Mongoose findOne scope doubts

Question

Mongoose findOne scope doubts

I'm new to this javascript thing and have i have some doubts about mongoose 'find' scope's.

I wrote the code below to try to understand the problem.

The code below searches into a Shopping collection and then search for the Stores assigned to this Shopping.

storeMap is a hashmap of storeId => storeObject, but when the Store.findOne scope end, storeMap seems to rollback to an empty array...

 var storeMap = {};
 Shopping.findOne({ name: shoppingName }, function(err, shopping){
                    shopping.stores.forEach(function(storeId) {
                            Store.findOne({_id: storeId}, function(err, store) {
                                    if(err) console.log(err);
                                    console.log(store); //prints out store data
                                    storeMap[storeId] = store;
                                    console.log(storeMap); //prints out store data
                            });
                            console.log(storeMap); //prints out an empty array
                    });
            });

So, why my storeMap array is printing an empty array instead of a store array?

javascript
arrays
node.js
mongodb
mongoose

Answer 1

Store.findOne, like many things in node.js, is asynchronous and takes a callback. Your callback is where storeMap gets set:

function(err, store) {
    if(err) console.log(err);
    console.log(store); //prints out store data
    storeMap[storeId] = store;
    console.log(storeMap); //prints out store data
}

And this only runs once Store.findOne finishes its work, which could take a long time.

The following line, console.log(storeMap); executes immediately, however--before the callback runs. Thus, storeMap is still empty.

I recommend looking at a few examples/explanations of the callback pattern in node.js, it's fundamental to understand.