WebSockets

TL;DR

WebSockets provide a way to open an interactive, two-way communication session between a user’s browser and a server. Unlike HTTP, where the client must request data, WebSockets allow the server to push real-time updates to the client.

Mental Model

How It Works

1. The Handshake: The connection starts as a standard HTTP GET request with specific headers (Upgrade: websocket).
2. The Upgrade: If the server supports WebSockets, it responds with an HTTP 101 Switching Protocols status.
3. The Connection: The original TCP connection is kept alive indefinitely. Both parties can send lightweight “frames” of data back and forth without the heavy overhead of HTTP headers.

Node.js does not have a built-in WebSocket module. You must use popular libraries like ws or socket.io.

Example (using ws library)

// npm install ws
const { WebSocketServer } = require('ws');

const wss = new WebSocketServer({ port: 8080 });

wss.on('connection', (ws) => {
  console.log('New client connected!');

  // Listen for messages from this client
  ws.on('message', (data) => {
    console.log('Received: %s', data);
    
    // Echo it back
    ws.send(`Server received: ${data}`);
  });

  // Push a message to the client independently
  ws.send('Welcome to the WebSocket server');
});

Common Interview Questions

WebSockets vs Server-Sent Events (SSE)?

• WebSockets are bi-directional (full-duplex). Ideal for chat apps or multiplayer games.
• Server-Sent Events (SSE) are uni-directional (server to client only), operating over standard HTTP. Ideal for live news feeds or simple real-time dashboards where the client doesn’t need to send rapid data back.

What is Socket.io and why use it over raw WebSockets (ws)?

Socket.io is a library that wraps WebSockets. It provides automatic reconnections, broadcasting to “rooms”, and fallback polling to standard HTTP if the network blocks WebSockets. Raw ws provides none of these but is faster and lighter.