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.

Practice

1. What is the main purpose of the EventEmitter class in Node.js?

easy

A. To manage file system operations

B. To handle HTTP requests and responses

C. To allow parts of a program to send and listen for events

D. To create and manage database connections

Solution

1. Step 1: Understand the role of EventEmitter

   The EventEmitter class is designed to let different parts of a program communicate by sending and listening for events.

2. Step 2: Compare with other options

   Options B, C, and D relate to other Node.js modules like HTTP, FS, and database modules, not EventEmitter.

3. Final Answer:

   To allow parts of a program to send and listen for events -> Option C

4. Quick Check:

   EventEmitter = event communication [OK]

Hint: EventEmitter is about events, not HTTP or files [OK]

Common Mistakes:

• Confusing EventEmitter with HTTP or file system modules
• Thinking EventEmitter manages databases
• Assuming EventEmitter runs code synchronously

2. Which of the following is the correct way to listen for an event named data using an EventEmitter instance called emitter?

easy

A. emitter.on('data', callback)

B. emitter.listen('data', callback)

C. emitter.emit('data', callback)

D. emitter.addEvent('data', callback)

Solution

1. Step 1: Identify the method to listen for events

   The on method is used to register a callback to listen for a named event.

2. Step 2: Check other methods

   emit sends events, not listens. listen and addEvent are not valid EventEmitter methods.

3. Final Answer:

   emitter.on('data', callback) -> Option A

4. Quick Check:

   Listen with on() = emitter.on('data', callback) [OK]

Hint: Use on() to listen, emit() to send events [OK]

Common Mistakes:

• Using emit() to listen instead of send
• Using non-existent methods like listen()
• Confusing method names for event handling

3. What will be the output of the following code?

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

medium

A. greet Alice

B. No output

C. Error: greet event not found

D. Hello, Alice!

Solution

1. Step 1: Understand event registration

   The code registers a listener for the 'greet' event that prints a greeting with the given name.

2. Step 2: Understand event emission

   When emit('greet', 'Alice') runs, it calls the listener with 'Alice', printing "Hello, Alice!".

3. Final Answer:

   Hello, Alice! -> Option D

4. Quick Check:

   emit triggers on() callback [OK]

Hint: emit calls on() listeners with arguments [OK]

Common Mistakes:

• Expecting no output if event name is wrong
• Confusing emit with on
• Thinking emit returns a value

4. Identify the error in this code snippet:

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

medium

A. The event name 'start' is invalid

B. The event listener is registered after emitting the event

C. emit() requires a callback function as second argument

D. The EventEmitter class is not imported correctly

Solution

1. Step 1: Check event listener registration timing

   The listener for 'start' is added after the event is emitted, so it misses the event.

2. Step 2: Verify other options

   EventEmitter is imported correctly, emit() does not require a callback, and 'start' is a valid event name.

3. Final Answer:

   The event listener is registered after emitting the event -> Option B

4. Quick Check:

   Listeners must be added before emit() [OK]

Hint: Add listeners before emitting events [OK]

Common Mistakes:

• Emitting before listener registration
• Thinking emit needs a callback argument
• Assuming event names have restrictions

5. You want to create an EventEmitter that counts how many times an event ping is emitted and logs the count each time. Which code correctly implements this?

hard

A. const EventEmitter = require('events');
const emitter = new EventEmitter();
let count = 0;
emitter.on('ping', () => {
count++;
console.log(`Ping count: ${count}`);
});
emitter.emit('ping');
emitter.emit('ping');

B. const EventEmitter = require('events');
const emitter = new EventEmitter();
let count = 0;
emitter.emit('ping', () => {
count++;
console.log(`Ping count: ${count}`);
});

C. const EventEmitter = require('events');
const emitter = new EventEmitter();
let count = 0;
emitter.on('ping', count++);
emitter.emit('ping');

D. const EventEmitter = require('events');
const emitter = new EventEmitter();
let count = 0;
emitter.on('ping', () => count++);
emitter.emit('ping');
console.log(`Ping count: ${count}`);

Solution

1. Step 1: Understand event counting logic

   We need to increase count and log it inside the event listener each time 'ping' is emitted.

2. Step 2: Analyze each option

   const EventEmitter = require('events'); const emitter = new EventEmitter(); let count = 0; emitter.on('ping', () => { count++; console.log(`Ping count: ${count}`); }); emitter.emit('ping'); emitter.emit('ping'); correctly registers a listener that increments and logs count on each 'ping'. const EventEmitter = require('events'); const emitter = new EventEmitter(); let count = 0; emitter.emit('ping', () => { count++; console.log(`Ping count: ${count}`); }); wrongly uses emit with a callback. const EventEmitter = require('events'); const emitter = new EventEmitter(); let count = 0; emitter.on('ping', count++); emitter.emit('ping'); passes count++ directly, which is incorrect. const EventEmitter = require('events'); const emitter = new EventEmitter(); let count = 0; emitter.on('ping', () => count++); emitter.emit('ping'); console.log(`Ping count: ${count}`); increments count but logs outside the event, so only logs once.

3. Final Answer:

   Option A code correctly counts and logs on each ping event -> Option A

4. Quick Check:

   Increment and log inside on() callback [OK]

Hint: Increment count inside on() callback, not outside [OK]

Common Mistakes:

• Passing callback to emit() instead of on()
• Incrementing count outside event listener
• Using count++ directly as listener instead of function