const PENDING_STATE = "pending";const FULLFILL_STATE = "fullfilled";const REJECTED_STATE = "refected";class Promise {  constructor(executor) {    if (typeof executor !== "function") {      throw new Error("Promise executor must be a function.");    }    this.__state = PENDING_STATE;    this.__chains = [];    const resolve = res => {      // 当且仅当状态是 pending 状态时才可以调用resolve或者reject，防止二次调用。      if (this.__state !== PENDING_STATE) return;      // resolve要处理promise，判断是否有then属性      if (res && typeof res.then === "function") {        return res.then();      }      this.__state = FULLFILL_STATE;      this.__internalValue = res;      for (const { onFullFilled } of this.__chains) {        onFullFilled(res);      }    };    const reject = err => {      if (this.__state !== PENDING_STATE) return;      this.__state = REJECTED_STATE;      this.__internalValue = err;      for (const { onRejected } of this.__chains) {        onRejected(err);      }    };    // 规范中，Promise初始化就调用executor    try {      executor(resolve, reject);    } catch (err) {      // 如果处理器函数抛出一个同步错误，我们认为这是一个失败状态      reject(err);    }  }  then(onFullFilled, onRejected) {    return new Promise((resolve, reject) => {      // 参考上述的constructor实现      const _onFullFilled = res => {        try {          resolve(onFullFilled(res));        } catch (err) {          reject(err);        }      };      const _onRejected = err => {        try {          reject(onRejected(res));        } catch (err) {          reject(err);        }      };      if (this.__state === FULLFILL_STATE) {        _onFullFilled(this.__internalValue);      } else if (this.__state === REJECTED_STATE) {        _onRejected(this.__internalValue);      } else {        this.__chains.push({          onFullFilled: _onFullFilled,          onRejected: _onRejected        });      }    });  }}