JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <div id="thisOne">
        <div>
            <div>
                <div>
                    <div id="a">
                        first element
                    </div>
                </div>
            </div>
        </div>
        <div>
            <div id="b">
                second element
            </div>
        </div>
    </div>
</div>

JavaScript

var a = $('#a'),
        b = $('#b'),
        closestCommonAncestor = a.parents().has(b).first();

alert(closestCommonAncestor.prop('id'));