toArray undefined in NodeJS and MongoDB - node.js

Question

I am trying to run an aggregation MongoDB and Nodejs, but I have some difficulties in running the project.
When I enter the following command in the MongoDB shell:
db.data.aggregate([{$match: {}},{$group: {'_id': '$State', 'total': {'$sum': 1}} }]).toArray()
then I am getting the expected output.
However, when I use the following little Nodejs program
var MongoClient = require('mongodb').MongoClient;
MongoClient.connect('mongodb://localhost:27017/weather', function(err, db) {
if(err) throw err;
console.log("Connected correctly to server");
var col=db.collection('data');
col.aggregate([{$match: {}},{$group: {'_id': '$State', 'total': {'$sum': 1}} }])
.toArray(function(err, result) {
if(err) throw err;
console.log(result);
});
db.close();
});
then I am getting the error message:
'TypeError: Cannot read property 'toArray' of undefined'
Could somebody please help me?
Many thanks in advance, Andi

Answer 1

As #ExplosionPills correctly pointed out, your code won't work as the logging is done asynchronously and after the connection is closed so you could try removing the db.close() line or create a function that makes use of a callback function to returns the aggregation results:
var aggregateStates = function(db, callback) {
db.collection('data').aggregate(
[
{ $group: { "_id": "$State", "total": { $sum: 1 } } }
]
).toArray(function(err, result) {
console.log(result);
callback(result);
});
};
Call the aggregateStates function:
var MongoClient = require('mongodb').MongoClient;
MongoClient.connect('mongodb://localhost:27017/weather', function(err, db) {
aggregateStates(db, function() {
db.close();
});
});

Answer 2

According to mongo-native driver doc, aggregate() returns null. Therefore, toArray() cannot be called from what it returns. However, aggregate() takes a callback which has the result of aggregate() if successful. So here is the revised code:
var MongoClient = require('mongodb').MongoClient;
MongoClient.connect('mongodb://localhost:27017/weather', function(err, db) {
if(err) throw err;
console.log("Connected correctly to server");
var col=db.collection('data');
col.aggregate([
{$match: {}},
{$group: {'_id': '$State', 'total': {'$sum': 1}} }
], function(err, result) {
if(err) {
db.close();
throw err;
}
console.log(result);
db.close();
});
});