Capybara-webkit

example_spec.rb

require 'spec_helper'

describe "JS Example" do
  it "doesn't use javascript driver" do

  end

  it "uses javascript driver", :js => true do

  end
end

Gemfile

group :test, :development do
  gem 'capybara'
  gem 'capybara-webkit'
  gem 'database_cleaner'
end

spec_helper.rb

Capybara.default_driver = :selenium
Capybara.javascript_driver = :webkit

RSpec.configure do |config|
  config.use_transactional_fixtures = false

  config.before(:each) do
    if example.metadata[:js]
      DatabaseCleaner.strategy = :truncation
      DatabaseCleaner.clean_with(:truncation)
    else
      DatabaseCleaner.strategy = :transaction
      DatabaseCleaner.clean_with(:transaction)
    end

    DatabaseCleaner.start
  end

  config.after(:each) do
    DatabaseCleaner.clean       # Truncate the database
    Capybara.reset_sessions!    # Forget the (simulated) browser state
    Capybara.use_default_driver # Revert Capybara.current_driver to Capybara.default_driver
  end
end

just FYI, also need this atop your spec_helper.rb

require 'capybara/rails'