Custom failure app for devise responding to JS

devise.rb

# config/initializers/devise.rb
class CustomFailureApp < Devise::FailureApp
  def http_auth_body
    case request_format
    when :js, 'js'
      'alert(" not authorized !");'
    when :json, 'json'
      { error: i18n_message }
    else
      i18n_message
    end
  end
end

Devise.setup do |config|
  # ...
  config.warden do |manager|
    manager.failure_app = CustomFailureApp
  end
  # ...
end