摘要：極簡版滿足的使用方式生成實例對象的方式通過類直接調用靜態方法，目前靜態方法僅支持親測使用，歡迎指教，互相學習，鏈接，歡迎。附贈利用構造函數手寫的方法，鏈接。

極簡版Promise 滿足的使用方式

生成實例對象的方式：new MyPromise()

通過類直接調用靜態方法：MyPromise.resolve()，目前靜態方法僅支持resolve & reject

親測使用OK，歡迎指教，互相學習，github鏈接，歡迎star。
附贈利用構造函數手寫Promise 的方法，github鏈接。

class MyPromise {
  constructor(fn) {
    // 定義Promise 三種狀態
      this.states = {
          PENDING: "PENDING", RESOLVED: "RESOLVED", REJECTED: "REJECTED"
      }
      // 定義傳遞到then的value
      this.value = null
      // 定義當前Promise運行狀態
      this.state = this.states.PENDING
      // 定義Promise失敗狀態的回調函數集合
      this.resolvedCallBacks = []
      // 定義Promise成功狀態的回調函數集合
      this.rejectedCallBacks = []
      // 為靜態方法定義其內部使用的指向實例的that  
      MyPromise.that = this
      try {
      // 執行 new MyPromise() 內傳入的方法
          fn(MyPromise.resolve, MyPromise.reject)
      } catch (error) {
          MyPromise.reject(this.value)
      }
  }
    // 靜態resolve方法，MyPromise實例不可訪問；
    //支持類MyPromise訪問，例：MyPromise.resolve("success").then(e=>e)
  static resolve(value) {
      // 由于靜態方法內部的this指向 類 而不是 實例，所以用下面的方法訪問實例對象
      const that = MyPromise.that
      // 判斷是否是MyPromise實例訪問resolve
      const f = that instanceof MyPromise
      // MyPromise實例對象訪問resolve
      if (f && that.state == that.states.PENDING) {
          that.state = that.states.RESOLVED
          that.value = value
          that.resolvedCallBacks.map(cb => (that.value = cb(that.value)))
      }
      // MyPromise類訪問resolve
      if (!f) {
          const obj = new MyPromise()
          return Object.assign(obj, {
              state: obj.states.RESOLVED,
              value
          })
      }
  }
   // 靜態reject方法，MyPromise實例不可訪問；
   //支持類MyPromise訪問，例：MyPromise.reject("fail").then(e=>e)
  static reject(value) {
      const that = MyPromise.that
      const f = that instanceof MyPromise
      if (f && that.state == that.states.PENDING) {
          that.state = that.states.REJECTED
          that.value = value
          that.rejectedCallBacks.map(cb => (that.value = cb(that.value)))
      }
      if (!f) {
          const obj = new MyPromise()
          return Object.assign(obj, {
              state: obj.states.REJECTED,
              value
          })
      }
  }
  // 定義在MyPromise原型上的then方法
  then(onFulfilled, onRejected) {
      const { PENDING, RESOLVED, REJECTED } = this.states
      const f = typeof onFulfilled == "function" ? onFulfilled : c => c;
      const r =
          typeof onRejected == "function"
              ? onRejected
              : c => {
                  throw c;
              };

      switch (this.state) {
          case PENDING:
              // ‘PENDING’狀態下向回調函數集合添加callback
              this.resolvedCallBacks.push(f)
              this.rejectedCallBacks.push(r)
              break;
          case RESOLVED:
              // 將回調函數的返回值賦值給 實例的 value，滿足鏈式調用then方法時傳遞value
              this.value = f(this.value)
              break;
          case REJECTED:
              // 同上
              this.value = r(this.value)
              break;
          default:
              break;
      }
      // 滿足鏈式調用then，返回MyPromise實例對象
      return this
  }
}

MyPromise.resolve("success").then((e) => {
  console.log(e);
  return e + 1
}).then( res=> {
  console.log(res);
})
new MyPromise(resolve => {
  setTimeout(() => {
      resolve(1);
  }, 2000);
})
  .then(res1 => {
      console.log(res1);
      return 2;
  })
  .then(res2 => console.log(res2 ));