After starting the Rails server in a freshly generated Rails 3.1 project you could see an error message such as

/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)

Rails professionals since 2007

Our laser focus on a single technology has made us a leader in this space. Need help?

• We build a solid first version of your product
• We train your development team
• We rescue your project in trouble

Read more Show archive.org snapshot

Just add a JavaScript runtime to your Gemfile and the error vanishes.

Examples:

gem 'therubyracer'
gem 'extjs'