SOURCE

<button class="add-li">添加</button>
<ul class="ul">
	<li>1111111111</li>
	<li>2222222222</li>
	<li>固定0</li>
</ul>

<script>
    window.onload=function(){
        let ul = document.querySelector('ul'),
            first = ul.querySelector('li');
        console.log(ul);
        document.querySelector('.add-li').onclick = function(){
            let li = document.createElement('li');
            li.innerText = Math.random().toString(36).slice(2);
            ul.insertBefore(li,first);
        }
    }
</script>

    ul{
        width:200px;
        height:200px;
        overflow:auto;
    }
    li:last-child{
        position:sticky;
        background: seashell;
        bottom: 0px;
    }