simple ES6 event system

eventSystem.js

class eventSystem {
  constructor() {
    this.listeners = new Map;
  }
  hasListener(type,func) {
    return this.listeners.has(type) && this.listeners.get(type).has(func);
  }
  addListener(type,func) {
    if(!this.listeners.has(type)) this.listeners.set(type,new Set);
    this.listeners.get(type).add(func);
  }
  removeListener(type,func) {
    if(this.listeners.has(type)) {
      const typeGroup = this.listeners.get(type);
      if(typeGroup.has(func)) typeGroup.delete(func);
      if(typeGroup.size == 0) this.listeners.delete(type);
    }
  }
  makeHandle(type,func) {
    const eventsys = this;
    return {
      off() {
        eventsys.off(type,func);
        return this;
      },
      on() {
        this.off();
        eventsys.on(type,func);
        return this;
      },
      once() {
        eventsys.once(type,func);
        return this;
      }
    }
  }
  on(type,func) {
    this.addListener(type,func);
    return this.makeHandle(type,func);
  }
  once(type,func) {
    if(this.hasListener(type,func)) this.removeListener(type,func);
    const wrap = (...args) => {
      func(...args);
      this.removeListener(type,wrap);
    }
    this.addListener(type,wrap);
    return this.makeHandle(type,func);
  }
  off(type,func) {
    this.removeListener(type,func);
    return this.makeHandle(type,func);
  }
  emit(type,...args) {
    if(this.listeners.has(type)) {
      const typeGroup = this.listeners.get(type);
      typeGroup.forEach(Listener => {
        Listener(...args);
      })
    }
  }
}