• We all know Rails 3.1 is rocking with jQuery, CoffeeScript by default.
• The rake utility won’t work if you create application with rails 3.1 error would be following type….

rake aborted!
Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes.

Solution
 

Step-1 To declare therubyracer gem in Gemfile of your application which is in application’s  root directory. 

gem ‘therubyracer’

Step-2 Then run bundle install command to install therubyracer gem.

$ bundle install
 

• Now above error will disapper.

Note:- If you create application with rails 3.1 with protopye javascript then therubyracer gem doesn’t require, but rails 3.1 uses CoffeeScript,  jQuery by default. A JavaScript runtime is needed for Linux Ubuntu. It is not needed for Mac OS X or Windows.