Rails server does not start?

when i give the command "rails s" following error comes please help me! still i'm in rails project and then give this command

root@ubuntu:~# cd mon
root@ubuntu:~/mon# rails s
/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-1.1.3/lib/bundler/runtime.rb:68:in `require'
    from /usr/lib/ruby/gems/1.8/gems/bundler-1.1.3/lib/bundler/runtime.rb:68:in `require'
    from /usr/lib/ruby/gems/1.8/gems/bundler-1.1.3/lib/bundler/runtime.rb:66:in `each'
    from /usr/lib/ruby/gems/1.8/gems/bundler-1.1.3/lib/bundler/runtime.rb:66:in `require'
    from /usr/lib/ruby/gems/1.8/gems/bundler-1.1.3/lib/bundler/runtime.rb:55:in `each'
    from /usr/lib/ruby/gems/1.8/gems/bundler-1.1.3/lib/bundler/runtime.rb:55:in `require'
    from /usr/lib/ruby/gems/1.8/gems/bundler-1.1.3/lib/bundler.rb:119:in `require'
    from /root/mon/config/application.rb:7
    from /usr/lib/ruby/gems/1.8/gems/railties-3.2.3/lib/rails/commands.rb:53:in `require'
    from /usr/lib/ruby/gems/1.8/gems/railties-3.2.3/lib/rails/commands.rb:53
    from /usr/lib/ruby/gems/1.8/gems/railties-3.2.3/lib/rails/commands.rb:50:in `tap'
    from /usr/lib/ruby/gems/1.8/gems/railties-3.2.3/lib/rails/commands.rb:50
    from script/rails:6:in `require'
    from script/rails:6

Solution

add

gem 'therubyracer'

to your Gemfile and run

bundle

command The 'rails new' command creates a new Rails application with a default directory structure and configuration at the path you specify.

You can specify extra command-line arguments to be used every time
'rails new' runs in the .railsrc configuration file in your home directory,
or in $XDG_CONFIG_HOME/rails/railsrc if XDG_CONFIG_HOME is set.

Note that the arguments specified in the .railsrc file don't affect the
defaults values shown above in this help message.