nodejs事件模块EventEmitter实现原理

function EventEmitter(){
  this._events = Object.create(null);
}

EventEmitter.prototype.on = function(eventName, callback){
  if(!this._events){
    this._events = Object.create(null);
  }
  if(eventName !== 'newListener'){
    this.emit('newListener', callback);
  }
  if(this._events[eventName]){
    this._events[eventName].push(callback);
  }else{
    this._events[eventName] = [callback];
  }
}

EventEmitter.prototype.once = function(eventName, callback){
  let one = ()=>{
    callback();
    this.off(eventName, one);
  }
  one.l = callback;
  this.on(eventName, one);
}

EventEmitter,prototype.off = function(eventName,callback){
  if(this._events[eventName]){
    this._events[eventName].filter(fn=>{
      return fn != callback && fn.l != callback;
    })
  }
}

EventEmitter.prototype.emit = function(eventName, ...args){
  if(this._events[eventName]){
    this._events[eventName].forEach(fn => fn(...args));
  }
}

module.exports = EventEmitter;