const EventEmitter = (() => {
  const listeners = new WeakMap();

  return class {
    constructor() {
      listeners.set(this, {});
    }

    on(event, fn) {
      const map = listeners.get(this);
      (map[event] ??= []).push(fn);
      return this;
    }

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

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

bus
  .on("data", val => log.push(`A:${val}`))
  .on("data", val => log.push(`B:${val}`))
  .on("done", ()  => log.push("done"));

bus.emit("data", 1).emit("data", 2).emit("done");

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