Events vs callbacks decision in Node.js - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

When to prefer events over callbacks?

In Node.js, which scenario best justifies using events instead of callbacks?

A When you want to execute a single asynchronous task and get its result once.

B When you need to handle multiple asynchronous actions that can happen at different times independently.

C When you want to block the main thread until a task finishes.

D When you want to write synchronous code without any asynchronous behavior.

💡 Hint

Think about how events allow multiple listeners and callbacks handle one response.

Events are useful when multiple things can happen over time and you want to react to each occurrence separately. Callbacks are simpler for one-time responses.

❓ component_behavior

intermediate

Output of event emitter with multiple listeners

What will be printed when this Node.js code runs?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('ping', () => console.log('First listener'));
emitter.on('ping', () => console.log('Second listener'));
emitter.emit('ping');

A No output

B First listener

C Second listener

D First listener\nSecond listener

💡 Hint

Remember that multiple listeners can respond to the same event.

Both listeners are registered for the 'ping' event, so emitting 'ping' calls both callbacks in order.

📝 Syntax

advanced

Identify the error in callback usage

What error does this code produce?

Node.js

function fetchData(callback) {
  setTimeout(() => {
    callback(null, 'data');
  }, 100);
}

fetchData((error, result) => {
  if (error) {
    console.log('Error:', error);
  } else {
    console.log('Result:', result);
  }
});

fetchData('not a function');

A TypeError: callback is not a function

B SyntaxError: Unexpected token

C No error, prints 'Result: data' twice

D ReferenceError: callback is not defined

💡 Hint

Check what happens when a non-function is passed as callback.

The second call passes a string instead of a function, so when setTimeout tries to call it, a TypeError occurs.

❓ state_output

advanced

State after event and callback usage

What is the value of the variable count after this code runs?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();
let count = 0;

emitter.on('increment', () => { count += 1; });

function doWork(callback) {
  emitter.emit('increment');
  callback();
}

doWork(() => { count += 10; });

A 11

B 10

C 0

D 1

💡 Hint

Consider the order of event emission and callback execution.

The event listener increments count by 1, then the callback adds 10, total 11.

🔧 Debug

expert

Why does the listener not run on the second emit?

Given this code, why does the second `emitter.emit('data')` not print anything?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();

emitter.once('data', () => console.log('Data received'));

emitter.emit('data');
emitter.emit('data');

A The listener is removed before any emit happens.

B The listener is never registered because of a syntax error.

C The listener runs only once, so the second emit does nothing.

D The event name is misspelled in the emit calls.

💡 Hint

Check how the 'once' method works for event listeners.

The 'once' method registers a listener that runs only the first time the event is emitted. The second emit triggers no listener.