<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title></title>
</head>
<body>
    <button>click</button>
    <script>
        document.getElementsByTagName('button')[0].onclick = function() {
            alert(2)
        }
        document.getElementsByTagName('button')[0].click()
    </script>
</body>
</html>