server.js

const dotenv = require("dotenv");
const connectDatabase = require("./config/database");
const cloudinary = require("cloudinary");

// Handling uncaught errors
process.on("uncaughtException", (err) => {
  console.log(`Error: $(err.message)`);
  console.log("Shutting down server due to uncaught exception");

  process.exit(1);
});

//config

dotenv.config({ path: "backend/config/config.env" });

//connecting to database
connectDatabase();
cloudinary.config({
  cloud_name: process.env.CLOUDINARY_NAME,
  api_key: process.env.CLOUDINARY_API_KEY,
  api_secret: process.env.CLOUDINARY_API_SECRET,
});

const server = app.listen(process.env.PORT, () => {
  console.log(`server is working on ${process.env.PORT}`);
});

//unhandled promise rejection

process.on("unhandledRejection", (err) => {
  console.log(`Error: ${err.message}`);
  console.log("Shutting down server due to unhandled promise rejection");
  server.close(() => {
    process.exit(1);
  });
});

databse.js

const { connect } = require("net");

const connectDatabase = () => {
  mongoose
    .connect(process.env.DB_URI, {
      useNewUrlParser: true,
      useUnifiedTopology: true,
    })
    .then((data) => {
      console.log(`Mongodb connected with server ${data.connection.host}`);
    });
};

module.exports = connectDatabase;

config.env

PORT=4000
DB_URI = "mongodb://localhost:27017/Ecommerce"

process.env.PORT is coming undefined instead of 4000. The same problem is coming for DB_URI and other process.env variables. If I write 4000 instead of process.env.PORT and "mongodb://localhost:27017/Ecommerce" instead of process.env.DB_URI it works fine. Below is the Error in my terminal.

server is working on undefined Error: The uri parameter to openUri() must be a string, got "undefined". Make sure the first parameter to mongoose.connect() or mongoose.createConnection() is a string. Shutting down server due to unhandled promise rejection

How can I resolve this issue?