby
require 'opal'
require 'opal-jquery'
require 'sprockets'

e = Sprockets::Environment.new
e.append_path 'src'
e.register_engine '.opal', Opal::Processor
e.register_engine '.rb', Opal::Processor
Opal.paths.each{|p| e.append_path p}
# parse and emit src/doi.js.rb
puts e['doi.js'].to_s

doi.js.rb

//= require jquery.min.js
require 'opal'
require 'opal-jquery'

Document.ready? {
  Element["body"].append [1,2,3,4,5].map{|i| i*2}.reduce(0){|m,v| m+v}
}

assets is probably a poor choice of name for the directory, since it has so much rails baggage.