Server-Sent Events alternative in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding WebSocket as an SSE Alternative

Which of the following best describes why WebSocket is often used as an alternative to Server-Sent Events (SSE) in Node.js applications?

A WebSocket allows two-way communication between client and server, unlike SSE which is one-way from server to client.

B WebSocket only supports text data, while SSE supports binary data.

C WebSocket requires less server resources because it uses HTTP/2, while SSE uses HTTP/1.1.

D WebSocket automatically retries connections without any client-side code, unlike SSE.

💡 Hint

Think about the direction of data flow and interaction between client and server.

WebSocket supports full-duplex communication, meaning both client and server can send messages independently. SSE only allows the server to push updates to the client.

❓ component_behavior

intermediate

Behavior of a WebSocket Server in Node.js

Given the following Node.js WebSocket server code snippet, what will happen when a client sends a message to the server?

Node.js

import { WebSocketServer } from 'ws';

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

wss.on('connection', ws => {
  ws.on('message', message => {
    ws.send(`Server received: ${message}`);
  });
});

A The server will broadcast the received message to all connected clients.

B The server will echo back the received message prefixed with 'Server received: ' to the same client.

C The server will close the connection after receiving the message.

D The server will ignore the message and not respond.

💡 Hint

Look at the ws.send call inside the message event handler.

The server listens for messages from a client and sends a response back only to that client with the message prefixed.

📝 Syntax

advanced

Correct WebSocket Client Connection Syntax

Which of the following Node.js client code snippets correctly establishes a WebSocket connection to ws://localhost:8080 and logs messages received from the server?

A

const ws = new WebSocket('ws://localhost:8080');
ws.on('message', function(data) { console.log(data); });

B

const ws = new WebSocket('ws://localhost:8080');
ws.addEventListener('message', event => console.log(event.data));

C

import WebSocket from 'ws';
const ws = new WebSocket('http://localhost:8080');
ws.onmessage = data => console.log(data);

D

import WebSocket from 'ws';
const ws = new WebSocket('ws://localhost:8080');
ws.on('message', data => console.log(`Received: ${data}`));

💡 Hint

Consider the correct import and event listener syntax for Node.js WebSocket client.

Option D correctly imports the WebSocket class from 'ws' and uses the 'on' method with 'message' event and a callback that logs the data.

🔧 Debug

advanced

Debugging WebSocket Server Connection Issue

A developer writes this WebSocket server code but clients cannot connect. What is the most likely cause?

Node.js

import { WebSocketServer } from 'ws';

const wss = new WebSocketServer({ port: 3000 });

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

A The server is listening on port 3000 but clients are connecting to port 8080.

B The WebSocketServer constructor requires a 'host' option to accept connections.

C The server must call wss.listen() to start listening for connections.

D The ws.send call should be inside a 'message' event handler.

💡 Hint

Check if the client connection URL matches the server port.

The server listens on port 3000 but if clients try to connect to a different port like 8080, connections will fail.

❓ state_output

expert

Output of WebSocket Server Broadcast Implementation

Consider this Node.js WebSocket server code that broadcasts messages to all clients except the sender. What will be logged on the server console after two clients connect and one sends 'Hello'?

Node.js

import { WebSocketServer } from 'ws';

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

wss.on('connection', ws => {
  console.log('Client connected');
  ws.on('message', message => {
    console.log(`Received: ${message}`);
    wss.clients.forEach(client => {
      if (client !== ws && client.readyState === 1) {
        client.send(`Broadcast: ${message}`);
      }
    });
  });
});

A

Client connected
Received: Hello
Client connected

B

Received: Hello
Client connected
Client connected

C

Client connected
Client connected
Received: Hello

D

Client connected
Client connected
Received: Broadcast: Hello

💡 Hint

Consider the order of events when clients connect and send messages.

Each client connection triggers 'Client connected' log. When a message is received, 'Received: Hello' is logged. Broadcast messages are sent to clients but not logged on server.