WebSocket gateway creation in NestJS

Introduction

A WebSocket gateway lets your app talk instantly with users without waiting for them to ask. It helps send and get messages in real time.

You want to build a chat app where messages appear instantly.

You need live updates on a dashboard without refreshing the page.

You want to notify users immediately when something changes.

You are making a multiplayer game that needs fast communication.

You want to stream data like stock prices or sports scores live.

Syntax

NestJS

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

@WebSocketGateway()
export class MyGateway {
  @WebSocketServer()
  server: Server;

  @SubscribeMessage('message')
  handleMessage(@MessageBody() data: string): string {
    return `Received: ${data}`;
  }
}

@WebSocketGateway() marks the class as a WebSocket gateway.

@SubscribeMessage('event') listens for messages with the given event name.

Examples

This listens for 'chat' messages and replies with a simple string.

NestJS

import { WebSocketGateway, SubscribeMessage, MessageBody } from '@nestjs/websockets';

@WebSocketGateway()
export class ChatGateway {
  @SubscribeMessage('chat')
  onChat(@MessageBody() message: string): string {
    return `Chat message: ${message}`;
  }
}

This gateway runs on port 3001 and can send messages to all connected clients.

NestJS

import { WebSocketGateway, WebSocketServer } from '@nestjs/websockets';
import { Server } from 'socket.io';

@WebSocketGateway(3001)
export class EventsGateway {
  @WebSocketServer()
  server: Server;

  sendEvent() {
    this.server.emit('event', 'Hello everyone!');
  }
}

Sample Program

This gateway listens for a 'greet' message with a name and replies with a hello message including that name.

NestJS

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

@WebSocketGateway()
export class SimpleGateway {
  @WebSocketServer()
  server: Server;

  @SubscribeMessage('greet')
  handleGreet(@MessageBody() name: string): string {
    return `Hello, ${name}!`;
  }
}

Important Notes

Make sure to install @nestjs/websockets and socket.io packages.

Use @WebSocketServer() to access the server and send messages to clients.

Event names in @SubscribeMessage() must match what clients send.

Summary

WebSocket gateways let your NestJS app handle real-time messages easily.

Use decorators to listen for and respond to events.

You can send messages back to one or all connected clients.

Practice

1. What is the main purpose of a WebSocket gateway in a NestJS application?

easy

A. To serve static files like images and CSS

B. To handle real-time communication between server and clients

C. To manage database connections

D. To handle HTTP request routing

Solution

1. Step 1: Understand WebSocket gateway role

   A WebSocket gateway is designed to manage real-time, two-way communication between the server and clients.

2. Step 2: Compare with other server roles

   Managing database connections, serving static files, or HTTP routing are handled by other parts of NestJS, not WebSocket gateways.

3. Final Answer:

   To handle real-time communication between server and clients -> Option B

4. Quick Check:

   WebSocket gateway = real-time communication [OK]

Hint: WebSocket gateways are for live message exchange [OK]

Common Mistakes:

• Confusing WebSocket gateway with HTTP routing
• Thinking it manages database connections
• Assuming it serves static files

2. Which decorator is used to define a WebSocket gateway class in NestJS?

easy

A. @Gateway()

B. @SocketGateway()

C. @WebSocketGateway()

D. @WebSocketServer()

Solution

1. Step 1: Identify the correct decorator for gateway class

   The official NestJS decorator to create a WebSocket gateway class is @WebSocketGateway().

2. Step 2: Differentiate from other decorators

   @WebSocketServer() is used inside the class to inject the server instance, not to define the class itself. The others are incorrect or do not exist.

3. Final Answer:

   @WebSocketGateway() -> Option C

4. Quick Check:

   Gateway class decorator = @WebSocketGateway() [OK]

Hint: Gateway class always uses @WebSocketGateway() [OK]

Common Mistakes:

• Using @WebSocketServer() to decorate the class
• Confusing with non-existent decorators
• Omitting parentheses on the decorator

3. Given this NestJS WebSocket gateway code, what will be logged when a client sends a 'message' event with payload 'Hello'?

import { WebSocketGateway, SubscribeMessage, MessageBody } from '@nestjs/websockets';

@WebSocketGateway()
export class ChatGateway {
  @SubscribeMessage('message')
  handleMessage(@MessageBody() data: string): string {
    console.log('Received:', data);
    return `Echo: ${data}`;
  }
}

medium

A. Received: Hello

B. Echo: Hello

C. message event received

D. No output logged

Solution

1. Step 1: Analyze the handleMessage method

   The method logs the string 'Received:' followed by the data payload, which is 'Hello'.

2. Step 2: Understand what is returned vs logged

   The return value is 'Echo: Hello', but only the console.log outputs to the server console. The client receives the return value, but the question asks about what is logged.

3. Final Answer:

   Received: Hello -> Option A

4. Quick Check:

   Console logs 'Received: Hello' [OK]

Hint: Look for console.log to find logged output [OK]

Common Mistakes:

• Confusing returned value with logged output
• Assuming client message is logged
• Ignoring the console.log statement

4. Identify the error in this WebSocket gateway code snippet:

import { WebSocketGateway, SubscribeMessage } from '@nestjs/websockets';

@WebSocketGateway()
export class MyGateway {
  @SubscribeMessage('chat')
  onChat(data: string) {
    return `Message: ${data}`;
  }
}

medium

A. Method onChat must be async

B. Incorrect event name in @SubscribeMessage decorator

C. WebSocketGateway decorator requires a port number

D. Missing @MessageBody() decorator on the method parameter

Solution

1. Step 1: Check method parameter decorators

   In NestJS WebSocket gateways, to receive the message payload, the parameter must be decorated with @MessageBody().

2. Step 2: Verify other parts

   The event name 'chat' is valid, @WebSocketGateway() can be used without a port, and the method does not need to be async unless awaiting something.

3. Final Answer:

   Missing @MessageBody() decorator on the method parameter -> Option D

4. Quick Check:

   Parameter needs @MessageBody() [OK]

Hint: Always decorate message parameters with @MessageBody() [OK]

Common Mistakes:

• Omitting @MessageBody() on parameters
• Thinking port number is mandatory
• Assuming method must be async

5. How can you broadcast a message to all connected clients from within a NestJS WebSocket gateway?

hard

A. Use the injected server instance with this.server.emit('event', data)

B. Return the message string from the event handler method

C. Call client.send(data) on a single client only

D. Use @SubscribeMessage('broadcast') decorator on the method

Solution

1. Step 1: Understand broadcasting in NestJS gateways

   To send a message to all clients, you use the server instance injected with @WebSocketServer() and call emit on it.

2. Step 2: Differentiate from other options

   Returning a string sends a response only to the sender. Calling client.send() targets one client. The decorator @SubscribeMessage() only listens for events, it does not broadcast.

3. Final Answer:

   Use the injected server instance with this.server.emit('event', data) -> Option A

4. Quick Check:

   Broadcast = this.server.emit() [OK]

Hint: Broadcast with this.server.emit() inside gateway [OK]

Common Mistakes:

• Returning message instead of emitting
• Sending to one client only
• Misusing decorators for broadcasting