Execute nodejs synchronously - node.js

Question

I am trying to write a function that accepts the mongodb collection name as the paramater and return an instance of the collection so that it can be used to perform CRUD operartions. But when I am trying to return the instance of the collection it returns 'undefined' as the return statement is executed before the MongoClient.connect function finishes its execution.
module.exports.dbConnection = function(collectionName)
{
var MongoClient = require('mongodb').MongoClient;
var url = "mongodb://127.0.0.1/test";
var collName;
MongoClient.connect(url, function(err, db)
{
var collName = db.collection(collectionName);
console.log(collName)
});
return collName;
}
Can I get assitance on how I can go about solving this issue.
Thanks

Answer 1

If you are using at least version 7.10 of Node you can accomplish this using async functions and promises.
// You can return a promise and resolve the promise once connected
module.exports.dbConnection = function dbConnection(collectionName) {
const MongoClient = require('mongodb').MongoClient;
const url = "mongodb://127.0.0.1/test";
return new Promise((resolve, reject) => {
MongoClient.connect(url, function (err, db) {
if (err) {
return reject(err);
}
resolve(db.collection(collectionName));
});
});
}
// You can then call the function within an async function (Node v7.10 and above)
async function fnThatCallsDbConnection() {
try {
const collName = await dbConnection('someCollection');
} catch(e){
// do something with error
}
}
Something additional you can do is cache your database connection so you don't need to connect every time - here's a way you could do it:
let cachedDB;
module.exports.dbConnection = function dbConnection(collectionName) {
const MongoClient = require('mongodb').MongoClient;
const url = "mongodb://127.0.0.1/test";
return new Promise((resolve, reject) => {
if (cachedDB) {
resolve(cachedDB.collection(collectionName));
} else {
MongoClient.connect(url, function (err, db) {
if (err) {
return reject(err);
}
cachedDB = db;
resolve(db.collection(collectionName));
});
}
});
}

Answer 2

The proper way to accomplish this is to use a callback. Accept a callback argument, and then pass your desired information to that function when the operation is complete.
module.exports.dbConnection = function(collectionName, cb)
{
var MongoClient = require('mongodb').MongoClient;
var url = "mongodb://127.0.0.1/test";
var collName;
MongoClient.connect(url, function(err, db)
{
var collName = db.collection(collectionName);
cb(collName); // invoke our callback
});
}
And you use it like this:
dbConnection('collName', function (coll) {
console.log(coll);
// do something else with our collection
})