MongoDB – TypeError: Cannot read properties of null (reading 'collection') – Javascript

indexHelpers.js

var db = require("../config/connection");
var collections = require("../config/collections");
var ObjectId = require("mongodb").ObjectId;

module.exports = {
  addPerson: (personDetails) => {
    return new Promise((resolve, reject) => {
     db.get().collection(collections.USER_DETAILS)
        .insertOne(personDetails)
        .then((details) => {
          resolve(details);
        });
    });
  },
};
 

In the indexhelpers.js the db.get() function return null… so when I try to call db.get().collection() it returns the error TypeError: Cannot read properties of null (reading ‘collection’)

So this is my connection .js.I couldn’t find out why i get this error

connection.js

const mongoClient = require("mongodb").MongoClient;
const state = {
  db: null,
};
module.exports.connect = function (done) {
  const url = "mongodb://localhost://27017";
  const dbname = "CMS";
  mongoClient.connect(url, { useUnifiedTopology: true }, (err, data) => {
    if (err) return done(err);
    state.db = data.db(dbname);
  });

  done();
};

module.exports.get = function () {
  return state.db;
};

app.js

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');
var db = require("./config/connection");

var indexRouter = require('./routes/index');
var hbs = require("express-handlebars");
var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'hbs');
//handlebars as the template engine
app.engine(
  "hbs",
  hbs.engine({
    extname: "hbs",
    defaultLayout: "layout",
    layoutsDir: __dirname + "/views/"
  })
);

app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));
db.connect((err) => {
  if (err) console.log("connection error" + err);
  else console.log("--------Database Connected--------");
});

app.use('/', indexRouter);


// catch 404 and forward to error handler
app.use(function(req, res, next) {
  next(createError(404));
});

// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

I tried console.log(db.get()) which returns null. actually db.get() must return some values but I am not getting0 it