alerts_and_confirms.rb

module AlertConfirmer
  class << self
    def reject_confirm_from &block
      handle_js_modal 'confirm', false, &block
    end

    def accept_confirm_from &block
      handle_js_modal 'confirm', true, &block
    end

    def accept_alert_from &block
      handle_js_modal 'alert', true, &block
    end

    def get_alert_text_from &block
      handle_js_modal 'alert', true, true, &block
      get_modal_text 'alert'
    end

    def get_modal_text(name)
      page.evaluate_script "window.#{name}Msg;"
    end

    private

    def handle_js_modal name, return_val, wait_for_call = false, &block
      modal_called = "window.#{name}.called"
      page.execute_script "
      window.original_#{name}_function = window.#{name};
      window.#{name} = function(msg) { window.#{name}Msg = msg; window.#{name}.called = true; return #{!!return_val}; };
      #{modal_called} = false;
      window.#{name}Msg = null;"

      block.call

      if wait_for_call
        timed_out = false
        timeout_after = Time.now + Capybara.default_wait_time
        loop do
          if page.evaluate_script(modal_called).nil?
            raise 'appears that page has changed since this method has been called, please assert on page before calling this'
          end
          
          break if page.evaluate_script(modal_called) || 
            (timed_out = Time.now > timeout_after)

          sleep 0.001
        end
        raise "#{name} should have been called" if timed_out
      end
    ensure
      page.execute_script "window.#{name} = window.original_#{name}_function"
    end
  end
end

example_usage.rb

AlertConfirmer.get_alert_text_from do
  click_buttton 'make an alert happen!'
end.should == 'totally alerted'