Adding to an array asynchronously in Node.js

Question

I'm pretty new to this type of programming and I'm having some trouble populating an array from a nested call. I'm pretty sure this needs to be done using callbacks, but I'm having trouble wrapping my brain around it. Closures must also come into play here. I tried searching the web for a similar example but didn't find much.

Here is my original code. I tried a few different approaches but didn't pull it off.

TaskSchema.statics.formatAssignee = function(assignees) {
  var users = [];

  assignees.forEach(function(uid) {
    mongoose.model('User').findById(uid, function(err, user) {
      users.push({
          name: user.name.full
        , id: user.id
      });
    });
  });

  return users;
}

Answer 1

I really like the following pattern (recursion is the most elegant solution to async loops):

TaskSchema.statics.formatAssignee = function(assignees, callback) {
  var acc = []
    , uids = assignees.slice()
  (function next(){
    if (!uids.length) return callback(null, acc);

    var uid = uids.pop()
    mongoose.model('User').findById(uid, function(err, user) {
      if (err) return callback(err);
      acc.push({
        name: user.name.full
      , id: user.id
      });
      next();
    });
  })();
}