Promise

首先声明定义类并声明Promise状态与值，有以下几个细节需要注意。

• executor为执行者
• 当执行者出现异常时触发拒绝状态
• 使用静态属性保存状态值
• 状态只能改变一次，所以在resolve与reject添加条件判断
• 因为 resolve或rejected方法在executor中调用，作用域也是executor作用域，这会造成this指向window，现在我们使用的是class定义，this为undefined。

class HD {
  static PENDING = "pending";
  static FULFILLED = "fulfilled";
  static REJECTED = "rejected";
  constructor(executor) {
    this.status = HD.PENDING;
    this.value = null;
    try {
      executor(this.resolve.bind(this), this.reject.bind(this));
    } catch (error) {
      this.reject(error);
    }
  }
  resolve(value) {
    if (this.status == HD.PENDING) {
      this.status = HD.FULFILLED;
      this.value = value;
    }
  }
  reject(value) {
    if (this.status == HD.PENDING) {
      this.status = HD.REJECTED;
      this.value = value;
    }
  }
}

下面测试一下状态改变

<script src="HD.js"></script>
<script>
  let p = new HD((resolve, reject) => {
    resolve("123");
  });
  console.log(p);
</script>