Utility setup

db.js

const mongodb = require('mongodb')

const MongoClient = mongodb.MongoClient
const mongoDbUrl = 'mongodb://localhost/database_name' // Standard URI connection scheme: mongodb://[username:password@]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]



let _db

const initDb = callback => {
  if (_db) {
    console.log('Database is already initialized!')
    return callback(null, _db)
  }
  MongoClient.connect(mongoDbUrl, {useNewUrlParser: true })
    .then(client => {
      _db = client
      callback(null, _db)
      console.log('Database connected!')
    })
    .catch(err => {
      callback(err)
    })
}

const getDb = () => {
  if (!_db) {
    throw Error('Database not initialzed')
  }
  return _db
}

module.exports = {
  initDb,
  getDb
}

index.js

const db = require('./db')

db.initDb((err, db) => {
  if (err){
    console.log(err)
  } else {
    app.listen(3000)
  }
})1