Express app.listen() Method

Posted in Express Tutorial

Updated on Feb 17, 2024

By Mari Selvan

👁️ 132 - 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!