Custom event emitter classes in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this custom event emitter code?

Consider this Node.js custom event emitter class. What will be printed when the code runs?

Node.js

import { EventEmitter } from 'node:events';

class MyEmitter extends EventEmitter {
  constructor() {
    super();
    this.on('ping', () => {
      console.log('pong');
    });
  }
}

const emitter = new MyEmitter();
emitter.emit('ping');

A ping

B undefined

C pong

D Error: event not handled

💡 Hint

Think about what happens when the 'ping' event is emitted and how the listener responds.

The 'ping' event triggers the listener which logs 'pong' to the console.

📝 Syntax

intermediate

Which option correctly adds a listener to a custom event emitter?

Given a custom event emitter instance, which code snippet correctly adds a listener for the 'data' event?

Node.js

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();

A emitter.on('data', () => console.log('received'));

B emitter.addListener('data', console.log('received'));

C emitter.listen('data', () => console.log('received'));

D emitter.on('data', console.log('received'));

💡 Hint

Remember the standard method to add event listeners in Node.js EventEmitter.

The correct method is on with a callback function. Option A uses this correctly.

🔧 Debug

advanced

Why does this custom event emitter code throw an error?

Examine the code below. Why does it throw an error when emitting the 'start' event?

Node.js

import { EventEmitter } from 'node:events';

class MyEmitter extends EventEmitter {
  constructor() {
    super();
  }
}

const emitter = new MyEmitter();
emitter.emit('start');

A The constructor is missing a call to super(), causing an error.

B No listener is registered for 'start', so emit returns false but no error is thrown.

C The event name 'start' is invalid and causes an error.

D emit throws an error if no listeners exist for the event.

💡 Hint

Think about what happens when an event is emitted but no listeners are attached.

Emitting an event with no listeners returns false but does not throw an error.

❓ state_output

advanced

What is the value of count after emitting events?

What is the value of count after the code runs?

Node.js

import { EventEmitter } from 'node:events';

class Counter extends EventEmitter {
  constructor() {
    super();
    this.count = 0;
    this.on('increment', () => {
      this.count += 1;
    });
  }
}

const counter = new Counter();
counter.emit('increment');
counter.emit('increment');
counter.emit('increment');

const count = counter.count;

A 3

B 0

C undefined

D Error: count is not defined

💡 Hint

Each 'increment' event increases count by 1. How many times is it emitted?

The 'increment' event is emitted 3 times, so count increases from 0 to 3.

🧠 Conceptual

expert

Which option best explains why custom event emitters improve Node.js apps?

Why do developers use custom event emitter classes in Node.js applications?

A They enforce strict type checking on event names and listener arguments.

B They automatically optimize CPU usage by managing event loops internally.

C They replace the need for callbacks and promises entirely in asynchronous code.

D They allow decoupling of code by letting parts communicate via events without direct calls.

💡 Hint

Think about how events help different parts of an app work together without tight connections.

Custom event emitters let different parts of an app communicate by sending and listening for events, reducing direct dependencies.