Promise状态、原因（二）

1. Promise 有pending、fulfilled、rejected 三种状态
2. Promise 的状态只能从 pending 到 fulfilled 或 rejected，一旦状态敲定，就不可以更改了

  <script>
    const PENDING = "pending";
    const FULFILLED = "fulfilled";
    const REJECTED = "rejected";
    class FXPromise {
      // 1. 添加状态
      state = PENDING;
      // 2. 添加原因
      result = undefined;

      constructor(fn) {
        const resolveFunc = (res) => {
          // 3.没有敲定状态，就敲定状态，以及添加原因
          if (this.state === PENDING) {
            this.result = res;
            this.state = FULFILLED;
            console.log("resolve ", res);
          }
        };
        const rejectFunc = (res) => {
          if (this.state === PENDING) {
            this.result = res;
            this.state = REJECTED;
            console.log("reject ", res);
          }
        };
        fn(resolveFunc, rejectFunc);
      }
    }

    new FXPromise((resolve, reject) => {
      console.log(1);
      reject(222);
      resolve(111);
    });
  </script>