Promise Tree traversal

promise-tree.js

function when(value, func) {
	var promise = Promise.cast(value);

	if (func) {
		promise = promise.then(function (value) {
			return Promise.cast(func(value)).then(function (newValue) {
				return newValue === undefined ? value : newValue;
			});
		});
	}
	
	return promise;
}

function traverse(node, handlers) {
	if (handlers instanceof Function) {
		handlers = {leave: handlers};
	}

	return when(node, handlers.enter).then(function (node) {
		if (typeof node !== 'object' || node === null || node === traverse.skip) {
			return node;
		}

		var promises = Object.keys(node).map(function (key) {
			var subNode = node[key];

			return traverse(subNode, handlers).then(function (newSubNode) {
				if (newSubNode !== traverse.skip) {
					if (newSubNode !== subNode) {
						node[key] = newSubNode;
					}
				} else {
					delete node[key];
				}
			});
		});

		return Promise.all(promises).then(function () {
			return node;
		});
	}).then(function (node) {
		return when(node, handlers.leave);
	});
}

traverse.skip = {};