MongoClient.connect only once - node.js

Question

So in the following functions I'm always establishing a new connection with my mongodb. How would I have to change my code so it only connects once at the beginning and not in every of these functions.
function getData(callback){
arrayOfArticles = [];
MongoClient.connect(url, { useNewUrlParser: true }, callback, function(err, db) {
if (err) throw err;
let dbo = db.db('testdb');
article = dbo.collection('testname').find({}).toArray(function(err, article) {
if (err) throw err;
db.close();
for (var i = 0, len = article.length; i < len; i++){
arrayOfArticles.push(article[i].name);
}
callback(null, arrayOfArticles);
});
});
function getPrice(callback){
arrayOfPrices = [];
MongoClient.connect(url, { useNewUrlParser: true }, callback, function(err, db) {
if (err) throw err;
let dbo = db.db('testdb');
article = dbo.collection('testcollection').find({}).toArray(function(err, arrayOfPrices) {
if (err) throw err;
db.close();
callback(null, arrayOfPrices);
});
});
function getDealerData(callback){
dealerData = [];
MongoClient.connect(url, { useNewUrlParser: true }, callback, function(err, db) {
if (err) throw err;
let dbo = db.db('Dealer');
article = dbo.collection('Dealer').find({}).toArray(function(err, dealerData) {
if (err) throw err;
db.close();
callback(null, dealerData);
});
});

Answer 1

It's not a good practice to create connection for every task.
I would suggest to create a separate file for create connection and available the connection on everywhere
Example
//db.js
MongoClient.connect(url, {
useNewUrlParser: true
}, callback, function (err, db) {
if (err) throw err;
global.dbo = db.db('testdb');
});
And in your main server file I am assuming app.js and require it on top for all middlewares
//app.js
<--All depandent module -->
require('db.js'); // change the path according to your structure.
And now dbo will be available to all your app and just use it everywhere.
It's also good practice to use single connection and for load, Mongo itself creates pull to handle the concurrency
As per Mongo official comment:
You open do MongoClient.connect once when your app boots up and reuse the db object. It's not a singleton connection pool each .connect creates a new connection pool. So open it once an reuse across all requests.
Update
I have tried to make your one function here
function getPrice(callback){
arrayOfPrices = [];
dbo.collection('testcollection').find({}).toArray(function(err, arrayOfPrices) {
if (err) throw err;
callback(null, arrayOfPrices);
});
}

Answer 2

I found this block of code, derived from the previous answer to be very useful...
Rather than a separate file it stores the collection in the app locals system.
MongoClient.connect(url, { useNewUrlParser: true })
.then(client => {
const dbo = client.db('myDB');
const collection = dbo.collection('numbers');
app.listen(port2, () => console.info(`REST API running on port ${port2}`));
app.locals.collection = collection; // this line stores the collection from above so it is available anywhere in the app, after small delay.
}).catch(error => console.error(error));