方法一：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<button id="btn">点击</button>
</body>
<script>
var btn = document.getElementById('btn')
btn.onclick = function() {
alert('点击事件')
}
</script>
</html>
方法二：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<button id="btn">点击</button>
</body>
<script>
var btn = document.getElementById('btn')
btn.addEventListener('click', function() {
alert('点击事件')
})
</script>
</html>
方法三
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<button onclick="btn()">点击</button>
</body>
<script>
function btn() {
alert('点击事件')
}
</script>
</html>