7. Event Emitter II - Once

Extend EventEmitter with once semantics while keeping dispatch deterministic under mutation.

This is an API-contract problem, not a syntax problem. Interviewers are testing whether you can preserve event ordering and correctness even when listeners are removed during emit.

Requirements

Implement:

class EventEmitter {
  on(event, handler) {}
  once(event, handler) {}
  off(event, handler) {}
  emit(event, ...args) {}
}

Contract

• on, once, and off must return this for chaining.
• once(event, handler) registers a listener that can run at most once.
• off(event, handler) removes all listeners (including once listeners) registered with that handler.
• emit(event, ...args) executes listeners in registration order and returns an array of return values in that same order.

Mutation Safety During Emit

emit must iterate over a snapshot of listeners for that event. If listeners are removed while dispatch is running (including once self-removal), all listeners that were scheduled at the start of the emit must still run exactly once in order.

Important Edge Cases

• A once listener must be removable via off(event, handler) before its first execution.
• Removing a listener during an active emit must not skip or reorder remaining listeners in that cycle.
• A once listener must never execute more than once, even under nested or re-entrant emits.

Example

const ee = new EventEmitter();

ee.on('sum', (a, b) => a + b);
ee.once('sum', (a, b) => a * b);

ee.emit('sum', 2, 3); // [5, 6]
ee.emit('sum', 2, 3); // [5]