Tutorials

Express app.listen() Method

Posted in Express Tutorial

Updated on Nov 24, 2024

By Mari Selvan

370 - Views

4 mins

1 Comment

Photo Credit to CodeToFun

Introduction

Express.js provides a straightforward way to create web servers in Node.js, and the app.listen() method is a crucial part of this process.

In this guide, we'll explore the app.listen() method, its syntax, and best practices for starting and configuring your Express.js server.

Syntax

The syntax for the app.listen() method is simple:

syntax.js

Copied

const server = app.listen(port, [hostname], [backlog], [callback])

port: The port number on which the server will listen for incoming requests.
hostname (optional): The hostname or IP address on which the server will listen. Defaults to 'localhost'.
backlog (optional): The maximum length of the queue of pending connections. Defaults to 511.
callback (optional): A callback function that is executed once the server is listening.

How app.listen() Works

The app.listen() method binds and listens for connections on the specified host and port. When a client makes a request, the server processes it and sends back the appropriate response.

example.js

Copied

const express = require('express');
const app = express();
const port = 3000;

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

const server = app.listen(port, () => {
  console.log(`Server is running on http://localhost:${port}`);
});

In this example, the server is set up to listen on port 3000, and a callback logs a message once the server is running.

Use Cases

Basic Server Setup:

Use app.listen() to start a basic server that responds to requests on a specified port.

example.js

Copied

const express = require('express');
const app = express();
const port = 3000;

app.get('/', (req, res) => {
  res.send('Welcome to my Express server!');
});

const server = app.listen(port, () => {
  console.log(`Server is running on http://localhost:${port}`);
});

Dynamic Port Assignment:

Dynamically assign the port based on the environment variable or use a default value.

example.js

Copied

const express = require('express');
const app = express();
const port = process.env.PORT || 3000;

app.get('/', (req, res) => {
  res.send('Dynamic Port Assignment');
});

const server = app.listen(port, () => {
  console.log(`Server is running on http://localhost:${port}`);
});

Best Practices

Port Availability Check:

Check if the specified port is available before calling app.listen() to avoid conflicts.

example.js

Copied

const express = require('express');
const app = express();
const port = 3000;

const server = app.listen(port, () => {
  console.log(`Server is running on http://localhost:${port}`);
});

server.on('error', (error) => {
  if (error.code === 'EADDRINUSE') {
    console.error(`Port ${port} is already in use`);
  }
});

Separate Server Setup:

Separate the server setup logic from the app.listen() call for better modularity.

example.js

Copied

const express = require('express');
const app = express();
const port = 3000;

app.get('/', (req, res) => {
  res.send('Separate Server Setup');
});

const startServer = () => {
  const server = app.listen(port, () => {
    console.log(`Server is running on http://localhost:${port}`);
  });
};

startServer();

Graceful Shutdown:

Implement a graceful shutdown mechanism to handle server termination.

example.js

Copied

const express = require('express');
const app = express();
const port = 3000;

app.get('/', (req, res) => {
  res.send('Graceful Shutdown');
});

const server = app.listen(port, () => {
  console.log(`Server is running on http://localhost:${port}`);
});

// Graceful shutdown example
process.on('SIGTERM', () => {
  server.close(() => {
    console.log('Server gracefully terminated');
    process.exit(0);
  });
});

Conclusion

The app.listen() method in Express.js is fundamental for starting your server and listening for incoming requests. By understanding its syntax, examples, and best practices, you can ensure a robust and well-configured Express.js server for your web applications.

Now, go ahead and set up your Express.js server with confidence!