firtChild 获取第一个子节点
firtElementChild 获取第一个元素子节点
lastChild
lastElementChild
nextSibling
nextElementSibling
previousSibling
previousElementSibling
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<!--
firtChild 获取第一个子节点
firtElementChild 获取第一个元素子节点
lastChild
lastElementChild
-->
<!--
nextSibling
nextElementSibling
previousSibling
previousElementSibling
-->
<div>hello</div>
<ul id="app">
<li>html</li>
<li>css</li>
</ul>
<div>
world
</div>
<script>
var app = document.getElementById("app");
console.log(app.previousElementSibling)
</script>
</body>
</html>