devise.md

#Devise gem

Devise failture app responding to JS reqest

if you use RJS (js format Rails calls) you may find that Devise responds with non JS response when 401 - not authenticated

# 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

https://gist.github.com/equivalent/385bdc3b788b484ade0bdf9cc23ad584