rails 5, remote: true form empty data

Question

I am trying to migrate Rails 4.2 application to Rails 5.1. In rails 4.2 we heavily used JQuery. At the moment I am struggling to make the forms, with remote: true attribute work properly. As an example this is a simple form where user select the country

= simple_form_for(:user_data,
  url: user_path(@user),
  remote: true,
  method: :patch,
  data: {'user-update' => true},
  dataType: 'json',
  html: wrapper: :horizontal_form ) do |f|
  .panel.panel-default
    .panel-body
      .col-md-8
        = f.input :country, label: 'Country',
      = f.button :submit  

The controller

def update
  @user.update!(user_params)
  render json: @user
end

I have tried to add the respond_to with js response format, but in that case it tries to convert the @user to executable javascript. As of the event handler looks like this

  $('form[data-user-update]')
  .on('ajax:success', function(e, data, status, xhr) {
     // The data variable is empty
  })
  .on('ajax:error', function(e, error, status, xhr) {
    $('.simple_form').renderFormErrors('user_data', error.responseJSON);
  });

EDIT

Response Header

HTTP/1.1 200 OK
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block
X-Content-Type-Options: nosniff
Content-Type: application/json; charset=utf-8
ETag: W/"f53889092c58dc37054386c9504ad1ff"
Cache-Control: max-age=0, private, must-revalidate
X-Request-Id: 712b59ff-8011-4b10-a905-83e559b47452
X-Runtime: 0.101165
Transfer-Encoding: chunked

Request Header

Accept:text/javascript, application/javascript, application/ecmascript, application/x-ecmascript, */*; q=0.01
Accept-Encoding:gzip, deflate, br
Accept-Language:en-US,en;q=0.8,et;q=0.6
Connection:keep-alive
Content-Length:71
Content-Type:application/x-www-form-urlencoded; charset=UTF-8
Host:localhost:3000
Origin:http://localhost:3000
Referer:http://localhost:3000/users/470d573b-b0f1-4822-b036-7d37be6672d6
X-Requested-With:XMLHttpRequest

Response

{ 
  firstName:'John',
  lastName: 'Smith',
  country: 'USA'
}

Answer 1

Just figured it out while running into the very same problem.

As per http://edgeguides.rubyonrails.org/working_with_javascript_in_rails.html#dealing-with-ajax-events

I assume you're using rails-ujs instead of older jquery-ujs. In jquery-ujs the code would work and data, status, xhr would be returned; rails-ujs however only returns one attribute - event and additional things are accessed through array on event.details

As per example in the guide

document.body.addEventListener('ajax:success', function(event) {
  var detail = event.detail;
  var data = detail[0], status = detail[1],  xhr = detail[2];
})

You can access response by calling event.detail[0]

If you've got quite a bit of code relying on this functionality, probably a better bet is to change rails-ujs back to jquery-ujs