Question 1

What is WebSocket?

Accepted Answer

WebSocket is a communication protocol providing full-duplex, bidirectional communication over a single TCP connection. Unlike HTTP's request-response model, WebSocket allows both client and server to send messages independently after the initial handshake.

Question 2

How does WebSocket differ from HTTP?

Accepted Answer

HTTP is request-response based where clients initiate all communication. WebSocket provides persistent, bidirectional connections where both parties can send data anytime. WebSocket has lower overhead after connection, making it ideal for real-time applications.

Question 3

What is the WebSocket handshake?

Accepted Answer

The WebSocket handshake starts as an HTTP request with Upgrade header. The server responds with 101 Switching Protocols, and the connection upgrades from HTTP to WebSocket. After handshake, both parties communicate using WebSocket frames.

Question 4

What is the difference between ws:// and wss://?

Accepted Answer

ws:// is unencrypted WebSocket (port 80), while wss:// is WebSocket over TLS/SSL (port 443). Always use wss:// in production for security—it encrypts data in transit and prevents man-in-the-middle attacks, similar to HTTPS vs HTTP.

Question 5

What are WebSocket ready states?

Accepted Answer

WebSocket has four ready states: CONNECTING (0) during handshake, OPEN (1) when connected and ready, CLOSING (2) during close handshake, and CLOSED (3) when connection is terminated. Check readyState before sending messages.

Question 6

What is the difference between WebSocket and Server-Sent Events?

Accepted Answer

WebSocket is bidirectional—both client and server can send messages. SSE is unidirectional—only server pushes to client. SSE uses HTTP, works with proxies better, and auto-reconnects. Choose WebSocket for chat apps, SSE for live feeds.

Question 7

What is Socket.IO?

Accepted Answer

Socket.IO is a library that enhances WebSocket with automatic reconnection, fallback transports (long-polling), rooms/namespaces, and broadcasting. It's not pure WebSocket—it adds its own protocol layer for reliability features.

Question 8

How do you handle WebSocket reconnection?

Accepted Answer

Implement automatic reconnection with exponential backoff: start with short delays (1s), increase after each failure (2s, 4s, 8s), cap at maximum (30s). Track connection state, queue messages during disconnection, and resync after reconnecting.

Question 9

What is WebSocket heartbeat/ping-pong?

Accepted Answer

Heartbeat is periodic messages to detect broken connections. The protocol has built-in ping/pong frames. Client or server sends ping, other responds with pong. If no response within timeout, connection is considered dead and should be closed.

Question 10

How do you scale WebSocket across servers?

Accepted Answer

Use a pub/sub system like Redis to broadcast messages between server instances. Each server subscribes to channels and forwards messages to connected clients. Sticky sessions or shared state ensure clients reconnect to servers with their data.

Question 11

How do you authenticate WebSocket connections?

Accepted Answer

Pass JWT token in query string or first message after connection. Validate token on server during handshake or immediately after. Can also use cookies if same-origin. Reject connections with invalid or missing credentials immediately.

Question 12

What are WebSocket close codes?

Accepted Answer

Close codes indicate why connection ended: 1000 (normal), 1001 (going away), 1002 (protocol error), 1003 (unsupported data), 1006 (abnormal—no close frame), 1011 (server error). Custom codes 4000-4999 available for application use.

Question 13

What are WebSocket rooms and channels?

Accepted Answer

Rooms are logical groups of connections for targeted broadcasting. Clients join rooms by topic/user ID. Server broadcasts to room members only. Implemented via Socket.IO rooms or manually with Maps tracking room memberships.

Question 14

How do you handle large WebSocket messages?

Accepted Answer

Large messages can be chunked into smaller frames—WebSocket supports fragmentation. Set maxPayload limits on server. For binary data, stream in chunks with sequence numbers. Consider compression (permessage-deflate extension) for text.

Question 15

How do you debug WebSocket connections?

Accepted Answer

Use browser DevTools Network tab (WS filter) to see frames. Chrome shows message content, timing, and close codes. Server-side, log connection events and messages. Tools like wscat test connections from command line.

WebSockets Interview Questions

Related Topics

WebSocket Fundamentals(5 questions)

What is WebSocket and how does it differ from HTTP?

What is the WebSocket handshake process?

What are the advantages of WebSocket over HTTP polling?

What is the difference between WebSocket and Server-Sent Events (SSE)?

What are WebSocket frames and message types?

Connection Lifecycle(5 questions)

How do you establish a WebSocket connection in JavaScript?

What are the different WebSocket ready states?

How do you handle WebSocket connection errors?

How do you properly close a WebSocket connection?

What are WebSocket close codes and when would you use them?

Message Handling(4 questions)

How do you send and receive messages over WebSocket?

What is the difference between text and binary WebSocket messages?

How do you handle large messages in WebSocket?

How do you implement message serialization and deserialization?

Scalability and Architecture(4 questions)

How do you scale WebSocket connections across multiple servers?

What is the pub/sub pattern in WebSocket architecture?

How do you handle WebSocket connections behind a load balancer?

What are WebSocket rooms and channels?

Reliability and Reconnection(4 questions)

How do you implement automatic reconnection for WebSocket?

What is heartbeat/ping-pong in WebSocket and why is it important?

How do you handle network interruptions gracefully?

How do you implement message acknowledgment and delivery guarantees?

Security(4 questions)

What are the security considerations for WebSocket connections?

How do you authenticate WebSocket connections?

What is the difference between ws:// and wss://?

How do you prevent WebSocket hijacking attacks?

Libraries and Implementation(5 questions)

What is Socket.IO and how does it enhance WebSocket?

What is the difference between native WebSocket and Socket.IO?

How do you implement WebSocket in Node.js?

What are the best practices for WebSocket error handling?

How do you debug WebSocket connections?