$ rails  server

.rvm/gems/ruby-1.9.3-head/gems/execjs-1.2.4/lib/execjs/runtimes.rb:45:in `autodetect’: Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)

—————————————

해결책: Gemfile 에  gem ‘therubyracer’ 를 추가하고
‘ bundle install’  명령을 실행

$ vi Gemfile
 gem ‘therubyracer’

$ bundle install
…..
Installing therubyracer (0.9.3) with native extensions