Single or multiple connections to MongoDB is better?

Question

I'm building my Node.js application by Express. I have two choices to write code in order to connect to my MongoDB.

• First is to open a connection each time a query is performed

https://codeshare.io/a3AQXg

• Second is to create one single connection when starting app

https://codeshare.io/GkmR14

Then

function productRepository(db) {
    this.db = db;
};

productRepository.prototype.insert = function(item) {
    return new Promise((resolve, reject) => {
        this.db.collection('product').insertOne(item, function(err, result) {
            if (err) {
                reject(err);
            } else {
                resolve(result);
            }
        });
    });
};

module.exports = productRepository;

And

module.exports = function(app, db) {

    var productRepository = require('../model/product');
    var productRepoInstance = new productRepository(db);

    app.get('/test', function(req, res) {
        productRepoInstance.insert({ createdAt: new Date() }).then(
            (result) => res.send({ result: 1 }),
            (error) => {
                console.log(error);
                res.send({ result: 0 });
            });
    });
};

I wonder which is better, and why?