Setting up WebSocket server in Express

Introduction

WebSocket lets your server and browser talk instantly without waiting. It helps build real-time apps like chats or live updates.

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

You need live notifications on a website without refreshing the page.

You want to show real-time data like stock prices or sports scores.

You want a game where players see each other's moves immediately.

Syntax

Express

const express = require('express');
const http = require('http');
const WebSocket = require('ws');

const app = express();
const server = http.createServer(app);
const wss = new WebSocket.Server({ server });

wss.on('connection', (ws) => {
  ws.on('message', (message) => {
    // handle incoming message
  });
  ws.send('Welcome!');
});

server.listen(3000);

Use http.createServer to create a server for both HTTP and WebSocket.

The WebSocket.Server listens on the same server to handle WebSocket connections.

Examples

This creates a WebSocket server that listens on port 8080 directly, without Express HTTP server.

Express

const wss = new WebSocket.Server({ port: 8080 });

Sends a welcome message to each client when they connect.

Express

wss.on('connection', (ws) => {
  ws.send('Hello client!');
});

Logs any message received from a client.

Express

ws.on('message', (msg) => {
  console.log('Received:', msg);
});

Sample Program

This code sets up an Express server with a WebSocket server on the same port. When a client connects, it sends a welcome message. It also listens for messages from the client, logs them, and replies back with the same message.

Express

const express = require('express');
const http = require('http');
const WebSocket = require('ws');

const app = express();
const server = http.createServer(app);
const wss = new WebSocket.Server({ server });

app.get('/', (req, res) => {
  res.send('WebSocket server is running');
});

wss.on('connection', (ws) => {
  ws.send('Welcome to WebSocket server!');

  ws.on('message', (message) => {
    console.log('Received:', message);
    ws.send(`You said: ${message}`);
  });
});

server.listen(3000, () => {
  console.log('Server started on http://localhost:3000');
});

Important Notes

WebSocket connections stay open, unlike normal HTTP requests that close after response.

Always handle errors and client disconnects to keep your server stable.

Summary

WebSocket server lets your app send and receive messages instantly.

Use http.createServer with Express to share one server for HTTP and WebSocket.

Listen for 'connection' and 'message' events to interact with clients.

Practice

1. What is the main purpose of setting up a WebSocket server with Express?

easy

A. To enable real-time two-way communication between client and server

B. To serve static files like images and CSS

C. To handle database queries asynchronously

D. To render HTML templates on the server

Solution

1. Step 1: Understand WebSocket functionality

   WebSocket allows instant two-way communication between client and server, unlike HTTP which is one-way per request.

2. Step 2: Identify Express role

   Express is used to create the HTTP server, but WebSocket adds real-time messaging capability on top.

3. Final Answer:

   To enable real-time two-way communication between client and server -> Option A

4. Quick Check:

   WebSocket = real-time two-way communication [OK]

Hint: WebSocket means instant two-way chat between client and server [OK]

Common Mistakes:

• Confusing WebSocket with static file serving
• Thinking WebSocket handles database queries
• Assuming WebSocket renders HTML

2. Which code snippet correctly creates an HTTP server to use with WebSocket in Express?

easy

A. const server = http.createServer(app);

B. const server = express();

C. const server = new WebSocket.Server(app);

D. const server = express.createServer();

Solution

1. Step 1: Recognize Express app and HTTP server creation

   Express app is created by calling express(), but to use WebSocket, you need an HTTP server wrapping the app.

2. Step 2: Identify correct server creation method

   http.createServer(app) creates an HTTP server that can be shared with WebSocket.

3. Final Answer:

   const server = http.createServer(app); -> Option A

4. Quick Check:

   Use http.createServer(app) for WebSocket [OK]

Hint: Use http.createServer(app) to share server with WebSocket [OK]

Common Mistakes:

• Using express.createServer() which does not exist
• Creating WebSocket.Server with app directly
• Assigning express() to server without HTTP wrapper

3. Given this code snippet, what will be logged when a client sends the message 'Hello'?

const WebSocket = require('ws');
const http = require('http');
const express = require('express');
const app = express();
const server = http.createServer(app);
const wss = new WebSocket.Server({ server });
wss.on('connection', ws => {
  ws.on('message', message => {
    console.log(`Received: ${message}`);
  });
});
server.listen(3000);

medium

A. No output

B. message

C. Error: ws.on is not a function

D. Received: Hello

Solution

1. Step 1: Understand event listeners setup

   The server listens for 'connection' events, then for each connection listens for 'message' events.

2. Step 2: Analyze message event handler

   When a message arrives, it logs 'Received: ' plus the message content.

3. Final Answer:

   Received: Hello -> Option D

4. Quick Check:

   Message event logs 'Received: message' [OK]

Hint: Message event logs 'Received: ' plus message text [OK]

Common Mistakes:

• Expecting raw 'message' string without prefix
• Confusing event names or handlers
• Thinking ws.on is undefined

4. Identify the error in this WebSocket server setup code:

const WebSocket = require('ws');
const express = require('express');
const app = express();
const server = http.createServer();
const wss = new WebSocket.Server({ server });
server.listen(3000);

medium

A. WebSocket.Server should not receive server option

B. http module is not imported

C. express() should be passed to WebSocket.Server

D. server.listen should be called before creating WebSocket.Server

Solution

1. Step 1: Check module imports

   The code uses http.createServer() but does not import the 'http' module, causing a ReferenceError.

2. Step 2: Verify server creation and usage

   http.createServer() requires the http module; without import, server is undefined.

3. Final Answer:

   http module is not imported -> Option B

4. Quick Check:

   Missing http import causes error [OK]

Hint: Always import http before using http.createServer() [OK]

Common Mistakes:

• Forgetting to import http module
• Passing express app directly to WebSocket.Server
• Calling listen before server creation

5. You want to broadcast a message to all connected WebSocket clients when one client sends a message. Which code snippet correctly implements this inside the 'connection' event handler?

wss.on('connection', ws => {
  ws.on('message', message => {
    // Broadcast message to all clients
    ???
  });
});

hard

A. ws.clients.forEach(client => client.send(message));

B. wss.broadcast(message);

C. wss.clients.forEach(client => { if(client.readyState === WebSocket.OPEN) client.send(message); });

D. server.clients.forEach(client => client.send(message));

Solution

1. Step 1: Identify how to access all clients

   The WebSocket.Server instance 'wss' has a 'clients' set containing all connected clients.

2. Step 2: Filter clients by open connection and send message

   Only clients with readyState === WebSocket.OPEN can receive messages safely.

3. Final Answer:

   wss.clients.forEach(client => { if(client.readyState === WebSocket.OPEN) client.send(message); }); -> Option C

4. Quick Check:

   Broadcast uses wss.clients with readyState check [OK]

Hint: Use wss.clients and check readyState before sending [OK]

Common Mistakes:

• Using ws.clients which does not exist
• Calling non-existent broadcast method
• Accessing server.clients which is undefined