Setting up WebSocket server in Express - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the WebSocket library.

Express

const WebSocket = require('[1]');

Drag options to blanks, or click blank then click option'

Aws

Bhttp

Cexpress

Dsocket.io

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'express' instead of 'ws' for WebSocket import.

Using 'socket.io' which is a different library.

The WebSocket server library is called ws. We import it using require('ws').

2fill in blank

medium

Complete the code to create a WebSocket server on top of an existing HTTP server.

Express

const wss = new WebSocket.Server({ [1] });

Drag options to blanks, or click blank then click option'

Ahost: 'localhost'

Bserver: httpServer

Cport: 8080

Dpath: '/ws'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 'port' instead of 'server' when attaching to an HTTP server.

Using 'host' or 'path' options incorrectly here.

To attach a WebSocket server to an existing HTTP server, use the server option with the HTTP server instance.

3fill in blank

hard

Fix the error in the WebSocket connection event handler to correctly receive the client socket.

Express

wss.on('connection', function([1]) {
  console.log('Client connected');
});

Drag options to blanks, or click blank then click option'

Aclient

BwsServer

Csocket

Dconnection

Attempts:

3 left

💡 Hint

Common Mistakes

Using the server object as the parameter instead of the client socket.

Naming the parameter 'connection' which is the event name, not the argument.

The event handler receives the client socket as the first argument, commonly named socket.

4fill in blank

hard

Fill both blanks to send a message to the client when a connection is established.

Express

wss.on('connection', function(socket) {
  socket.[1]('[2]');
});

Drag options to blanks, or click blank then click option'

Asend

Bemit

Cmessage

DHello, client!

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'emit' which is from other libraries like socket.io.

Passing an event name instead of a message string.

Use the send method on the client socket to send a message string.

5fill in blank

hard

Fill all three blanks to handle incoming messages and log them.

Express

wss.on('connection', function(socket) {
  socket.on('[1]', function([2]) {
    console.log('Received:', [3]);
  });
});

Drag options to blanks, or click blank then click option'

Amessage

Bdata

Dconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'connect' event instead of 'message'.

Using wrong parameter names or forgetting to log the data.

The event to listen for incoming messages is message. The callback receives the message data, commonly named data.

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