If, like me, you find the idea of needing two commands to run your Ruby & Javascript test suites objectionable; try adding this to your Rakefile:

namespace :test do
  desc 'run NPM tests'
  Rake::TestTask.new(:npm_test) do |t|
    puts `npm test`
  end
end

Rake::Task[:test].enhance { Rake::Task["test:npm_test"].invoke }

This snippet extends the standard rake test task to include npm test.