Mongodb.connect does not execute callback function - node.js

Question

Im trying to connect to my Mongodb and insert some documents if they are not already in the db. It works fine with the first inserts but in the function existInDatabase it sometimes does not execute the callback function.
var MongoClient = require('mongodb').MongoClient;
var mongoData = require('./mongoData');
var exports = module.exports = {};
var dbName = 'checklist';
MongoClient.connect(mongoData.ConString, {
useNewUrlParser: true
}, function(err, db) {
if (err) throw err;
for (var key in mongoData.Customers) {
if (!existsInDatabase(mongoData.Customers[key], 'Customers')) {
db.db(dbName).collection('Customers').insertOne(mongoData.Customers[key], function(err, res) {
if (err) throw err;
console.log('1 document inserted');
db.close();
});
}
}
for (var key in mongoData.Categorys) {
if (!existsInDatabase(mongoData.Customers[key], 'Customers')) {
db.db(dbName).collection('Categorys').insertOne(mongoData.Categorys[key], function(err, res) {
if (err) throw err;
console.log('1 document inserted');
db.close();
});
}
}
});
function existsInDatabase(obj, collection) {
var result = false;
MongoClient.connect(mongoData.ConString, {
useNewUrlParser: true
}, function(err, db) {
db.db(dbName).collection(collection).find({}).forEach(function(doc) {
if (doc.id == obj.id) {
result = true;
}
}, function(err) {
console.log(err);
});
});
return result;
}

Answer 1

I have made a few changes to your code. It seems you are new to async programming, spend some time to understand the flow. Feel free for any further query. Here is your code.
// Welcome to aync programming
// Here no one waits for the slow processes
var MongoClient = require('mongodb').MongoClient;
var mongoData = require('./mongoData');
var exports = module.exports = {};
var dbName = 'checklist';
// Make the connection for once only
MongoClient.connect(mongoData.ConString, { useNewUrlParser: true },
function(err, db) {
if (err) throw err;
var myDB = db.db(dbName); // create DB for once
for (var key in mongoData.Customers) {
//make call to the function and wait for the response
existsInDatabase(mongoData.Customers[key], 'Customers', function(err, result) {
//once the response came excute the next step
if (result) {
myDB.collection('Customers').insertOne(mongoData.Customers[key], function(err, res) {
if (err) throw err;
console.log('1 document inserted');
});
}
});
}
for (var key in mongoData.Categorys) {
//make call to the function and wait for the response
existsInDatabase(mongoData.Customers[key], 'Customers', function(err, result) {
//once the response came excute the next step
if (result) {
myDB.collection('Categorys').insertOne(mongoData.Categorys[key], function(err, res) {
if (err) throw err;
console.log('1 document inserted');
});
}
});
}
// Both the for loop will work randomly without any order
function existsInDatabase(obj, collection, cb) {
var result = false;
myDB.collection(collection).findOne({ id: obj.id }, function(err, result)
{
if (err) {
//this cb will work only when db operation is complited
cb(err);
} else if (result) {
cb(null, true);
} else {
cb(null, false);
}
});
}
});
This code may result in some error. Feel free to ask more questions over it

Answer 2

db.db(dbName).collection(collection).find({}) returns a cursor per the docs. You are missing .toArray():
db.db(dbName).collection(collection).find({}).toArray()...