直接上代码

<script>
window.onscroll = function () {
var topScroll = document.documentElement.scrollTop;//滚动的距离,距离顶部的距离
var bignav = document.getElementsByClassName("aside")[0];//获取到导航栏id
var left = document.getElementsByClassName("aside")[0].getBoundingClientRect().left;
var width=document.getElementsByClassName("aside")[0].getBoundingClientRect().width;
if (topScroll > 210 &&
Math.abs(topScroll-document.documentElement.scrollHeight)>600) { //当滚动距离大于250px时执行下面的东西
bignav.style.position = 'fixed';
bignav.style.top = '10px';
bignav.style.left = '' + left + 'px';
bignav.style.width = '' + width + 'px';
bignav.style.zIndex = '9999';
} else {//当滚动距离小于250的时候执行下面的内容，也就是让导航栏恢复原状
bignav.style.position = 'static';
}
}
</script>​