Find the closest ancestor element that has a specific class

How can I find an element’s ancestor that is closest up the tree that has a particular class, in pure JavaScript? For example, in a tree like so:

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

Then I want div.near.ancestor if I try this on the p and search for ancestor.