How to Get websockets working with NestJS

In NestJS, using WebSocket typically involves working with libraries such as Socket.IO or ws alongside NestJS's abstraction layer for easy integration and maintenance. NestJS provides a module named @nestjs/websockets that includes decorators and classes required for interacting with WebSocket.

1. Install necessary packages

First, ensure that you have installed the @nestjs/websockets module and the socket.io library (if you choose to use Socket.IO):

bash
npm install @nestjs/websockets @nestjs/platform-socket.io socket.io

2. Create Gateway

In NestJS, you can create a Gateway, which is a class decorated with @WebSocketGateway(), handling WebSocket connections. For example:

typescript
import { WebSocketGateway, WebSocketServer, SubscribeMessage, MessageBody, ConnectedSocket } from '@nestjs/websockets';
import { Server, Socket } from 'socket.io';

@WebSocketGateway({ cors: true })
export class EventsGateway {
  @WebSocketServer()
  server: Server;

  @SubscribeMessage('message')
  handleMessage(@MessageBody() data: string, @ConnectedSocket() client: Socket): void {
    client.emit('message', data); // Echo the message back to the client
  }
}

In this example, the EventsGateway class uses the @WebSocketGateway decorator to create a WebSocket server. We listen for the message event and define a handler function handleMessage to process received messages.

3. Register Gateway in Module

Next, you need to register this Gateway in a NestJS module:

typescript
import { Module } from '@nestjs/common';
import { EventsGateway } from './events.gateway';

@Module({
  providers: [EventsGateway],
})
export class AppModule {}

This way, the EventsGateway will be recognized by the NestJS framework and automatically start the WebSocket server upon application startup.

4. Connect WebSocket Client

Clients can use the socket.io-client library or other WebSocket client libraries to connect to the server:

javascript
// Using socket.io-client in the client-side code
const socket = io('http://localhost:3000');
socket.on('message', function(data) {
  console.log('Received message:', data);
});
socket.emit('message', 'Hello, server!'); // Send a message to the server

The above client-side code example demonstrates using socket.io-client to connect to the NestJS service and listen for the message event. The client also sends a message event to the server using emit.

5. Using Advanced Features

The NestJS WebSocket module also supports advanced features such as namespaces/rooms, exception filters, pipes, interceptors, and guards, enabling developers to build WebSocket applications with complex logic and security.

For example, if you want to send messages only to clients in a specific room, you can do the following:

typescript
@SubscribeMessage('joinRoom')
async handleJoinRoom(@MessageBody() room: string, @ConnectedSocket() client: Socket) {
  client.join(room); // Join the specified room
}

@SubscribeMessage('leaveRoom')
async handleLeaveRoom(@MessageBody() room: string, @ConnectedSocket() client: Socket) {
  client.leave(room); // Leave the specified room
}

async sendMessageToRoom(room: string, message: string) {
  this.server.to(room).emit('message', message); // Emit a message to all clients in the specified room
}

In this example, we create event handlers for joining and leaving rooms, as well as a function to send messages to all clients in a specified room.

By following these steps, you can set up and use WebSocket communication in NestJS. Of course, adjustments and optimizations may be needed based on the specific application context.