Lowest Hiding Element

You're given an array of DOM element nodes, targets. Find the single deepest element that is an ancestor of all of them — their lowest common ancestor. Setting that one element to display: none would hide every target at once, and no element below it could do the same. Think of it as: what is the smallest box you can collapse that still swallows every item on the list?

Signature

function lowestHidingElement(targets: Element[]): Element | null;

targets is an array of element nodes that already live in a document. The return value is one element from those elements' shared ancestry (it may be one of the targets itself), or null when no single element contains them all.

Examples

// <ul id="list"><li id="a">a</li><li id="b">b</li></ul>
lowestHidingElement([a, b]);
// → <ul id="list">   (the shared parent — hiding it hides both li's)

// <div id="outer"><span id="inner">x</span></div>
lowestHidingElement([outer, inner]);
// → <div id="outer">  (outer is an ancestor of inner, so it's the answer)

lowestHidingElement([inner]);
// → <span id="inner"> (a lone target hides itself)

// two elements in completely separate trees
lowestHidingElement([nodeInTreeA, nodeInTreeB]);
// → null              (no element contains both)

Notes

• Deepest, not just any ancestor. Many elements are ancestors of all the targets (the body, the html element). You want the lowest one — the one furthest from the root.
• A single target hides itself. lowestHidingElement([el]) returns el. Hiding el hides el.
• Targets can be ancestors of each other. If one target contains another, the containing target is the answer — it's already an ancestor of everything below it.
• All targets count. The answer must account for every element in the array, not just the first two. A later, far-flung target can pull the answer higher up the tree.
• Disjoint trees return null. If the elements don't all share at least one common ancestor, there is no single hiding element. An empty array also returns null.
• Order doesn't matter. [a, b, c] and [c, a, b] produce the same result.