js 获取滚动条的高度

function getScrollTop() {
    var scroll_top = 0;
    if (document.documentElement && document.documentElement.scrollTop) {
        scroll_top = document.documentElement.scrollTop;
    }
    else if (document.body) {
        scroll_top = document.body.scrollTop;
    }
    return scroll_top;
}

参考链接： https://www.jianshu.com/p/4c37a2a56586