<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>登录</title>
</head>
<body>
<button id="btn" >按钮</button>
<script>
var btn = document.getElementById("btn");
btn.onclick=fun;
function fun(){
alert("I love you");
}
</script>
</body>
</html>
