关于前端:promise实现原理面试

//创立一个Promise的类   class Promise{    constructor(executer){//构造函数constructor外面是个执行器       this.status = 'pending';//默认的状态 pending       this.value = undefined//胜利的值默认undefined       this.reason = undefined//失败的值默认undefined       //状态只有在pending时候能力扭转       let resolveFn = value =>{        //判断只有期待时能力resolve胜利         if(this.status == pending){          this.status = 'resolve';          this.value = value;        }      }      //判断只有期待时能力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){      //如果状态胜利调用onFufilled       if(this.status = 'resolve'){        onFufilled(this.value);      }      //如果状态失败调用onReject       if(this.status = 'reject'){        onReject(this.reason);      }    }  }