TypeError: Cannot read property 'db' of null

This is because the mongo.connect function is asynchronous. You will need to include the app.post function inside of the mongo.connect callback.

Something kind of like this should work:

let express = require('express')
let mongodb = require('mongodb')
let app = express()

app.use(express.urlencoded({extended: false}))

let connectionString = 'mongodb+srv://olumide:[email protected]/todoApp?retryWrites=true&w=majority'
mongodb.connect(connectionString, {useNewUrlParser: true, useUnifiedTopology: true}, function(err, client){
    const db = client.db()

    app.post('/create-item', function(req,res){
       db.collection('item').insertOne({text: req.body.item}, function(){
        res.send("thanks for submitting the form")
       })
    })

    app.listen(3000)
})

I figured out, in newer versions of MongoDB (3 and higher) they have essentially changed the way of connecting node server to the database. To establish a reusable connection (So that we can access the connected database from any other file), I created an async function in my db.js file where the connection is established and then exported it. In the end of the file, I have called the function. The code is as follows:

const {MongoClient} = require('mongodb')


const client = new MongoClient('mongodb+srv://todoAppUser:<password>@cluster0.6lvjr.mongodb.net/myDatabase?retryWrites=true&w=majority')

async function start(){
  await client.connect()
  console.log("Connected")
  module.exports = client.db()
  const app = require('./app')
  app.listen(3000)
}

  start()

and while calling it from another file:

const productCollection = require('./db').collection("product");

This code gives me no error and works perfectly fine. With the help of the above code, one can use this conveniently while following the MVC (Model-View-Controller) framework.

use mongoose connection as a different javascript file and import it to express script file database.js

let mongoose = require('mongoose');
const server = '127.0.0.1:27017'; // REPLACE WITH YOUR DB SERVER
const database = 'test';      // REPLACE WITH YOUR DB NAME
class Database {
    constructor() {
      this._connect()
    }
    
  _connect() {
       mongoose.connect(`mongodb://${server}/${database}`,{ useUnifiedTopology: true ,useNewUrlParser: true, useFindAndModify: false})
         .then(() => {
           console.log('Database connection successful')
         })
         .catch(err => {
           console.error('Database connection error')
         })
    }
  }
  
  module.exports = new Database()

strong text