Broadcasting to connected clients in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when broadcasting a message to all clients?

Consider a Node.js server using WebSocket. When the server broadcasts a message to all connected clients, what will each client receive?

Node.js

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

wss.on('connection', function connection(ws) {
  ws.on('message', function incoming(message) {
    wss.clients.forEach(function each(client) {
      if (client.readyState === WebSocket.OPEN) {
        client.send(message);
      }
    });
  });
});

A No clients receive any message because broadcast is not implemented.

B Only the client that sent the message receives it back.

C Only clients connected after the message is sent receive it.

D All connected clients receive the exact same message sent by one client.

💡 Hint

Think about how the server loops through all clients and sends the message.

The server loops through all connected clients and sends the received message to each one that is open. This means every client connected at that moment gets the message.

📝 Syntax

intermediate

Identify the syntax error in broadcasting code

Which option contains a syntax error that prevents broadcasting messages to all clients?

Node.js

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

wss.on('connection', ws => {
  ws.on('message', message => {
    wss.clients.forEach(client => {
      if (client.readyState === WebSocket.OPEN) {
        client.send(message)
      }
    });
  });
});

A Missing closing parenthesis for wss.on('connection'...)

B Using parentheses instead of curly braces in forEach callback

C Missing semicolon after client.send(message)

D Using assignment operator '=' instead of comparison '===' in if condition

💡 Hint

Check the if condition inside the forEach loop carefully.

Missing semicolon after client.send(message) can cause issues in some JavaScript environments.

🔧 Debug

advanced

Why does broadcasting fail when clients disconnect?

Given this broadcasting code, why might some clients not receive messages after disconnecting and reconnecting?

Node.js

wss.on('connection', ws => {
  ws.on('message', message => {
    wss.clients.forEach(client => {
      if (client.readyState === WebSocket.OPEN) {
        client.send(message);
      }
    });
  });
});

A The server does not remove closed clients from wss.clients, so readyState check fails.

B Clients reconnecting get a new WebSocket instance not yet in wss.clients.

C wss.clients only contains clients connected at server start, not new ones.

D Disconnected clients remain in wss.clients, causing errors when sending.

💡 Hint

Think about how the server tracks connected clients and their states.

The server's wss.clients set includes all clients, but some may be closed. The readyState check prevents sending to closed clients, but if the server does not clean up properly, it may cause issues.

❓ state_output

advanced

What is the state of clients after broadcasting a message?

After broadcasting a message to all clients, what is the state of each client's WebSocket connection?

Node.js

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

A All clients remain OPEN and receive the message.

B Clients change state to CLOSING after receiving the message.

C Clients become CLOSED immediately after message is sent.

D Clients' state is UNKNOWN after sending the message.

💡 Hint

Sending a message does not close the connection.

Sending a message over WebSocket does not change the connection state. Clients remain OPEN unless explicitly closed.

🧠 Conceptual

expert

Which option best describes broadcasting scalability challenges?

When broadcasting messages to many connected clients in Node.js WebSocket server, what is a key scalability challenge?

A Broadcasting automatically queues messages on clients, so no server load increase occurs.

B Broadcasting to many clients can cause high CPU and memory usage due to many simultaneous sends.

C WebSocket protocol limits the number of clients to 100, so scalability is not an issue.

D Broadcasting messages is handled by the browser, so server resources are unaffected.

💡 Hint

Consider what happens on the server when sending many messages at once.

Sending messages to many clients simultaneously can increase CPU and memory usage on the server, potentially causing delays or crashes if not managed properly.