I am getting error on my postman application Error: connect ECONNREFUSED 127.0.0.1:3000

Question

app.js file code I am trying to connect mongodb
const express = require('express');
const dotenv = require('dotenve');
const bodyParser = require('body-parser');
const app = express();
app.use(bodyParser.json());
const connectDB = require('./config/db');
dotenv.config({ path: './config/config.env'});
connectDB();
app.use('/', require('./routes/index'));
app.listen(3000);

this the code. I watched numerous video on YouTube