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;
}

Adrien · Accepted Answer · 2011-10-05 15:30:29Z

6

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();
    });
  })();
}

edited Oct 5, 2011 at 15:30

answered Oct 4, 2011 at 21:14

Adrien

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1 Comment

sir-pinecone Over a year ago

Thanks Adrien, this did the trick. I ended up using this with the async library mentioned below to work with the array. I suppose I can replace this recursive function with with a call to async.map, but at least I now know how to solve this problem without depending on a library.

Chance · Accepted Answer · 2011-10-06 03:59:55Z

2

Check out async, it has an async foreach loop.

Edit

Here is the foreach method from the async library

async.forEach = function (arr, iterator, callback) {
    if (!arr.length) {
        return callback();
    }
    var completed = 0;
    _forEach(arr, function (x) {
        iterator(x, function (err) {
            if (err) {
                callback(err);
                callback = function () {};
            }
            else {
                completed += 1;
                if (completed === arr.length) {
                    callback();
                }
            }
        });
    });
};
var _forEach = function (arr, iterator) {
    if (arr.forEach) {
        return arr.forEach(iterator);
    }
    for (var i = 0; i < arr.length; i += 1) {
        iterator(arr[i], i, arr);
    }
};

edited Oct 6, 2011 at 3:59

answered Oct 4, 2011 at 20:30

Chance

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3 Comments

Mark Bolusmjak Over a year ago

If you look at the source, you'll see basically the same pattern of: do stuff, increment counter, if counter==input.length then invoke callback I have mentioned. Basically a choice of DIY or library, but the idea is the same.

Chance Over a year ago

@z5h Right, it becomes a recursive loop.

sir-pinecone Over a year ago

Hey Chance, I checked out async and it looks very useful. Thanks for the recommendation.

Mark Bolusmjak · Accepted Answer · 2011-10-04 20:26:34Z

0

you could do something like:
Give formatAssignee a callback.
Count down how many users you need to push onto users.
After you push the last one, invoke the callback with the parameter users.

answered Oct 4, 2011 at 20:26

Mark Bolusmjak

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