javascript">function MyPromise(fn) {
    pending = '待定';
    fulfilled = '成功';
    rejected = '失败';

    // 定义初始化状态
    this.status = pending;
    this.res = null;
    this.ret = null;

    // resolve 时的函数数组
    this.resolveCallBack = [];
    // reject 时的函数数组
    this.rejectCallBack = [];

    // 定义 resolve 和 reject 函数
    const resolve = (res) => {
        setTimeout(() => {
            if (this.status === pending) {
                // 变更状态
                this.status = fulfilled;
                // 保存值
                this.res = res;
                this.resolveCallBack.forEach((callback) => callback(this.res));
            }
        }, 1000);
    };
    const reject = (ret) => {
        setTimeout(() => {
            if (this.status === pending) {
                // 变更状态
                this.status = rejected;
                // 保存值
                this.ret = ret;
                this.rejectCallBack.forEach((callback) => callback(this.ret));
            }
        }, 1000);
    };
    // 因为初始化的时候该函数就被执行了,并且接受两个函数作为参数
    // 初始化时如果遇到错误会直接 reject 出去
    try {
        fn(resolve, reject);
    } catch (error) {
        reject(error);
    }
}
// 定义then方法
MyPromise.prototype.then = function (onFulfilled, onRejected) {
    // .then 方法会返回一个新的 Promise 对象
    return new MyPromise((resolve, reject) => {
        if (this.status === pending) {
            // onFulfilled 和  onRejected 是异步完成的,这里只要把函数 push 进去就可以啦
            resolve(this.resolveCallBack.push(onFulfilled));
            this.rejectCallBack.push(onRejected);
        }
        if (this.status === fulfilled) {
            onFulfilled(this.res);
        }
        if (this.status === rejected) {
            onRejected(this.ret);
        }
    });
};

//测试
const promise = new MyPromise(function (resolve, reject) {
    throw new Error('报错');
    reject(3);
    console.log(1);
})
    .then((res) => {
        console.log(res);
    }, (ret) => {
        console.log(ret);
    })
    .then((res) => {
        console.log(4);
    });
console.log(2);
//   最终输出的结果为: 2、报错、4
// 因为直接抛出错误了,所以后面的 reject 和 console 就不会再执行了

还有很多边界条件需要判断和优化的,这里只是简单实现一下