why env file in node js is not working to connect mongodb

1    // Create .env file 
2    inside .env 
3    MONGO_URI=mongo+src://username:password@cluster0......majority
4    //
5   // Inside app.js or server.js 
6    const dotenv = require('dotenv')
7    dotenv.config()
8
9    const mongoose = require('mongoose')
10
11    mongoose.connect(process.env.MONGO_URI, {
12        useNewUrlParser: true,
13        useUnifiedTopology: true
14    });
15    const db = mongoose.connection
16    db.on('error', error => console.error(error))
17    db.once('open', MONGO_URI => console.log('Connected to Database'))
18

1mongoose.connect(process.env.MONGO_URI).then(() => {
2  console.log("DB Connected");
3});
4
5mongoose.connection.on("error", err => {
6 console.log(`DB Connection Error: ${err.message}`);
7});
8