MongoDB read and create backend api

index.js

require('dotenv').config()
const { MongoClient } = require('mongodb')

const connectMongoDB = () => MongoClient.connect(process.env.MONGODB)

const getContacts = (req, res) => {
  return connectMongoDB()
    .then(
      db => db.collection('contacts')
        .find({})
        .toArray()
        .then(documents => ({db, documents}))
    )
    .then(({db, documents}) => {
      db.close()
      return documents
    })
    .then(contacts => res.json(contacts))
    .catch(err => res.status(400).send(err.toString()))
}

const createContact = (req, res) => {
  return connectMongoDB()
    .then(
      db => db.collection('contacts').insertOne(req.body)
        .then(result => db)
    )
    .then(db => db.close())
    .then(() => res.json({result: 'ok'}))
    .catch(err => res.status(400).send(err.toString()))
}

module.exports.handler = (req, res) => {
  if (req.method === 'POST') {
    return createContact(req, res)
  }

  return getContacts(req, res)
}