function backToTop () {
    cancelAnimationFrame(timer);
    // 获取当前毫秒数
    const startTime = Date.now();     
    // 获取当前页面的滚动高度
    const scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
    // 滚动周期时间
    let interval = 500;
    timer = requestAnimationFrame(function scrollFunc(){
        // 已执行时间
        let executeTime = Date.now() - startTime;
        // 剩余滚动距离
        document.documentElement.scrollTop = document.body.scrollTop = scrollTop - (executeTime / interval * scrollTop);
        timer = requestAnimationFrame(scrollFunc);
        if(executeTime >= interval){
            cancelAnimationFrame(timer);
        }
    });
}