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'>d</div></td></tr>
          <tr id='res2'><td class='fc-id5 ui-widget-content fc-resource2 '><div class=day-content'>d</div></td></tr>
          <tr id='res3'><td class='fc-id6 ui-widget-content fc-resource3 '><div class=day-content'>d</div></td></tr>
      </tbody>
  </table>

JavaScript

var id = 3
    var td = $('.fc-resource'+ id); // this id will be the unique id coming from ajax.
    var th = td.closest('tbody').prev('thead').find('> tr > th:eq(' + td.closest('tr').index() + ')').html();
    alert(th);