How to reuse mongodb connection through Promise

dbUtils.js

'use strict'

const assert = require('assert')

module.exports = {

	getProductsCount: (req, res) => {

        return new Promise((resolve, reject) => {

			try {

				const db = req.app.locals.db

				db.collection('products').find({}).count((ec, count) => {
					resolve( count )
				})

			} catch (err) {
				return reject(err + '')
			}
		})
	},

	getProducts: (req, res, _limit) => {

	    return new Promise((resolve, reject) => {

	    	try {

	    		const db = req.app.locals.db

	    		db.collection('products').find({}).limit(_limit).toArray((err, docs) => {
	    			assert.equal(err, null);
	    		    resolve(docs)
	    		})

	    	} catch (err) {
	    		return reject(err + '')
	    	}
	    })
	}
}

promise_mongodg.js

'use strict'

const express = require('express'),
    app = express(),
    MongoClient = require('mongodb').MongoClient,
    assert = require('assert');

const dbUtils = require('./helpers/dbUtils')

const config = {
	port: 3000,
	database: {
		url: 'mongodb://localhost:27017/store'
	}
}

app.use((err, req, res, next) => {
    if (res.headersSent) { return next(err) }
	res.status(500)
	res.render('error', { error: err })
})
// ------------------------------------------------------------------------

app.get('/', (req, res, next) => {
	console.time('get / req');
	res.send({
		count_all: `http://localhost:${config.port}/db/products.count`,
		get_all: `http://localhost:${config.port}/db/products.all`,
		get_all_limit: `http://localhost:${config.port}/db/products.all/2`
	})
	console.timeEnd('get / req')
})

app.get('/db/products.count', (req, res, next) => {

	console.time('get /db/products.count')

	dbUtils.getProductsCount(req, res)
		.then((result) => {
			res.send({productsCount: result})
			console.timeEnd('get /db/products.count')
		}).catch((e) => {
			console.error(e + '')
			console.timeEnd('get /db/products.count')
		})
})

app.get('/db/products.all/:limit*?', (req, res, next) => {

	let limit = ('limit' in req.params) ? Number(req.params.limit) : null

	console.time('get /db/products.all/' + limit)

	dbUtils.getProducts(req, res, limit)
		.then((result) => {
		    res.send({products: result})
		    console.timeEnd('get /db/products.all/' + limit)
		}).catch((e) => {
		    console.error(e + '')
		    console.timeEnd('get /db/products.all/' + limit)
		})
})

app.get('*', (req, res) => {
	res.status(404).send('Not found')
})
// ------------------------------------------------------------------------

MongoClient.connect(config.database.url, (err, db) => {
	if (err) {
		console.error(`Failed to connect to the database. ${err.stack}`)
	}
	app.locals.db = db
	app.listen(config.port, () => {
		console.info(`Node.js app is listening at http://localhost:${config.port}`)
	})
})

// https://stackoverflow.com/questions/10656574/how-do-i-manage-mongodb-connections-in-a-node-js-web-application/14464750#14464750
// https://medium.com/@tarkus/how-to-open-database-connections-in-a-node-js-express-app-e14b2de5d1f8
// https://www.npmjs.com/package/mongodb
// https://mongodb.github.io/node-mongodb-native/api-generated/cursor.html
// http://2ality.com/2014/09/es6-modules-final.html