Implement Custom EventEmitter (Pub/Sub Pattern)

Problem Statement

Implement a custom `EventEmitter` class supporting `on(event, listener)`, `off(event, listener)`, `emit(event, ...args)`, and `once(event, listener)`. Ensure unsubscribing works reliably and listeners do not leak.

Target Complexity

Time:

O(1) on, O(N) emit and off

Space:

O(N) listener storage

Interview Talking Points:

• Explain snapshot iteration (`[...listeners]`) to avoid infinite loops if a listener unsubscribes or subscribes inside its own callback.
• Discuss memory leak prevention via returning unsubscribe teardown callbacks.

Solution Editor (JavaScript)

Ready to run

Test Results

Subscribes, emits with arguments, and unsubscribes correctly

Test #1

once() listener fires exactly once then auto-unsubscribes

Test #2