手写 promise - 后续处理

const MyPromise = (() => {
    const PENDING = "pending",
        RESOLVED = "resolved",
        REJECTED = "rejected",
        //不能让外部访问
        PromiseValue = Symbol("PromiseValue"),//状态数据
        PromiseStatus = Symbol("PromiseStatus"),//当前状态
        changeStatus = Symbol("changeStatus"),
        thenables = Symbol("thenables"),
        cachables = Symbol("cachables"),
        settleHandle = Symbol("settleHandle");

    return class MyPromise {
        [changeStatus](newStatus, newValue, queue) {
            if (this[PromiseStatus] != PENDING) {
                //状态无法变更
                return;
            };
            this[PromiseStatus] = newStatus;
            this[PromiseValue] = newValue;
            //执行响应队列中的函数
            queue.forEach(handler => handler(newValue));
        };

        // executor 未决阶段（pending 状态）下的处理函数
        constructor(executor) {
            this[PromiseStatus] = PENDING;
            this[PromiseValue] = undefined;

            this[thenables] = [];//后续处理函数的数组 --> resolved
            this[cachables] = [];//后续处理函数的数组 --> rejected

            const resolve = data => {
                this[changeStatus](RESOLVED, data, this[thenables]);
            };

            const reject = reason => {
                this[changeStatus](REJECTED, reason, this[cachables]);
            };

            try {
                executor(resolve, reject);
            } catch (err) {
                reject(err);
            };
        };

        // handler 后续处理函数
        // immediatelyStatus 需要立即执行的状态
        // queue 作业队列 
        [settleHandle](handler, immediatelyStatus, queue) {
            if (typeof handler !== "function") return;
            
            if(this[PromiseStatus] == immediatelyStatus) {
                setTimeout(() => {
                    handler(this[PromiseValue])
                }, 0);
            } else {
                queue.push(handler);
            }
        }
        
        then(thenable, cachable) {
            this[settleHandle](thenable, RESOLVED, this[thenables]);
            this.catch(cachable);
        }

        catch(cachable) {
            this[settleHandle](cachable, REJECTED, this[cachables]);
        }
    };
})();

const pro = new MyPromise((resolve, reject) => {
    setTimeout(() => {
        reject(1);
    }, 3000);
});

pro.then(data => {
    console.log(1, data);
});

pro.then(data => {
    console.log(2, data);
});

pro.catch(err => {
    console.log(311, err);
});

console.log(pro);