JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css">
<p>Clicking open will open the modal and pass the first textarea contents to the second textarea within the modal.</p>
<p><a href="#my_modal" data-toggle="modal">Open</a></p>

<div class="modal" id="my_modal">
      <div class="modal-body">
        <textarea class="second"></textarea>
      </div>
</div>

<p><textarea class="first">First Textarea</textarea></p>

JavaScript

$('#my_modal').on('show.bs.modal', function() {
    var first = $(".first").val();
    $(".second").val(first);
});