How to install JavaScript runtime in Rails

*** Exception PhusionPassenger::UnknownError in PhusionPassenger::Rack::ApplicationSpawner (Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable))

Solution: Install Node.js
install on RHEL:

wget http://nodejs.tchol.org/repocfg/el/nodejs-stable-release.noarch.rpm
yum localinstall --nogpgcheck nodejs-stable-release.noarch.rpm
yum install nodejs-compat-symlinks npm

Its also possible to add gem 'therubyracer' to the projects Gemfile and bundle install it.