Capture javascript errors in Cucumber+Capybara+Webdriver tests

gistfile1.rb

# in features/support/env.rb
require 'selenium/webdriver'

# we need a firefox extension to start intercepting javascript errors before the page
# scripts load
Capybara.register_driver :selenium do |app|
  profile = Selenium::WebDriver::Firefox::Profile.new
  # see https://github.com/mguillem/JSErrorCollector
  profile.add_extension File.join(Rails.root, "features/support/extensions/JSErrorCollector.xpi")
  Capybara::Selenium::Driver.new app, :profile => profile
end

After do |scenario|
  if page.driver.to_s.match("Selenium")
    errors = page.execute_script("return window.JSErrorCollector_errors.pump()")

    if errors.any?
      puts '-------------------------------------------------------------'
      puts "Found #{errors.length} javascript #{pluralize(errors.length, 'error')}"
      puts '-------------------------------------------------------------'
      errors.each do |error|
        puts "    #{error["errorMessage"]} (#{error["sourceName"]}:#{error["lineNumber"]})"
      end
      raise "Javascript #{pluralize(errors.length, 'error')} detected, see above"
    end

  end
end