用 Promise 实现每隔 1 秒输出 1，2，3

const oneToThree = () => {
    const arr = [1, 2, 3];
    arr.reduce((prev, next) => {
        return prev.then(() => {
            return new Promise((resolve) => {
                setTimeout(() => {
                    console.log(next);
                    resolve();
                }, 1000);
            })
        });
    }, Promise.resolve())
};

oneToThree();