Sinatra create action with ajax and corresponding JS

horses_controller.rb

# Split a horses create action into separate methods for success and failure.

def post_horses_success
  if request.xhr?
    content_type :json
    as_html = erb :'/horses/_horse_list_item', layout: false, locals: { horse: @horse }
    { horse: @horse, as_html: as_html }.to_json
  else
    redirect "/horses/#{@horse.id}"
  end
end

def post_horses_failure
  status 422
  @errors = @horse.errors.full_messages
  if request.xhr?
    erb :'/horses/_horse_form_errors', layout: false
  else
    erb :"/horses/new"
  end
end

post '/horses' do
  @horse = Horse.new(params[:horse])
  if @horse.save
    post_horses_success
  else
    post_horses_failure
  end
end

horses_create.js

/*
* START - horses create action
*/
var createHorseFormSuccessHandler = function(response){
  $('ul.list').append(response.as_html)
}

var createHorseFormFailHandler = function(response, status, error, $form){
  $(response.responseText).insertBefore($form)
}

var createHorseFormAlwaysHandler = function($form){
  $form.find('button').prop('disabled', false)
}

var createHorseFormSubmitHandler = function(e){
  e.preventDefault()
  console.log('createHorseFormSubmitHandler invoked')
  var $form = $(this)
  var method = $form.attr('method')
  var url = $form.attr('action')
  var data = $form.serialize()

  $('section.errors').remove()
  $form.find('button').prop('disabled', true);

  var $ajax = $.ajax({
    method: method,
    url: url,
    data: data,
    dataType: "JSON"
  })

  $ajax.done(createHorseFormSuccessHandler)
  $ajax.fail(function(response, status, error){
    createHorseFormFailHandler(response, status, error, $form)
  })
  $ajax.always(function(){
    createHorseFormAlwaysHandler($form)
  })
}
/*
* END - horses create action
*/