手寫極簡版Promise

極簡版Promise 知足的使用方式

• 生成實例對象的方式：new MyPromise()
• 經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連接，歡迎star。
附贈利用構造函數手寫Promise 的方法，github連接。

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 = []
      // 爲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) {
      // 因爲靜態方法內部的this指向 類 而不是 實例，因此用下面的方法訪問實例對象
      const that = MyPromise.that
      // 判斷是不是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，知足鏈式調用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;
      }
      // 知足鏈式調用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 ));