获取body滚动条的宽度:
var scrollbarWidth = document.documentElement.clientWidth - document.body.clientWidth
获取元素的滚动条宽度:
<div class="wrap" style="width:200px;height:200px;border:1px solid #ccc;overflow:auto;">
<div class="inner" style="width:100%;height:1000px;background-color:#f00;"></div>
</div>
<script type="text/javascript">
var ele = document.getElementsByClassName('wrap')[0]
var scrollbarWidth = ele.offsetWidth - ele.clientWidth;
console.log(scrollbarWidth);
</script>