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(diào)用靜態(tài)方法，目前靜態(tài)方法僅支持親測使用，歡迎指教，互相學(xué)習(xí)，鏈接，歡迎。附贈利用構(gòu)造函數(shù)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(diào)用靜態(tài)方法：MyPromise.resolve()，目前靜態(tài)方法僅支持resolve & reject

親測使用OK，歡迎指教，互相學(xué)習(xí)，github鏈接，歡迎star。
附贈利用構(gòu)造函數(shù)手寫Promise 的方法，github鏈接。

class MyPromise {
  constructor(fn) {
    // 定義Promise 三種狀態(tài)
      this.states = {
          PENDING: "PENDING", RESOLVED: "RESOLVED", REJECTED: "REJECTED"
      }
      // 定義傳遞到then的value
      this.value = null
      // 定義當(dāng)前Promise運行狀態(tài)
      this.state = this.states.PENDING
      // 定義Promise失敗狀態(tài)的回調(diào)函數(shù)集合
      this.resolvedCallBacks = []
      // 定義Promise成功狀態(tài)的回調(diào)函數(shù)集合
      this.rejectedCallBacks = []
      // 為靜態(tài)方法定義其內(nèi)部使用的指向?qū)嵗膖hat  
      MyPromise.that = this
      try {
      // 執(zhí)行 new MyPromise() 內(nèi)傳入的方法
          fn(MyPromise.resolve, MyPromise.reject)
      } catch (error) {
          MyPromise.reject(this.value)
      }
  }
    // 靜態(tài)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(tài)方法內(nèi)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(tài)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(tài)下向回調(diào)函數(shù)集合添加callback
              this.resolvedCallBacks.push(f)
              this.rejectedCallBacks.push(r)
              break;
          case RESOLVED:
              // 將回調(diào)函數(shù)的返回值賦值給 實例的 value，滿足鏈?zhǔn)秸{(diào)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?zhǔn)秸{(diào)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 ));