JSFiddle - React, Tailwind, and code Playground

HTML

<table>
      <thead>
         <tr><th class='fc-id1 ui-widget-header'>09:00</th></tr>
         <tr><th class='fc-id2 ui-widget-header'>09:30</th></tr>
         <tr><th class='fc-id3 ui-widget-header'>10:00</th></tr>
      </thead>
      <tbody>
          <tr id='res1'><td class='fc-id4 ui-widget-content fc-resource1 '><div class='day-content'></div></td></tr>
          <tr id='res2'><td class='fc-id5 ui-widget-content fc-resource2 '><div class='day-content'></div></td></tr>
          <tr id='res3'><td class='fc-id6 ui-widget-content fc-resource3 '><div class='day-content'></div></td></tr>
      </tbody>
  </table>

JavaScript

var id = 2;
var td = $('.fc-resource' + id);
var tr = td.closest('tr');
var index = tr.index();
var th = td.closest('tbody').prev('thead').find('> tr > th:eq(' + index + ')').html();
alert(td);