How to center a bootstrap modal before showing?

Question

I'm working with a bootstrap modal and want to center it (width and height based on window width/height).

I've tried this:

(function(global) {
  let W = $(global).width(),
    H = $(global).height();

  // console.log($('.modal .modal-content').width()); // -2
  // console.log($('.modal .modal-content').height()); // -2

  $('.modal').on('shown.bs.modal', function() {
    let $content = $(this).find('.modal-content');

    let w = $content.width(),
      h = $content.height();

    $content.css({
      'margin-left': (W / 2) - (w / 2),
      'margin-top': (H / 2) - (h / 2)
    });
  });
}(window));

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>

<button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
  Launch demo modal
</button>

<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">Modal title</h4>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

It works. But the problem (for me) is: the code doesn't center the modal before showing. I want to center it first. I've also tried with show.bs.modal but it makes something wrong.

Then, I'm trying to get the content width and height before loading:

console.log($('.modal .modal-content').width()); // -2
console.log($('.modal .modal-content').height()); // -2

The modal has been appended to the body tag successful, but I'm not still getting the width and height.

Where can I edit? Thank you!