Setting up WebSocket server in Express - Try It Yourself

Setting up WebSocket server

📖 Scenario: You want to create a simple chat server that can send and receive messages instantly using WebSockets. This will allow multiple users to connect and chat in real time.

🎯 Goal: Build a basic WebSocket server using Express and the ws library that accepts client connections and can broadcast messages to all connected clients.

📋 What You'll Learn

Create an Express server

Integrate the ws WebSocket library

Set up a WebSocket server on top of the Express server

Broadcast messages received from one client to all connected clients

💡 Why This Matters

🌍 Real World

Real-time chat apps, live notifications, multiplayer games, and collaborative tools use WebSocket servers to instantly share data between users.

💼 Career

Understanding WebSocket setup with Express is valuable for backend developers building interactive web applications requiring live updates.

1

Create Express server

Write code to create an Express app and start a server listening on port 3000. Name the Express app app and the server server.

Express

const express = require('express');
const app = express();

// Your code here

Hint

Use express() to create the app and app.listen(3000) to start the server.

2

Import and configure WebSocket server

Import the ws library and create a WebSocket server named wss using new WebSocket.Server with the existing server as the HTTP server.

Express

const express = require('express');
const app = express();
const server = app.listen(3000);

// Your code here

Hint

Use require('ws') to import and pass { server } to the WebSocket.Server constructor.

3

Handle client connections and messages

Add an event listener on wss for the 'connection' event with a callback that receives a ws client. Inside, add a 'message' event listener on ws that receives message data.

Express

const express = require('express');
const app = express();
const server = app.listen(3000);

const WebSocket = require('ws');
const wss = new WebSocket.Server({ server });

// Your code here

Hint

Use wss.on('connection', callback) and inside it ws.on('message', callback).

4

Broadcast messages to all clients

Inside the 'message' event listener, add code to send the received message to every connected client using wss.clients.forEach. Only send if the client is open (client.readyState === WebSocket.OPEN).

Express

const express = require('express');
const app = express();
const server = app.listen(3000);

const WebSocket = require('ws');
const wss = new WebSocket.Server({ server });

wss.on('connection', (ws) => {
  ws.on('message', (message) => {
    // Your code here
  });
});

Hint

Loop over wss.clients and send the message only to clients with readyState === WebSocket.OPEN.

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