In response to http://andrewkelley.me/post/js-callback-organization.html I believe that the convention of next(err, val) is that if err is truthy it will get handled if not then you expect data in the second arg. I often see code that adds extra checks for if (err != null) but is not necessary, you can just pass in the error with your data if th…

callback-organization-redux.js

var async = require('async');

function getUserFriends(userName, next) {
  db.users.findOne({name:userName}, foundUser);

	function foundUser(err, user) {
		err ? next(err) : getFriendsById(user.id, gotFriends);

		function gotFriends(err, friends) {
			if (err || user.type != 'power user') {
				next(err, friends);
			} else {
				async.map(friends, getFriendsById, addFriendsOfFriends);
			}

			function addFriendsOfFriends(err, friendsOfFriends) {
				for2dArray(friendsOfFriends, function(row, val) {
					if (val && friends.indexOf(val) != -1) {
						friends.push(val);
					}
				});

				next(err, friends);
			}
		}
	}
}

function getFriendsById(userId, next) {
	db.friends.find({userId:userId}, next);
}

function for2dArray(twoDimensionalArray, fn) {
	for (var i = 0; i < twoDimensionalArray.length; i++) {
		for (var j = 0; j < twoDimensionalArray[i].length; j++) {
			fn(twoDimensionalArray[i], twoDimensionalArray[i][j], i, j);
		}
	}
}