Once listeners in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a 'once' listener in Node.js EventEmitter?

A 'once' listener is an event handler that runs only one time when the event is emitted, then it automatically removes itself.

beginner

How do you add a 'once' listener to an event in Node.js?

Use the once method on an EventEmitter instance, like emitter.once('eventName', callback).

intermediate

Why use a 'once' listener instead of a regular listener?

To handle an event only the first time it happens, avoiding repeated calls and automatically cleaning up the listener.

beginner

What happens if you emit an event multiple times but have a 'once' listener attached?

The 'once' listener runs only on the first emit. Subsequent emits do not trigger it because it removes itself after the first call.

intermediate

Can you remove a 'once' listener before it fires? How?

Yes, by using emitter.off('eventName', listenerFunction) or emitter.removeListener('eventName', listenerFunction) before the event is emitted.

Which method adds a listener that runs only once in Node.js EventEmitter?

A addListener()

B once()

C on()

D emit()

The once() method adds a listener that is called only the first time the event is emitted.

What happens to a 'once' listener after it is triggered?

A It is removed automatically

B It stays active for future events

C It throws an error

D It duplicates itself

A 'once' listener removes itself automatically after being called once.

How can you remove a 'once' listener before it fires?

A Use emitter.emit()

B You cannot remove it

C Use emitter.off() or emitter.removeListener() with the listener function

D Use emitter.once() again

You can remove any listener, including 'once' listeners, by calling off() or removeListener() with the exact listener function.

If an event is emitted three times but has a 'once' listener, how many times does the listener run?

A 1 time

B 2 times

C 3 times

D 0 times

The 'once' listener runs only once on the first event emission.

Which of these is NOT true about 'once' listeners?

A They run only once

B They automatically remove themselves

C They can be removed before firing

D They can be reused multiple times without re-adding

'Once' listeners do not run multiple times unless re-added explicitly.

Explain what a 'once' listener is and why you might use it in a Node.js application.

Think about events you want to handle only the first time they happen.

Describe how to add and remove a 'once' listener on an EventEmitter instance.

Focus on the methods and parameters involved.