js实现页面消息滚动效果

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<title>autoScroll</title>
</head>
<style>
.parent {
width: 300px;
height: 200px;
margin: 0 auto;
background: #242424;
overflow-y: scroll;
}
/*设置的子盒子高度大于父盒子，产生溢出效果*/
.child {
height: auto;
}
.child li {
height: 50px;
margin: 2px 0;
background: #009678;
}
</style>
<body>
<div id="parent" class="parent">
<div id="child1" class="child">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
<li>6</li>
<li>7</li>
<li>8</li>
<li>8</li>
<li>8</li>
<li>8</li>
<li>8</li>
<li>8</li>
<li>8</li>
</div>
<!-- <div id="child2" class="child"></div> -->
</div>
<script type="text/javascript">
(function () {
var parent = document.getElementById('parent');
var child1 = document.getElementById('child1');
setInterval(function () {
console.log(parent.scrollTop,child1.scrollHeight,parent.scrollHeight)
if(parent.scrollTop >= child1.clientHeight-parent.clientHeight) {
parent.scrollTop = 0;
} else {
parent.scrollTop++;
}
}, 20);
})()
</script>
</body>
</html>