**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** WebSockets and Real-Time Applications **Topic:** Using the Socket.IO library with NestJS **Overview** In this topic, we will explore how to use the Socket.IO library with NestJS to build real-time applications. Socket.IO is a popular library for real-time communication over WebSockets, and it provides a simple and efficient way to establish real-time connections between clients and servers. **What is Socket.IO?** Socket.IO is a JavaScript library that enables real-time communication between clients and servers over WebSockets. It provides a simple and efficient way to establish real-time connections, allowing for bidirectional communication between clients and servers. **Why use Socket.IO with NestJS?** NestJS is a popular framework for building scalable server-side applications, and Socket.IO is a perfect fit for building real-time applications with NestJS. By using Socket.IO with NestJS, you can build applications that provide real-time updates, notifications, and live updates to clients. **Setting up Socket.IO with NestJS** To use Socket.IO with NestJS, you need to install the `@nestjs/websockets` package and the `socket.io` package. You can install them using the following command: ```bash npm install @nestjs/websockets socket.io ``` **Creating a Socket.IO Service** To use Socket.IO with NestJS, you need to create a Socket.IO service that will handle the real-time connections. You can create a new service by running the following command: ```bash nest generate service socket ``` **Configuring Socket.IO** To configure Socket.IO, you need to create a new file called `socket.io.config.ts` in the `config` directory. In this file, you can configure the Socket.IO settings, such as the port and the namespace. ```typescript import { ConfigModule, ConfigService } from '@nestjs/config'; import { SocketIoModule, SocketIoOptions } from '@nestjs/websockets'; export const socketIoOptions: SocketIoOptions = { namespace: '/socket', server: { cors: { origin: '*', methods: ['GET', 'POST'], }, }, }; export const socketIoModule = SocketIoModule.forRoot(socketIoOptions); ``` **Creating a Socket.IO Controller** To handle the real-time connections, you need to create a new controller that will handle the Socket.IO events. You can create a new controller by running the following command: ```bash nest generate controller socket ``` **Handling Socket.IO Events** To handle the Socket.IO events, you need to create a new method in the controller that will handle the event. For example, you can create a method that will handle the `connection` event: ```typescript import { Controller, OnGatewayConnection, OnGatewayDisconnect, OnGatewayInit, WebSocketServer } from '@nestjs/websockets'; import { Server, Socket } from 'socket.io'; @Controller('socket') export class SocketController implements OnGatewayInit, OnGatewayConnection, OnGatewayDisconnect { @WebSocketServer() server: Server; afterInit(server: Server) { console.log('Socket.IO server initialized'); } handleConnection(client: Socket) { console.log('Client connected'); } handleDisconnect(client: Socket) { console.log('Client disconnected'); } } ``` **Broadcasting Messages** To broadcast messages to all connected clients, you can use the `server.to()` method. For example, you can broadcast a message to all connected clients: ```typescript this.server.to('socket').emit('message', 'Hello, world!'); ``` **Conclusion** In this topic, we have explored how to use the Socket.IO library with NestJS to build real-time applications. We have covered the basics of Socket.IO, including setting up Socket.IO with NestJS, creating a Socket.IO service, configuring Socket.IO, creating a Socket.IO controller, handling Socket.IO events, and broadcasting messages. **Exercise** Create a new NestJS application and install the `@nestjs/websockets` package and the `socket.io` package. Create a new Socket.IO service and configure it to use the `socket.io` package. Create a new Socket.IO controller and handle the `connection` event. Broadcast a message to all connected clients. **Leave a comment below if you have any questions or need help with this topic.** **Next topic:** Building chat applications and real-time notifications.