来实现一个js平滑滚动到顶部的小功能

// 平滑滚动到顶部
var scrollTopSmooth = function (position) {
    if (!window.requestAnimationFrame) {
        window.requestAnimationFrame = function (cb) {
            return setTimeout(cb, 10);
        };
    }
    var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
    var step = function () {
        var distance = position - scrollTop;
        scrollTop = scrollTop + distance / 5;
        if (Math.abs(distance) < 1) {
            window.scrollTo(0, position);
        } else {
            window.scrollTo(0, scrollTop);
            requestAnimationFrame(step);
        }
    };
    step();
}
$backToTop = document.querySelector('.top')
$backToTop.addEventListener('click', function () {
    scrollTopSmooth(0);
}, false);