EventEmitter ES6 class

EventEmitter.js

class EventEmitter{
  
  constructor(){
    this.events = null ;
  }

  on(name , callback , ctx){
    let events = this.events || (this.events = {});
    let handlers = events[name] || (events[name] = []);
    handlers.push({
      callback: callback,
      ctx: ctx
    })
  }

  off(name , callback ){
    this.events = this.events || {};
    let handlers = this.events[name];
    let left = [];
    let cb;
    if(arguments.length == 0){
      this.events = null;
      return;
    }
    if(arguments.length == 1){
      delete this.events[name];
      return;
    }
    for(let i = 0 ; i < handlers.length ; i ++){
      cb = handlers[i].callback ;
      cb = cb.fn || cb;
      if(cb !== callback){
        left.push(handlers[i]);
      }
    }
    if(left.length > 0){
      this.events[name]= left;
    }else{
      delete this.events[name];
    }

  }

  once(name , callback , ctx){
    let self = this;
    let proxy = function(...args){
      callback.apply(ctx , args);
      self.off(name , proxy , ctx);
    }
    proxy.fn = callback;
    this.on(name , proxy , ctx);

  }

  emit(name , ...args){
    let handlers = (this.events || [])[name];
    let cb , ctx;
    if(handlers && handlers.length > 0 ){
      let length = handlers.length ;
      for(let i = 0 ; i < length ; i++){
        cb = handlers[i].callback;
        ctx = handlers[i].ctx;
        cb.apply(ctx , args)
      }
    }

  }
}