index.js

var nextTick = function (fn) {
	if (typeof setImmediate === 'function')
		setImmediate(fn);
	else if (typeof process !== 'undefined' && process.nextTick)
		process.nextTick(fn);
	else
		setTimeout(fn, 0);
};

var makeIterator = function (tasks) {
	var makeCallback = function (index) {
		var fn = function () {
			if (tasks.length)
				tasks[index].apply(null, arguments);

			return fn.next();
		};
		
		fn.next = function () {
			return (index < tasks.length - 1) ? makeCallback(index + 1): null;
		};
		return fn;
	};
	return makeCallback(0);
};

var _isArray = Array.isArray || function(maybeArray){
	return Object.prototype.toString.call(maybeArray) === '[object Array]';
};

var waterfall = function (tasks, callback) {
	callback = callback || function () {};

	if (!_isArray(tasks)) { return callback (); }
	if (!tasks.length) { return callback (); }

	var wrapIterator = function (iterator) {
		return function (err) {
			var args = Array.prototype.slice.call(arguments);
			var next = iterator.next();

			if (next)
				args.push (wrapIterator (next));
			else
				args.push (callback);

			nextTick(function () {
				iterator.apply(null, args);
			});
		};
	};

	wrapIterator(makeIterator(tasks))();
};

exports = module.exports = waterfall;