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

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens after removing a listener?

Consider this Node.js EventEmitter code. What will be the output after the listener is removed and the event is emitted again?

Node.js

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

function greet() {
  console.log('Hello!');
}

emitter.on('sayHello', greet);
emitter.emit('sayHello');
emitter.removeListener('sayHello', greet);
emitter.emit('sayHello');

A (no output)\n(no output)

B Hello!\n(no output)

C (no output)\nHello!

D Hello!\nHello!

💡 Hint

Think about what happens when you remove a listener before emitting the event again.

The first emit triggers the listener and prints 'Hello!'. After removing the listener, the second emit has no listeners, so no output occurs.

📝 Syntax

intermediate

Identify the syntax error when removing a listener

Which option contains a syntax error when trying to remove a listener from an EventEmitter?

Node.js

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

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

emitter.on('data', onData);
emitter.removeListener('data', ???);

A emitter.removeListener('data', onData());

B emitter.removeListener('data', onData);

C emitter.removeListener('data', function onData() {});

D emitter.removeListener('data', 'onData');

💡 Hint

Remember that removeListener expects the exact function reference, not the result of calling it.

Option A calls onData() immediately and passes its return value (undefined) to removeListener, causing a runtime error or no removal. The others pass a function reference or invalid types but only A is a syntax/runtime misuse.

🔧 Debug

advanced

Why does the listener not get removed?

Given this code, why does the listener not get removed as expected?

Node.js

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

emitter.on('ping', () => console.log('Ping received'));
emitter.removeListener('ping', () => console.log('Ping received'));
emitter.emit('ping');

A Because the arrow functions are different instances, so removeListener does not match the original listener.

B Because removeListener requires the event name to be uppercase.

C Because the listener was never added due to a typo in the event name.

D Because removeListener only works with named functions, not arrow functions.

💡 Hint

Think about how JavaScript treats function expressions and references.

Each arrow function is a new function object. The listener passed to removeListener is not the same reference as the one added, so it does not remove the original listener.

❓ state_output

advanced

How many listeners remain after these operations?

After running this code, how many listeners remain for the 'update' event?

Node.js

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

function listener1() {}
function listener2() {}

emitter.on('update', listener1);
emitter.on('update', listener2);
emitter.removeListener('update', listener1);
emitter.removeAllListeners('update');
emitter.on('update', listener1);

A 2

B 0

C 1

D 3

💡 Hint

Remember what removeAllListeners does and what happens after it.

removeAllListeners('update') removes all listeners for 'update'. Then listener1 is added again, so only one listener remains.

🧠 Conceptual

expert

What error occurs when removing a listener with wrong function reference?

What error or behavior occurs if you call removeListener with a function that was never added as a listener?

Node.js

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

function foo() {}
function bar() {}

emitter.on('event', foo);
emitter.removeListener('event', bar);
emitter.emit('event');

A Removes all listeners for 'event' regardless of function.

B Throws a TypeError because 'bar' was never added.

C Throws a ReferenceError because 'bar' is not defined.

D No error; the original listener 'foo' remains and is called on emit.

💡 Hint

Consider what happens if you try to remove a listener that does not exist.

removeListener silently does nothing if the function is not found. The original listener remains and is called on emit.