TypeError: Cannot read property 'collection' of undefined in mongodb

Database connected.

But whenever I try to insert a a data from Submit FORM, I am facing this error. Can anyone help? I am using mongodb version 4.10.0

This is my models code ->

const  db = require("../database").db;

class User { 
    constructor(name) {
        this.name = name;
    }

    save() {
        db.collection("user").insertOne(this)
        .then( result => {
            console.log("Data Saved Properly");
        })
        .catch(err => {
            // console.log(err);
            throw err;
        })
    }
}

module.exports = User;

This is my database.js codes ->

> const mongodb = require('mongodb'); const MongoClient =
> mongodb.MongoClient;
> 
> let db;
> 
> const connectDB = (next) => {
>     MongoClient.connect('mongodb://localhost:27017/testNode')
>     .then( result => {
>         console.log("Connected");
>         db = result.db();
>         next();
>     })
>     .catch(err => {
>         console.log(err);
>     }) }
> 
> exports.db = db;
> 
> module.exports = connectDB;