SOURCE

class EventEmitter{
   constructor(){
       this.cache = {};
   } 
   on(name,func){
       if(this.cache[name]){
           this.cache[name].push(func);
       }else{
           this.cache[name] = [func];
       }
   }
   off(name,func){
       let tasks = this.cache[name];
       if(tasks){
           const index = tasks.findIndex(f =>{
               return f === func||f.callback === func;
           })
           if(index >= 0){
               tasks.splice(index,1);
           }
       }
   }
   emit(name,once = false,...args){
       if(this.cache[name]){
           let tasks = this.cache[name].splice();
           for(let fn of tasks){
               fn(...args);
           }
           if(once){
               delete this.cache[name];
           }
       }
   }

}