<!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>
<button >按钮</button>
<script>
let btn=document.getElementById('button')[0];
btn.onclick=function(){
alert('666');
}
</script>
</body>
</html>getElementsByTagName返回是数组集合形式。
把
var Btn = document.getElementsByTagName("button")
改成以下即可(后面一定要加0)！！！
var Btn = document.getElementsByTagName("button")[0]