Similar to jQuery .closest() but traversing descendants?

Question

If by “closest” descendant you mean the first child then you can do:

$('#foo').find(':first');

Or:

$('#foo').children().first();

Or, to look for the first occurrence of a specific element, you could do:

$('#foo').find('.whatever').first();

Or:

$('#foo').find('.whatever:first');

Really though, we need a solid definition of what “closest descendant” means.

E.g.

<div id="foo">
    <p>
        <span></span>
    </p>
    <span></span>
</div>

Which <span> would $('#foo').closestDescendent('span') return?