# WebSockets (Node.js / NestJS)

> Socket.io, gateways, events, rooms, namespaces, authentication, broadcasting

- 20 interview questions
- Mid-Level
- [Interview Questions: Node.js / NestJS](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions.md)

## 1. What is a WebSocket Gateway in NestJS?

**Answer**

A WebSocket Gateway is a class decorated with @WebSocketGateway() that handles WebSocket connections and real-time events. It acts as an entry point for bidirectional communication between server and clients. Gateways use Socket.io by default and allow handling incoming and outgoing events via decorators like @SubscribeMessage().

## 2. Which library does NestJS use by default for WebSockets?

**Answer**

NestJS uses Socket.io by default for WebSockets. Socket.io provides advanced features like rooms, namespaces, automatic reconnection and long-polling fallback. It's possible to use ws (native WebSocket) by specifying the adapter, but Socket.io remains the recommended choice for most use cases.

## 3. How to listen to a WebSocket event in a NestJS Gateway?

**Answer**

To listen to an event, use the @SubscribeMessage('eventName') decorator on a Gateway method. The method automatically receives data emitted by the client. The decorator handles event registration and allows returning a response to the client or broadcasting to multiple clients.

## 17 more questions available

- Which lifecycle method allows executing code when a client connects to the Gateway?
- What is a Room in Socket.io?

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