Build an event emitter

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Build an event emitter · UIHero

Build an event emitter

Create isolated ordered event channels with on, off, once, and unsubscribe behavior.

MediumJavaScript Design Patterns

55 min0% acceptance

Description

Implement createEventEmitter with independent event channels. Listeners run in subscription order, once listeners remove themselves, on returns an unsubscribe function, and listener mutation during emit cannot corrupt the active dispatch.

Requirements

The solution exports the required createEventEmitter API from the editable module.
Listener order works deterministically for the documented normal, boundary, and repeated-use scenarios.
Off works deterministically for the documented normal, boundary, and repeated-use scenarios.
Once works deterministically for the documented normal, boundary, and repeated-use scenarios.
Unsubscribe works deterministically for the documented normal, boundary, and repeated-use scenarios.
Mutation during emit works deterministically for the documented normal, boundary, and repeated-use scenarios.
Event isolation works deterministically for the documented normal, boundary, and repeated-use scenarios.

Mutation while emitting

Input

The first listener removes the second listener during an emit

Expected output

The current dispatch remains deterministic and later emits use the updated list

Explanation

Dispatching from a snapshot separates current iteration from future subscriptions.

Constraints

Preserve subscription order
Removing an unknown listener is a no-op
Do not expose the internal listener collections

Hints

Hint 1

Map each event name to its own ordered collection.

Hint 2

Iterate over a snapshot so unsubscribe calls cannot skip or duplicate current listeners.

Hint 3

Wrap once listeners while retaining enough identity to remove them through off.

eventspub-subcollections

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