如何实现promise

https://juejin.im/post/5d0da5c8e51d455ca0436271#heading-3

自定义实现及解析如下

class MyPromise {
    constructor (fn) {
        this.status = "pending";
        this.value = "";
        // 由于then是同步调用，因此无法拿到异步之后的状态
        // 因此需要将异步cb暂存起来以实现异步的效果，拿到异步的value
        this.fulfillArray = [];
        this.rejectArray = [];


        let resolveFn = (value) => {
            console.log(value);
            // setTimeout(() => { // 模拟异步
                this.status = "fulfilled";
                this.value = value;
                this.fulfillArray.map((item) => {
                    item(value);
                })
            // }, 0);
        }

        let rejectFn = (result) => {
            console.log(result);
            // setTimeout(() => { // 模拟异步
                this.status = "rejected";
                this.value = result;
                this.rejectArray.map((item) => {
                    item(result);
                })
            // }, 0)
        }

        fn(resolveFn, rejectFn);
    }

    then(resolveCb, rejectCb) { // 以同步的方式调用
        console.log("111111")
        return new MyPromise((resolve, reject) => {
            this.fulfillArray.push(() => {
                let x = resolveCb(this.value);

                resolve(x);
            });

            this.rejectArray.push(() => {
                let x = rejectCb(this.value);

                reject(x);
            });
        })
    }

    catch(rejectCb) {
        return this.then(null, rejectCb)
    }
}


let p1 = new MyPromise((resolve, reject) => {
    setTimeout(() => { // 异步请求
        let ran = Math.random();
        console.log("ran===", ran);
        ran > 0.5 ? resolve(100) : reject(-100)
    }, 0);
}).then((res) => {
    console.log("res===", res);
    return res;
}, (err) => {
    console.log("err===", err);
    return err;
}).then((res) => {
    console.log("res2===", res);
    return res;
}, (err) => {
    console.log("err2===", err);
    return err;
})

setTimeout(() => {
    console.log(p1)
}, 10);

运行结果如下：

111111 // 可见then调用是以同步的形式
111111 // 第二个then调用
// 开始执行异步
ran=== 0.36985480393587333
// 异步结束之后调用cb
-100
err=== -100
// 第二个异步mypromise
-100
err2=== -100
-100
MyPromise {
  status: 'rejected',
  value: -100,
  fulfillArray: [],
  rejectArray: [] }