To pass port in command line and also to close the server

app.js

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

app.get('/', (req, res) => {
    res.send({"mess": "hello world"});
});

const portArrayPointer = process.argv.indexOf("--port");

if (portArrayPointer) {
    app.set('port', process.argv[portArrayPointer+1] || 3001);    
}

const server = app.listen(app.get('port'), () => {
    console.log("App listening on port-->>>",app.get('port'));
});

server.close(() => {
    console.log("server closed");
});
// node app --port 3000