Boostrap 4 modal in modal

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css">
<script src="https://code.jquery.com/jquery-3.1.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/tether/1.4.0/js/tether.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/js/bootstrap.min.js"></script>
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#modal-a">
  Launch demo modal a
</button>

<div class="modal fade" id="modal-a" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <h5 class="modal-title" id="exampleModalLabel">Modal title</h5>
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
      </div>
      <div class="modal-body">
        <button type="button" class="btn btn-primary" data-toggle="modal" data-target="#modal-b">
  Launch another demo modal b
</button>
<p class="my-3">
Not good for fade In.
</p>
<p class="my-3">
who help to improve?
</p>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

<div class="modal fade" id="modal-b" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <h5 class="modal-title" id="exampleModalLabel">Modal title</h5>
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
      </div>
      <div class="modal-body">
        <button...

JavaScript

/*
 * Boostrap 4 modal in modal
 * By Roy Ng
 */
 
var modal_lv = 0;
$('.modal').on('shown.bs.modal', function (e) {
    $('.modal-backdrop:last').css('zIndex',1051+modal_lv);
    $(e.currentTarget).css('zIndex',1052+modal_lv);
    modal_lv++
});

$('.modal').on('hidden.bs.modal', function (e) {
    modal_lv--
});