手写极简版Promise

摘要：极简版满足的使用方式生成实例对象的方式通过类直接调用静态方法，目前静态方法仅支持亲测使用，欢迎指教，互相学习，链接，欢迎。附赠利用构造函数手写的方法，链接。

极简版Promise 满足的使用方式

生成实例对象的方式：new MyPromise()

通过类直接调用静态方法：MyPromise.resolve()，目前静态方法仅支持resolve & reject

亲测使用OK，欢迎指教，互相学习，github链接，欢迎star。
附赠利用构造函数手写Promise 的方法，github链接。

class MyPromise {
  constructor(fn) {
    // 定义Promise 三种状态
      this.states = {
          PENDING: "PENDING", RESOLVED: "RESOLVED", REJECTED: "REJECTED"
      }
      // 定义传递到then的value
      this.value = null
      // 定义当前Promise运行状态
      this.state = this.states.PENDING
      // 定义Promise失败状态的回调函数集合
      this.resolvedCallBacks = []
      // 定义Promise成功状态的回调函数集合
      this.rejectedCallBacks = []
      // 为静态方法定义其内部使用的指向实例的that  
      MyPromise.that = this
      try {
      // 执行 new MyPromise() 内传入的方法
          fn(MyPromise.resolve, MyPromise.reject)
      } catch (error) {
          MyPromise.reject(this.value)
      }
  }
    // 静态resolve方法，MyPromise实例不可访问；
    //支持类MyPromise访问，例：MyPromise.resolve("success").then(e=>e)
  static resolve(value) {
      // 由于静态方法内部的this指向 类 而不是 实例，所以用下面的方法访问实例对象
      const that = MyPromise.that
      // 判断是否是MyPromise实例访问resolve
      const f = that instanceof MyPromise
      // MyPromise实例对象访问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类访问resolve
      if (!f) {
          const obj = new MyPromise()
          return Object.assign(obj, {
              state: obj.states.RESOLVED,
              value
          })
      }
  }
   // 静态reject方法，MyPromise实例不可访问；
   //支持类MyPromise访问，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
          })
      }
  }
  // 定义在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’状态下向回调函数集合添加callback
              this.resolvedCallBacks.push(f)
              this.rejectedCallBacks.push(r)
              break;
          case RESOLVED:
              // 将回调函数的返回值赋值给 实例的 value，满足链式调用then方法时传递value
              this.value = f(this.value)
              break;
          case REJECTED:
              // 同上
              this.value = r(this.value)
              break;
          default:
              break;
      }
      // 满足链式调用then，返回MyPromise实例对象
      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 ));