EventEmitter class in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this EventEmitter code?

Consider this Node.js code using EventEmitter. What will it print when run?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('greet', () => {
  console.log('Hello!');
});
emitter.emit('greet');
emitter.emit('greet');

A Hello!

B Hello!\nHello!

C No output

D Error: Event not found

💡 Hint

Think about how many times the 'greet' event is emitted and how many listeners are attached.

The 'greet' event listener runs each time the event is emitted. Since emit is called twice, 'Hello!' prints twice.

❓ state_output

intermediate

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

Look at this code snippet. What is the final value of the variable count?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();
let count = 0;
emitter.on('increment', () => {
  count += 1;
});
emitter.emit('increment');
emitter.emit('increment');
emitter.emit('increment');

A 0

B 1

C 3

D Error: count is not defined

💡 Hint

Each 'increment' event increases count by 1.

The listener adds 1 to count each time 'increment' is emitted. It is emitted 3 times, so count is 3.

📝 Syntax

advanced

Which option causes a syntax error in EventEmitter usage?

Which of these code snippets will cause a syntax error when using EventEmitter?

A emitter.on 'event', () => console.log('Hi')

B emitter.emit('event')

C emitter.on('event', () => console.log('Hi'))

D emitter.once('event', () => console.log('Hi'))

💡 Hint

Check the syntax for calling methods in JavaScript.

Option A misses parentheses around the arguments, causing a syntax error.

🔧 Debug

advanced

Why does this EventEmitter code not print anything?

This code does not print anything. What is the reason?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.emit('start');
emitter.on('start', () => {
  console.log('Started');
});

A The listener is added after the event is emitted, so it misses the event.

B The event name 'start' is invalid and ignored.

C emit() does not trigger listeners in EventEmitter.

D console.log is disabled by default in EventEmitter.

💡 Hint

Think about the order of adding listeners and emitting events.

Listeners must be added before emitting events to catch them. Here, emit runs before listener is added.

🧠 Conceptual

expert

What happens if an EventEmitter listener throws an error and there is no 'error' listener?

In Node.js EventEmitter, if a listener throws an error and no 'error' event listener is registered, what happens?

A The EventEmitter automatically retries the listener.

B The error is silently ignored and the program continues.

C The error is logged to console but the program continues.

D The process crashes with an uncaught exception.

💡 Hint

Consider Node.js default behavior for unhandled errors in EventEmitter.

If no 'error' listener is registered, an error thrown in a listener causes the Node.js process to crash.