How to force javascript not to make callback functions parallel?

Question

How to force javascript not to make callback functions parallel?

I'm building a twitter clone using Node.js and MongoDB with mongoose. My Tweet model has body, user and created fields where user is the id of the user who has created the tweet. Now I'm building the API. I want when I make a GET request to receive a list of all the tweets (/api/tweets/) but except the user field (which returns only the id of the user) I want to get the whole user object so that I can display information about the tweet owner in my front-end part. I ended up with the following code.

exports.all = function (req, res, next) {
  Tweet.find({}, function (err, tweets) {
    if (err) return res.json(400, err);

    var response = [];
    tweets.forEach(function (element, index, array) {
      var tweet = {};
      tweet._id = element._id;
      tweet.created = element.created;
      tweet.body = element.body;
      User.findById(element.user, function (err, user) { // <- This line
        if (err) return res.json(400, err);

        tweet.user = user;
      });

      response.push(tweet);
    });

    return res.json(response);
  });
};

It works perfectly except that it doesn't add the user info. The problem is in the line I have marked. When javascript comes to that line, it tries to make it "parallel" and continues with the code without executing the callback function. But then it pushes the tweet object that doesn't have yet user info. How can I fix this?

javascript
node.js
callback
mongoose

Answer 1

You're going to want to use the async library. It will make your life much easier.

// inside `Tweet.find`
async.each(tweets, function(done) {
    // do stuff to tweets
    User.findById(element.user, function(err, user){
        if (err) return done(err);
        // do stuff with user
        done();
    });
}, function(err) {
   // called when done
   res.json(response);
});

Answer 2

The issue is that res.json sends the response so it doesn't matter that findById is being called. You need to call res.json once everything is done.

You can do this in several ways, but the easiest one with your existing code is to just keep a counter:

var tweetCount = 0;

tweets.forEach(/* snip */) {
    User.findById(element.user, function (err, user) {
        tweet.user = user;
        tweetCount++;
        response.push(tweet);

        if (tweetCount == tweets.length) {
            res.json(response);
        }
    });
});