如何封装Promise对象？

 <script>     class MyPromise {           constructor(executor) {         // 初始化state赋值为pending         this.state = "pending";         // 定义成功的 值         this.value = undefined;         //定义失败的 原因         this.reason = undefined;         //定义成功存起来的数组         this.onResolvedCallbacks = [];         //定义失败存起来的数组         this.onRejectedCallbacks = [];         let resolve = (value) => {           // state改变，resolve调用就会失败           if (this.state === "pending") {             this.value = value;             //resolve调用后,state转为fulfilled             this.state = "fulfilled";             // 一旦resolve执行，调用成功数组的函数             this.onResolvedCallbacks.forEach(function (fn) {               return fn(value);             });             //也可以使用es6语法 简写             //this.onResolvedCallbacks.forEach(fn => fn(value));           }         };         let reject = (reason) => {           // state改变,reject调用就会失败           if (this.state === "pending") {             this.reason = reason;             // reject调用后，state转为失败状态             this.state = "rejected";             // 一旦reject执行，调用失败数组的函数             this.onRejectedCallbacks.forEach(fn => fn(reason)); //简写           }         };          // new MyPromise后的实例具有状态， 默认状态是等待，当执行器调用resolve后， 实例状态为成功状态；当执行器调用reject后，实例状态为失败状态         try {           executor(resolve, reject);         } catch (error) {           reject(error);         }       }        // then中有两个参数 ，把第一个参数叫做then的成功回调，把第二个参数叫做then的失败回调，这两个参数也都是函数，当执行器调用resolve后，then中第一个参数函数会执行，当执行器调用reject后，then中第二个参数函数会执行       then(onFulfilled, onRejected) {         // 状态为fulfilled,执行onFulfilled,传入成功的值         if (this.state === "fulfilled") {           onFulfilled(this.value);         }         // 状态为rejected,执行onRejected,传入失败的原因         if (this.state === "rejected") {           onRejected(this.reason);         }         // 状态为pending时          if (this.state === "pending") {           // onFulfilled传入到成功数组           this.onResolvedCallbacks.push(onFulfilled);           // onRejected传入到失败数组           this.onRejectedCallbacks.push(onRejected);         }       }     } 	//当new MyPromise实例 会立即调用constructor(executor)     let p = new MyPromise((resolve, reject) => {       console.log("开始");       setTimeout(function () {         resolve("我成功了");       }, 2000);     });          p.then(       (resolve) => {         console.log("success:" + resolve);       },       (reject) => {         console.log("error:" + reject);       }     );     console.log("结束");   </script>