thanks @gf3

gistfile1.rb

namespace :js do  
  desc "Pack javascript src for production environment"
  task :pack => :environment do
    # Files to pack
    Dir.glob(File.join("public", "javascripts", "**", "*.js")).each do |filename|
      puts "packing #{filename}..."
      FileUtils.cp(filename, "#{filename}.orig")
      js = ""
      File.open(filename, "r") { |f| js = f.read }
      File.open(filename, "w") { |f| f.write(Packr.pack(js, :shrink_vars => true, :private => true)) }
    end
  end
end