Build subset of DOM tree from list of unordered DOM elements

dom-subset.js

// See https://twitter.com/LeaVerou/status/1288721937288638465
function buildGraph(elements) {
	let elements = new Set(elements);
	const className = "almanac-2020"; // anything sufficiently unique
	
	let map = new Map(); // keep pointers to each element's object
	let ret = [];
	
	for (let element of elements) {
		// Create initial objects
		map.set(element, {element, children: []});
		
		// Add class to elements so we can find them with .closest()
		// If that's not an option, we can always loop on .parentNode and check if elements.has(el)
		element.classList.add(className);
	}
	
	// Recursively build tree
	for (let element of elements) {
		let ancestor = element.parentNode.closest?.("." + className);
		let obj = map.get(element);

		if (ancestor) {
			let o = map.get(ancestor);
			o.children.push(obj)
		}
		else {
			// Top-level
			ret.push(obj);
		}
	}
		
	// CLeanup
	elements.forEach(el => el.classList.remove(className));

	return ret;
}