class EventEmitter {
  #listeners = new Map();

  on(event, listener) {
    if (!this.#listeners.has(event)) {
      this.#listeners.set(event, []);
    }
    this.#listeners.get(event).push(listener);
    return this;
  }

  emit(event, ...args) {
    const handlers = this.#listeners.get(event) ?? [];
    handlers.forEach(fn => fn(...args));
    return this;
  }

  once(event, listener) {
    const wrapper = (...args) => {
      listener(...args);
      this.off(event, wrapper);
    };
    return this.on(event, wrapper);
  }

  off(event, listener) {
    const updated = (this.#listeners.get(event) ?? []).filter(fn => fn !== listener);
    this.#listeners.set(event, updated);
    return this;
  }
}

const emitter = new EventEmitter();
const log = [];

emitter.on("data", val => log.push(`on:${val}`));
emitter.once("data", val => log.push(`once:${val}`));
emitter.on("data", val => log.push(`on2:${val}`));

emitter.emit("data", "A");
emitter.emit("data", "B");

console.log(log.join(", "));