Rails : ExecJS::RuntimeUnavailable

# rails s
/home/ericbbs/.rvm/gems/ree-1.8.7-2012.02/gems/execjs-1.3.1/lib/execjs/runtimes.
rb:50:in `autodetect': Could not find a JavaScript runtime. See https://github.c
om/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavaila
ble) 

發生(ExecJS::RuntimeUnavailable) 錯誤 

修改Gemfile 並加入以下二行

  gem 'execjs'
  gem 'therubyracer'

重新bundle install