Need to send response after forEach is done - node.js

Question

I'm working with NodeJS + Mongoose and I'm trying to populate an array of objects and then send it to the client, but I can't do it, response is always empty because it is sent before forEach ends.
router.get('/', isAuthenticated, function(req, res) {
Order.find({ seller: req.session.passport.user }, function(err, orders) {
//handle error
var response = [];
orders.forEach(function(doc) {
doc.populate('customer', function(err, order) {
//handle error
response.push(order);
});
});
res.json(response);
});
});
Is there any way to send it after the loop has finished?

Answer 1

Basically, you could use any solution for async control flow management like async or promises (see laggingreflex's answer for details), but I would recommend you to use specialized Mongoose methods to populate the whole array in one MongoDB query.
The most straightforward solution is to use Query#populate method to get already populated documents:
Order.find({
seller: req.session.passport.user
}).populate('customer').exec(function(err, orders) {
//handle error
res.json(orders);
});
But if, for some reason, you can't use this method, you could call Model.populate method yourself to populate an array of already fetched docs:
Order.populate(orders, [{
path: 'customer'
}], function(err, populated) {
// ...
});

Answer 2

One solution is to use Promises.
var Promise = require('bluebird');
Promise.promisifyAll(Order);
router.get('/', isAuthenticated, function(req, res) {
Order.findAsync({ seller: req.session.passport.user })
.then(function(orders) {
return Promise.all(orders.map(function(doc){
return Promise.promisify(doc.populate).bind(doc)('customer');
}));
}).then(function(orders){
// You might also wanna convert them to JSON
orders = orders.map(function(doc){ return doc.toJSON() });
res.json(orders);
}).catch(function(err){
//handle error
});
});
BlueBird's .promisifyAll creates an …Async version of all functions of an object, which saves you an extra step in configuring the initial promise. So instead of Order.find I used Order.findAsync in above example