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.

Practice

1. What happens when you remove an event listener in Node.js using removeListener or off?

easy

A. The function will no longer run when the event occurs.

B. The function runs twice when the event occurs.

C. The event stops emitting completely.

D. The listener is paused but still runs once.

Solution

1. Step 1: Understand event listeners in Node.js

   Listeners are functions that run when an event happens.

2. Step 2: Effect of removing a listener

   Removing a listener stops that function from running on the event.

3. Final Answer:

   The function will no longer run when the event occurs. -> Option A

4. Quick Check:

   Removing listener = function stops running [OK]

Hint: Removing listeners stops their function from running on events [OK]

Common Mistakes:

• Thinking the event stops emitting entirely
• Believing the listener runs once after removal
• Confusing removeListener with pausing the listener

2. Which of the following is the correct syntax to remove a listener named myFunc from an event emitter emitter?

easy

A. emitter.removeListener(myFunc);

B. emitter.removeListener('event', myFunc);

C. emitter.removeListener('event');

D. emitter.removeListener('event', myFunc());

Solution

1. Step 1: Identify correct method usage

   The removeListener method requires the event name and the exact function reference.

2. Step 2: Check each option

   emitter.removeListener('event', myFunc); correctly passes the event name and function reference without calling it.

3. Final Answer:

   emitter.removeListener('event', myFunc); -> Option B

4. Quick Check:

   Correct syntax = emitter.removeListener('event', myFunc); [OK]

Hint: Pass event name and function reference without parentheses [OK]

Common Mistakes:

• Calling the function instead of passing reference
• Omitting the event name
• Passing only the function without event name

3. Consider this code snippet:

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

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

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

What will be printed when this code runs?

medium

A. Hello!

B. Error: greet is not defined

C. Nothing is printed

D. Hello! Hello!

Solution

1. Step 1: Add and then remove the listener

   The listener greet is added then removed before emitting.

2. Step 2: Emit event after removal

   Since the listener was removed, no function runs on emit.

3. Final Answer:

   Nothing is printed -> Option C

4. Quick Check:

   Removed listener means no output [OK]

Hint: Removed listener means no function runs on emit [OK]

Common Mistakes:

• Expecting 'Hello!' to print after removal
• Thinking emit throws error if no listeners
• Assuming multiple prints without multiple listeners

4. What is wrong with this code if the listener is not removed as expected?

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

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

medium

A. Listeners cannot be removed once added.

B. The event name 'data' is misspelled in removeListener.

C. removeListener method does not exist on EventEmitter.

D. The function passed to removeListener is a different reference than the original listener.

Solution

1. Step 1: Check function references

   The listener added is an anonymous function; removeListener uses a new anonymous function, which is a different reference.

2. Step 2: Understand listener removal requirement

   To remove a listener, the exact same function reference must be passed.

3. Final Answer:

   The function passed to removeListener is a different reference than the original listener. -> Option D

4. Quick Check:

   Same function reference needed to remove listener [OK]

Hint: Use same function reference to remove listener [OK]

Common Mistakes:

• Passing a new anonymous function to removeListener
• Assuming removeListener removes all listeners for event
• Thinking removeListener method is missing

5. You have multiple listeners on an event 'update'. You want to remove only the listener logUpdate but keep others. Which approach correctly removes only logUpdate?

hard

A. emitter.off('update', logUpdate);

B. emitter.removeAllListeners('update');

C. emitter.removeListener('update');

D. emitter.off(logUpdate);

Solution

1. Step 1: Understand removing specific listeners

   To remove one listener, use off or removeListener with event name and function reference.

2. Step 2: Analyze options

   emitter.off('update', logUpdate); correctly calls off with event and function. emitter.removeAllListeners('update'); removes all listeners, which is not desired.

3. Final Answer:

   emitter.off('update', logUpdate); -> Option A

4. Quick Check:

   Remove specific listener with off(event, func) [OK]

Hint: Use off or removeListener with event and function to remove one listener [OK]

Common Mistakes:

• Using removeAllListeners which removes all listeners
• Calling removeListener without function reference
• Passing only function without event name