Listening on a port in Express

Introduction

Listening on a port lets your Express app accept requests from users or other programs. It opens a door for communication.

When you want your web app to be reachable on the internet or local network.

When testing your Express server locally to see how it responds.

When deploying your app so it can handle real user traffic.

When running multiple apps on different ports on the same machine.

When you want to specify a custom port instead of the default.

Syntax

Express

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

const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server is listening on port ${PORT}`);
});

The listen method starts the server and waits for connections on the given port.

The callback function runs once the server is ready, useful for logging.

Examples

Starts the server on port 3000 without a callback.

Express

app.listen(3000);

Starts the server and logs a message when ready.

Express

app.listen(3000, () => {
  console.log('Server started on port 3000');
});

Uses an environment variable for the port, with 3000 as fallback.

Express

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Listening on port ${PORT}`);
});

Sample Program

This program creates a simple Express server that responds with 'Hello, world!' when you visit the home page. It listens on port 3000 and logs a message when ready.

Express

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

app.get('/', (req, res) => {
  res.send('Hello, world!');
});

const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server is listening on port ${PORT}`);
});

Important Notes

Listening on a port is how your server accepts incoming requests.

If the port is already in use, the server will fail to start.

Common mistake: forgetting to call listen, so the server never starts.

Use environment variables for ports in real apps to avoid conflicts.

Summary

Listening on a port opens your app to receive requests.

Use app.listen(port, callback) to start the server.

Always handle the callback to confirm the server started.