Wraps the TreeWalker interface into a generator function, for simpler and more idiomatic usage.

treeIterator.js

/*
SYNTAX:
	Generator treeIterator(Node root, int whatToShow, bool function filter(Node))
	- root: The parent node. It is not returned.
	- whatToShow: Bitmask of the NodeFilter.SHOW_* static constants. Represents the types of nodes included.
	- filter: Predicate returning whether the node given as the first argument should be included in the iteration.
		Instead of a boolean, it may return a NodeFilter.FILTER_* static constant.
		A return value of `true` is implicitly converted to `NodeFilter.FILTER_ACCEPT`,
		and `false` becomes `NodeFilter.FILTER_REJECT`
	
EXAMPLE: Print the text of every non-whitespace text node in the page
	for (let node of treeIterator(document.body, NodeFilter.SHOW_TEXT, node => /\S/.test(node.nodeValue)))
		console.log(node.nodeValue);
*/

function* treeIterator(root, whatToShow, filter) {
	let treeWalker;
	if (typeof filter === 'function') {
		treeWalker = document.createTreeWalker(root, whatToShow, {
			acceptNode: node => {
				let ret = filter(node);
				if (ret === true)
					return NodeFilter.FILTER_ACCEPT;
				else if (ret === false)
					return NodeFilter.FILTER_REJECT;
				else
					return ret;
			}
		});
	} else {
		treeWalker = document.createTreeWalker(root, whatToShow);
	}
	
	while (treeWalker.nextNode())
		yield treeWalker.currentNode;
}