Keep your javascripts in app/javascripts. Read and serve in development and package before deployment

development.rb

# In config/environments/development.rb

config.middleware.use 'DevelopmentJavascript'

development_javascript.rb

# lib/development_javascript.rb

class DevelopmentJavascript
  def initialize(app)
    @app = app
  end

  def call(env)
    path = Rack::Utils.unescape(env['REQUEST_PATH'])

    return @app.call(env) unless path.match(%r{/app/javascripts/(.*\.js)})

    javascript_file = ::File.expand_path("../../app/javascripts/#{$1}", __FILE__)

    if ::File.exist?(javascript_file)
      size = ::File.size(javascript_file).to_s
      content = ::File.new(javascript_file, 'r').read
      headers = {'Content-Type' => 'text/javascript', 'Content-Length' => size}
      [200, headers, [content]]
    else
      @app.call(env)
    end
  end
end