<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<script src="js/jquery-3.3.1.js" type="text/javascript" charset="utf-8"></script>
</head>
<body>
<h1>1</h1><h2>2</h2><h3>3</h3><h4>4</h4><h4>5</h4>
<script type="text/javascript">
var i = new Set([...document.querySelectorAll("*")].map(val=>val.nodeName)).size;
console.log(i);
//document.querySelectorAll("*")
//NodeList(12) [html, head, meta, title, script, body, h1, h2, h3, h4, h4, script]
//[document.querySelectorAll("*")]
//[NodeList(12)]
//[...document.querySelectorAll("*")]
//[html, head, meta, title, script, body, h1, h2, h3, h4, h4, script]
//[...document.querySelectorAll("*")].map(val=>val.nodeName)
//(12) ["HTML", "HEAD", "META", "TITLE", "SCRIPT", "BODY", "H1", "H2", "H3", "H4", "H4", "SCRIPT"]
//new Set([...document.querySelectorAll("*")].map(val=>val.nodeName))
//Set(10) {"HTML", "HEAD", "META", "TITLE", "SCRIPT", …}
</script>
</body>
</html>
擴展運算符（...）可以將某些數據結構轉爲數組
map() 方法返回一個新數組，數組中的元素爲原始數組元素調用函數處理後的值
set 數組去重