promise实现原理（面试）

//创建一个Promise的类   class Promise{
    constructor(executer){//构造函数constructor里面是个执行器       this.status = 'pending';//默认的状态 pending       this.value = undefined//成功的值默认undefined       this.reason = undefined//失败的值默认undefined       //状态只有在pending时候才能改变       let resolveFn = value =>{
        //判断只有等待时才能resolve成功         if(this.status == pending){
          this.status = 'resolve';
          this.value = value;
        }
      }
      //判断只有等待时才能reject失败       let rejectFn = reason =>{
        if(this.status == pending){
          this.status = 'reject';
          this.reason = reason;
        }
      }    
      try{
        //把resolve和reject两个函数传给执行器executer         executer(resolve,reject);
      }catch(e){
        reject(e);//失败的话进catch       }
    }
    then(onFufilled,onReject){
      //如果状态成功调用onFufilled       if(this.status = 'resolve'){
        onFufilled(this.value);
      }
      //如果状态失败调用onReject       if(this.status = 'reject'){
        onReject(this.reason);
      }
    }
  }