program.js

function loadUsers(userIds, load, done) {
    var users = [];
    var relatives = {};
    var callbackInvokedNumber = 0;
    for (var i = 0; i < userIds.length; i++) {
        relatives['u' + userIds[i]] = i;
        load(userIds[i], function(user) {
            callbackInvokedNumber++;
            if (!user) {
                return;
            }
            users[relatives['u' + user.id]] = user;
            if (callbackInvokedNumber === userIds.length) {
                done(users);
            }
        });
    }
}

module.exports = loadUsers;