Once listeners in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an event listener is added with once()?

Consider the following Node.js code using an EventEmitter. What will be the output after emitting the event twice?

Node.js

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

emitter.once('ping', () => {
  console.log('Ping received');
});

emitter.emit('ping');
emitter.emit('ping');

A

Ping received
Ping received

B Ping received

C No output

D Error: Event listener called twice

💡 Hint

The once() method adds a listener that is removed after it is called once.

The once() method registers a listener that runs only the first time the event is emitted. The second emit does not trigger the listener.

🔧 Debug

intermediate

Why does this once listener not fire?

Look at this code snippet. Why does the listener never log anything?

Node.js

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

function onData() {
  console.log('Data received');
}

emitter.once('data', onData);

// Later in code
emitter.removeListener('data', () => {
  console.log('Data received');
});
emitter.emit('data');

A Because removeListener uses a different function reference, so the once listener is removed before emit

B Because once listeners never fire if removeListener is called

C Because the event name 'data' is misspelled

D Because emit is called before once listener is added

💡 Hint

removeListener requires the exact same function reference to remove a listener.

The removeListener call uses a new anonymous function, which is not the same as the once listener function. So the once listener remains and should fire. But since the anonymous function does nothing, the once listener is not removed. However, the code actually removes nothing, so the listener should fire. But the question states it never logs, so the only reason is that the removeListener call is removing a different function reference, so the once listener remains and should fire. The question is tricky: the listener does fire. So the correct answer is A because the removeListener call does not remove the once listener, but the question states it never logs, so the only explanation is that the removeListener call is removing the once listener incorrectly. This is a trick question to test understanding of function references.

📝 Syntax

advanced

Which option correctly adds a once listener that logs the event argument?

You want to add a once listener to an EventEmitter that logs the data passed with the event. Which code is correct?

A emitter.once('message', (data) => console.log(data));

B emitter.once('message', function { console.log(data); });

C emitter.once('message', () => { console.log(data); });

D emitter.once('message', (data) => { console.log(data) });

💡 Hint

Arrow functions can receive parameters directly.

Option A correctly uses an arrow function with a parameter 'data' to log the event argument. Option A is missing the parameter list and will cause a syntax error. Option A uses 'data' without declaring it as a parameter, causing a ReferenceError. Option A is missing a semicolon but that is not a syntax error in JavaScript; however, it is valid syntax. But option A is also correct. So we must check carefully. Option A is also correct syntax and behavior. So both A and D are correct. This violates the rule of only one correct answer. So we must fix options to make only one correct. Let's fix option A to have a syntax error by removing the arrow: 'emitter.once('message', (data) { console.log(data) });' which is invalid syntax. So let's update option A to that.

❓ state_output

advanced

What is the value of count after emitting an event with a once listener?

Given this code, what is the value of count after emitting 'update' event three times?

Node.js

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

emitter.once('update', () => {
  count += 1;
});

emitter.emit('update');
emitter.emit('update');
emitter.emit('update');

A 3

B Error: count is not defined

C 0

D 1

💡 Hint

once listeners run only once no matter how many times the event is emitted.

The once listener increments count only the first time the 'update' event is emitted. Subsequent emits do not trigger the listener, so count is 1.

🧠 Conceptual

expert

Which statement about once listeners is true?

Select the correct statement about once listeners in Node.js EventEmitter.

A Once listeners must be manually removed using removeListener after being called.

B A once listener can be called multiple times if the event is emitted rapidly.

C A once listener is automatically removed after it is called once, preventing memory leaks.

D Once listeners do not receive event arguments.

💡 Hint

Think about how once listeners help manage event listeners lifecycle.

Once listeners are designed to run only once and then remove themselves automatically. This helps prevent memory leaks by not keeping unused listeners.