JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
<li class="square" data-selected data-square="1">1</li>
<li class="square" data-square="2">2</li>
<li class="square" data-selected data-square="3">3</li>
</ul>

JavaScript

$('.square[data-selected]').attr('data-selected',function(){
  return $(this).attr('data-square');
});