Fan-out promise resolution

join-promises.js

function join(promises) {
	var promise = new Promise();
	promise.then(
		function(val) {
			for (var i = 0; i < promises.length; i++) {
				promises[i].resolve(val);
			}
		},
		function(err) {
			for (var i = 0; i < promises.length; i++) {
				promises[i].reject(err);
			}
		},
		function(update) {
			for (var i = 0; i < promises.length; i++) {
				promises[i].progress(update);
			}
		}
	);

	return promise;
}