javascript alert box rspec matcher for selenium

matchers.rb

Spec::Matchers.define :be_displaying_alert do
  match do |actual|
    class_name = begin
      actual.driver.browser.switch_to.alert.class
    rescue => exception
      exception.class
    end
    class_name == Selenium::WebDriver::Alert
  end

  failure_message_for_should do |actual|
    "Alert box not displayed."
  end

  failure_message_for_should_not do |actual|
    "Alert box displayed.  Don't wanna see that."
  end
end