JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/ui-lightness/jquery-ui.css">
<div id="outer">
    <div id="middle">
        <div id="inner"></div>
    </div>
</div>

CSS

div {
    padding: 10px;
    border: 2px solid blue;
}

JavaScript

var outer = document.getElementById('outer');
var middle = document.getElementById('middle');
var inner = document.getElementById('inner');

document.write('<br><br>middle is child of outer? ' + isChild(outer, middle));


document.write('<br><br>inner is child of outer? ' + isChild(outer, inner));

function isChild(node1, node2) {
    return node2 && node2.parentNode === node1;
}