going full circle 2

app.js

app.get('/users/:userId', (req, res) => {
  db.getUserAsync(req.params.userId, (err, user) => {
    db.getPostsByUserAsync(user.id, (err, posts) => {
      db.getCommentsByPostsAsync(posts, (err, comments) => {
        res.json(mergeData(user, posts, comments));
      });
    });
  });
});