Socket.IO Documentation

Socket.IO is a library for real-time, bidirectional, and event-based communication between web clients and servers. It is built on top of WebSockets and includes fallbacks for older browsers.

---

Installation

Install Socket.IO in both your server and client projects using npm or yarn.

Server-side Installation:

npm install socket.io

Client-side Installation:

npm install socket.io-client

Alternatively, use a CDN for the client:

<script src="https://cdn.socket.io/4.6.0/socket.io.min.js"></script>

---

Basic Usage

Server-side Example

const express = require('express');
const http = require('http');
const { Server } = require('socket.io');

const app = express();
const server = http.createServer(app);
const io = new Server(server);

io.on('connection', (socket) => {
    console.log('A user connected');

    // Handle custom events
    socket.on('message', (data) => {
        console.log('Message received:', data);
    });

    // Disconnect event
    socket.on('disconnect', () => {
        console.log('A user disconnected');
    });
});

server.listen(3000, () => {
    console.log('Server is running on port 3000');
});

Client-side Example

import { io } from 'socket.io-client';

const socket = io('http://localhost:3000');

// Listen for connection event
socket.on('connect', () => {
    console.log('Connected to server');
});

// Emit a message event
socket.emit('message', 'Hello, server!');

// Listen for disconnect
socket.on('disconnect', () => {
    console.log('Disconnected from server');
});

---

Features

1. Namespaces

Namespaces allow you to create multiple, independent communication channels.

Server-side:

const chatNamespace = io.of('/chat');
chatNamespace.on('connection', (socket) => {
    console.log('User connected to chat namespace');
});

Client-side:

const chatSocket = io('http://localhost:3000/chat');

2. Rooms

Rooms allow grouping sockets to broadcast messages to a subset of connected clients.

Joining a Room:

socket.join('room1');
socket.to('room1').emit('message', 'Hello, Room 1!');

Leaving a Room:

socket.leave('room1');

3. Broadcasting

Broadcast messages to all connected sockets except the sender.

socket.broadcast.emit('message', 'This is a broadcast message');

4. Middleware

Use middleware to intercept and process events.

Example Middleware:

io.use((socket, next) => {
    const token = socket.handshake.auth.token;
    if (isValidToken(token)) {
        next();
    } else {
        next(new Error('Authentication error'));
    }
});

---

Advanced Topics

Error Handling

Use the error event to handle errors:

socket.on('connect_error', (err) => {
    console.error('Connection error:', err.message);
});

Sending and Receiving Binary Data

Socket.IO supports binary data, such as files or images:

Sending:

socket.emit('file', fileBuffer);

Receiving:

socket.on('file', (data) => {
    console.log('Received file:', data);
});

Integration with Express

Combine Socket.IO with an Express app:

app.get('/', (req, res) => {
    res.sendFile(__dirname + '/index.html');
});

io.on('connection', (socket) => {
    console.log('User connected');
});

---

Debugging

Enable debugging logs using the DEBUG environment variable:

DEBUG=socket.io:* node app.js

---

Best Practices

1. Authentication: Use middleware to authenticate users.
2. Scalability: Use adapters like Redis for scaling across multiple servers.
3. Error Handling: Always handle errors gracefully.
4. Data Validation: Validate data sent by clients to prevent malicious input.

---

Resources

• Socket.IO Official Documentation
• GitHub Repository
• Socket.IO Debugging Guide

---

Conclusion

Socket.IO is a powerful tool for building real-time applications such as chat apps, gaming platforms, and live data dashboards. Its simplicity and flexibility make it an essential library for modern web development.