<body>
<button id="btn">按钮</button>
<script>
var btn = document.getElementById("btn");
btn.onclick = function(){
};
</script>
</body>
<script>
window.onload = function(){
var btn = document.getElementById("btn");
btn.onclick = function(){
};
};
</script>
原文：https://www.cnblogs.com/leiyanting/p/15217169.html