原生js返回顶部

let backToTop = function() {
    let scrollToptimer = setInterval(function() {
        let top = document.body.scrollTop || document.documentElement.scrollTop;
        let speed = top / 4;
        if (document.body.scrollTop !== 0) {
            document.body.scrollTop -= speed;
        } else {
            document.documentElement.scrollTop -= speed;
        }
        if (top === 0) {
            clearInterval(scrollToptimer);
        }
    }, 40);
};