Could not find a JavaScript runtime.

Rails3 Error 

$ APP/rails generate scaffold hoge name:string
Error
/usr/lib/ruby/gems/1.8/gems/execjs-1.3.0/lib/execjs/runtimes.rb:50:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)

	from /usr/lib/ruby/gems/1.8/gems/execjs-1.3.0/lib/execjs.rb:5

	from /usr/lib/ruby/gems/1.8/gems/coffee-script-2.2.0/lib/coffee_script.rb:1:in `require'

	from /usr/lib/ruby/gems/1.8/gems/coffee-script-2.2.0/lib/coffee_script.rb:1

	from /usr/lib/ruby/gems/1.8/gems/coffee-script-2.2.0/lib/coffee-script.rb:1:in `require'

	from /usr/lib/ruby/gems/1.8/gems/coffee-script-2.2.0/lib/coffee-script.rb:1

	from /usr/lib/ruby/gems/1.8/gems/coffee-rails-3.2.2/lib/coffee-rails.rb:1:in `require'

	from /usr/lib/ruby/gems/1.8/gems/coffee-rails-3.2.2/lib/coffee-rails.rb:1

	from /usr/lib/ruby/gems/1.8/gems/bundler-1.0.0/lib/bundler/runtime.rb:64:in `require'

	from /usr/lib/ruby/gems/1.8/gems/bundler-1.0.0/lib/bundler/runtime.rb:64:in `require'

	from /usr/lib/ruby/gems/1.8/gems/bundler-1.0.0/lib/bundler/runtime.rb:62:in `each'

	from /usr/lib/ruby/gems/1.8/gems/bundler-1.0.0/lib/bundler/runtime.rb:62:in `require'

	from /usr/lib/ruby/gems/1.8/gems/bundler-1.0.0/lib/bundler/runtime.rb:51:in `each'

	from /usr/lib/ruby/gems/1.8/gems/bundler-1.0.0/lib/bundler/runtime.rb:51:in `require'

	from /usr/lib/ruby/gems/1.8/gems/bundler-1.0.0/lib/bundler.rb:112:in `require'

	from /home/kamata/www/rails3/config/application.rb:7

	from /usr/lib/ruby/gems/1.8/gems/railties-3.2.3/lib/rails/commands.rb:24:in `require'

	from /usr/lib/ruby/gems/1.8/gems/railties-3.2.3/lib/rails/commands.rb:24

	from script/rails:6:in `require'

	from script/rails:6

solution
Edit Gemfile

gem 'execjs'

gem 'therubyracer'

run bundle install

$ APP/bundle install