JSFiddle - React, Tailwind, and code Playground

HTML

<table>
    <tr id="courseSection[0]" class="sectionRow"><td>stuff</td></tr>
    <tr id="courseSection[1]" class="sectionRow"><td>more stuff</td></tr>
</table>

JavaScript

var lastRowID = $(".sectionRow:last").attr('id');
document.write(lastRowID);