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

May I suggest a small improvement over @alexspeller suggestion.

class DriverJSError < StandardError; end

AfterStep do |scenario, step|
  errors = page.driver.browser.manage.logs.get(:browser)
    .select {|e| e.level == "SEVERE" && message.present? }
    .map(&:message)
    .to_a

  if errors.present?
    raise DriverJSError, errors.join("\n\n")
  end
end

Main difference here is that it only collects errors (in practice my instance of browser throws quite a lot of warnings, but errors are marked as "SEVERE" in Selenium) and also filters out entries with empty messages (I encountered entries with empty message, though not sure if there are any for "SEVERE" entries).

Not a geek, but would love to understand above lines.

.select {|e| e.level == "SEVERE" && message.present? }
.map(&:message)

To learn above lines, which instance of message variable are we referring to? should it be written as e.message.present?

@jacob-s-son ?

@anandchavan you're correct, it should be e.message.present?.

@jacob-s-son the to_a is not necessary