Very Basic EventEmitter implementation

EventEmitter.js

/* Very basic EventEmitter class */
class EventEmitter {
  constructor() {
    this.events = new Map();
  }
  on(eventname, callback) {
    if(this.events.has(eventname)) {
      const events = this.events.get(eventname);
      events.push(callback);
      return this.events.set(eventname, events);
    }
    return this.events.set(eventname, [callback]);
  }
  off(eventname, callback) {
    if(this.events.has(eventname)) {
      const events = this.events.get(eventname).filter(clbk => (clbk !== callback));
      return this.events.set(eventname, events);
    }
    return undefined;
  }
  emit(eventname, ...args) {
    if (this.events.has(eventname)) {
      this.events.get(eventname).forEach(function(callback) {
        callback.apply(this, args);
      });
    }
  }
}