❗️Что будет на выходе:

class EventEmitter {
  constructor() {
    this.events = {};
  }
  
  on(event, callback) {
    this.events[event] = this.events[event] || [];
    this.events[event].push(callback);
    return this;
  }
  
  emit(event, data) {
    if (this.events[event]) {
      this.events[event].forEach(cb => cb(data));
    }
    return this;
  }
}

const emitter = new EventEmitter();
emitter.on('test', x => console.log(x * 2))
       .on('test', x => console.log(x + 5))
       .emit('test', 10);

Ответ: 20 15

JavaScript test | #JavaScript