手写promise(面试版)

(function () {
   
    "use strict";

    function Promise(executor) {
   
        var self = this;
        if (typeof executor !== "function") throw new TypeError("Promise resolver is not a function");
        if (!(self instanceof Promise)) throw new TypeError("undefined is not a promise");

        // 实例默认具备两个属性：state状态 result值
        self.state = "pending";
        self.result = undefined;
        self.onFulfilledCallbacks = [];
        self.onRejectedCallbacks = [];

        // 修改实例的状态和值
        var change = function change(state, result) {
   
            // 状态一但不是pending，则不能再修改了
            if (self.state !== "pending") return;
            self.state = state;
            self.result = res