Implement EventEmitter (on/off/once/emit) — Node style

Question

Accepted Answer

**Approach:** Map `event -> Set<listener>`. `once` wraps listener to self-remove after first call. `emit` clones listeners before iterating to allow mutation during emit.

```javascript
class EventEmitter {
  constructor() { this.events = new Map(); }
  on(event, listener) {
    if (!this.events.has(event)) this.events.set(event, new Set());
    this.events.get(event).add(listener);
    return this;
  }
  off(event, listener) {
    if (!this.events.has(event)) return this;
    if (!listener) this.events.delete(event);
    else this.events.get(event).delete(listener);
    return this;
  }
  once(event, listener) {
    const wrapper = (...args) => { this.off(event, wrapper); listener(...args); };
    wrapper._original = listener;
    return this.on(event, wrapper);
  }
  emit(event, ...args) {
    const set = this.events.get(event);
    if (!set) return false;
    for (const fn of [...set]) fn(...args);
    return true;
  }
  removeAllListeners(event) {
    if (event) this.events.delete(

Implement EventEmitter (on/off/once/emit) — Node style

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