1. function Promise2(fn) {
    2.   let self = this;
    3.   self.PromiseValue = undefined;
    4.   self.status = "pending";
    5.   self.onResolvedCallbacks = [];
    6.   self.onRejectedCallbacks = [];
    8.   const resolve = (value) => {
    9.     if (self.status === "pending") {
    10.       self.status = "fulfilled";
    11.       self.PromiseValue = value;
    12.       self.onResolvedCallbacks.forEach((fn) => fn());
    13.     }
    14.   };
    16.   const reject = (reason) => {
    17.     if (self.status === "pending") {
    18.       self.status = "rejected";
    19.       self.PromiseValue = reason;
    20.       self.onRejectedCallbacks.forEach((fn) => fn());
    21.     }
    22.   };
    24.   try {
    25.     fn(resolve, reject);
    26.   } catch (error) {
    27.     reject(error);
    28.   }
    29. }
    31. Promise2.prototype.then = function(onFulfilled, onRejected) {
    32.   if (this.status === "pending") {
    33.     this.onResolvedCallbacks.push(() => {
    34.       this.PromiseValue = onFulfilled(this.PromiseValue);
    35.     });
    36.     this.onRejectedCallbacks.push(() => {
    37.       this.PromiseValue = onRejected(this.PromiseValue);
    38.     });
    39.   }
    40.   if (this.status === "fulfilled") {
    41.     try {
    42.       this.PromiseValue = onFulfilled(this.PromiseValue);
    43.     } catch (error) {
    44.       this.PromiseValue = error;
    45.       this.status = "rejected";
    46.     }
    47.   } else if (this.status === "rejected") {
    48.     try {
    49.       this.PromiseValue = onRejected(this.PromiseValue);
    50.       this.status = "fulfilled";
    51.     } catch (error) {
    52.       this.PromiseValue = error;
    53.     }
    54.   }
    55.   return this;
    56. };
    58. new Promise2((resolve, reject) => {
    59.     console.log("执行同步");
    60.     resolve(10);
    61. })
    62.   .then((value) => {
    63.     console.log("正在执行then");
    64.     console.log(value);
    65.     let n = value + 1;
    66.     return n;
    67.   })
    68.   .then((value) => {
    69.     console.log("执行第二个then");
    70.     console.log(value);
    71.   });