'ExecJS::RuntimeUnavailable' error after creating new Rails4 application

Create new rails4 application, run command: rails new interview -d mysql
It will automatically run bundle install.
Browse to interview folder and try to start rails server by command, 'rails s'.
It will give error:

ritesh@ritesh-ThinkPad-L420:~/projects/interview$ rails s
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/execjs-2.0.1/lib/execjs/runtimes.rb:51:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/execjs-2.0.1/lib/execjs.rb:5:in `<module:ExecJS>'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/execjs-2.0.1/lib/execjs.rb:4:in `<top (required)>'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/uglifier-2.2.1/lib/uglifier.rb:3:in `require'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/uglifier-2.2.1/lib/uglifier.rb:3:in `<top (required)>'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `require'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `block (2 levels) in require'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in `each'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in `block in require'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in `each'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in `require'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler.rb:132:in `require'
    from /home/ritesh/projects/interview/config/application.rb:7:in `<top (required)>'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/railties-4.0.0/lib/rails/commands.rb:76:in `require'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/railties-4.0.0/lib/rails/commands.rb:76:in `block in <top (required)>'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/railties-4.0.0/lib/rails/commands.rb:73:in `tap'
    from /home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/railties-4.0.0/lib/rails/commands.rb:73:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'

Solution:
To fix this issue, edit Gemfile and comment out the line:
        gem 'therubyracer', platforms: :ruby
After this run bundle install and you can start rails server.

Although I was able to fix it, but I am not sure why 'therubyracer' gem in Gemfile is commented ???

Where to start on Rails4 : Links, Resources, Tutorials, Tips, References

Tutorials:
http://net.tutsplus.com/tutorials/ruby/digging-into-rails-4/
http://edgeguides.rubyonrails.org/
http://ruby.railstutorial.org/ruby-on-rails-tutorial-book
https://blog.engineyard.com/2013/new-in-rails-4

Two most important links:
http://blog.wyeworks.com/2012/11/13/rails-4-compilation-links/
http://blog.remarkablelabs.com/2012/11/rails-4-countdown-to-2013


Videos:
http://railscasts.com/episodes/400-what-s-new-in-rails-4
http://railscasts.com/?tag_id=38&view=list

Upgrading to Rails4:
http://railscasts.com/episodes/415-upgrading-to-rails-4
http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html


Features:
Many features can be found on rails-3.1 features
Asset Pipeline : http://edgeguides.rubyonrails.org/asset_pipeline.html

Turbolinks : http://railscasts.com/episodes/390-turbolinks

Russian Doll Caching and Cache Digests : http://blog.remarkablelabs.com/2012/12/russian-doll-caching-cache-digests-rails-4-countdown-to-2013