7. Event Emitter II

Implement an EventEmitter class with on, off, once, and emit methods.

This version builds on the basic EventEmitter by adding:

1) once(event, handler) — handler runs at most once.

2) emit(event, ...args) returns an array of handler return values.

Requirements

Methods

• on(event, handler)
• Register a handler for an event.
• Return this (chainable).

• once(event, handler)
• Register a handler that runs at most once.
• Return this.

• off(event, handler)
• Remove the matching handler for the event (if present).
• Return this.

• emit(event, ...args)
• Call all handlers for the event in registration order.
• Pass ...args to each handler.
• Remove any once handlers after they run.
• Return an array of handler return values (in call order).

Example

const ee = new EventEmitter();

const calls = [];
ee.on('a', (x) => { calls.push('on'); return x + 1; });
ee.once('a', () => { calls.push('once'); return 99; });

const r1 = ee.emit('a', 1); // ['on' return, 'once' return] => [2, 99]
const r2 = ee.emit('a', 1); // once removed => [2]

// calls => ['on', 'once', 'on']