Some utility/example functions for sequential processing with Q.

gistfile1.js

function loop (fn, startValue) {
	var deferred = Q.defer(),
		finished = false;

	function end (val) {
		finished = true;
		deferred.resolve(val);
	}

	function error (reason) {
		finished = true;
		deferred.reject(reason);
	}

	function next (val) {
		try {
			var ret = fn(end, val);
		} catch (ex) {
			error(ex);
		}

		if (!finished) {
			Q(ret).then(next, error);
		}
	}

	Q(startValue).then(next, error);

	return deferred.promise;
}

function repeatNTimes (n, fn) {
	return loop(function (end) {
		if (n-- > 0) {
			return fn();
		} else {
			end();
		}
	});
}

function mapSequentially (array, fn, context) {
	var index = -1, len = array.length;
	var results = [];

	return loop(function (end, val) {
		if (~index) {
			results[index] = val;
		}

		while (++index < len && !(index in array));

		if (index < len) {
			return fn.call(context, array[index], index, array);

		} else {
			end(results);
		}
	});
}