Server port undefined in Node

Question

I am making a Node server and running it so I am getting undefined values from dotenv file.

Here is my project structure:

backend
       config
            config.env
       app.js
       server.js

Here is the code of config.js:

PORT = 5000

NODE_ENV = DEVELOPMENT

app.js:

const express = require('express')
const app = express();

module.exports = app

server.js:

const app = require('./app')
const dotenv = require('dotenv');

//setting up confg file
dotenv.config({path:'backend/config/config.env'})
app.listen(process.env.PORT,()=>{
    console.log(`server started on PORT:${process.env.PORT} in ${process.env.NODE_ENV} node.`)
})

Here is the output:

server started on PORT:undefined in undefined node.