JSFiddle - React, Tailwind, and code Playground

HTML

See your console for the result.<hr>

<div class="far ancestor">
    <div class="near ancestor">
        <p id="p">Where am I?</p>
    </div>
</div>

JavaScript

function findAncestor (el, cls) {
    while ((el = el.parentElement) && !el.classList.contains(cls));
    return el;
}

console.log(
    findAncestor(
        document.getElementById('p'), 'ancestor'));