JSFiddle - React, Tailwind, and code Playground

HTML

<input id="hidden1" type="hidden" value="5" class="includeds">
<h3>header</h3>
<ul class="groups">
    <li><input id="li1" type="checkbox" value="1"></li>
    <li><input id="li2" type="checkbox" value="2"></li>
    <li><input id="li3" type="checkbox" value="3"></li>
</ul>
<input id="hidden2" type="hidden" value="3" class="includeds">
<h3>header2</h3>
<ul class="groups">
    <li><input id="li4" type="checkbox" value="4"></li>
    <li><input id="li5" type="checkbox" value="5"></li>
    <li><input id="li6" type="checkbox" value="6"></li>
</ul>

JavaScript

$('input').click(function(){
    var value = $(this).closest('ul').prevAll('.includeds:first').val();
    alert(value);
});