JSFiddle - React, Tailwind, and code Playground

HTML

<ul id='sortable'>
   <li data-itemid='1'>One</li>
   <li data-itemid='2'>Two</li>
   <li data-itemid='1000'>One thousand</li>
   <li data-itemid='3'>Three</li>
   <li data-itemid='4'>Four</li>
</ul>

JavaScript

var max= Math.max.apply([], $('li[data-itemid]').map(function() {
  return $(this).data('itemid');
}).get());

console.log(max); //1000