app-controllers-sessions_controller.rb

class SessionsController < Devise::SessionsController
  def create
    if request.xhr?
      resource = warden.authenticate! :scope => resource_name

      if resource.nil?
        render :json => {
          :status => 'fail',
          :data   => {
          :cause    => 'invalid',
          :message  => I18n.t('devise.failure.invalid'),
        },
        }
      else
        sign_in resource_name, resource

        render :json => {
          :status => 'success',
          :data   => {:message => I18n.t('devise.sessions.signed_in')},
        }
      end
    else
      super
    end
  end
end

app-models-session_failure.rb

class SessionFailure < Devise::FailureApp
  def respond
    message = I18n.t 'devise.failure.invalid'
    cause   = 'invalid'

    if warden_options[:message] == :locked
      message = I18n.t 'devise.failure.locked'
      cause   = 'account_locked'
    end

    if request.xhr?
      self.status         = 200
      self.content_type   = 'json'
      self.response_body  = {
        :status => 'fail',
        :data   => { :message  => message, :cause => cause },
      }.to_json
    else
      super
    end
  end
end

config-initializers-devise.rb

Devise.setup do |config|
  config.http_authenticatable_on_xhr = false

  config.navigational_formats = [:html]

  config.warden do |manager|
    manager.failure_app = SessionFailure
  end
end