如何知道键盘按了F5刷新窗口

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html>
<head>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>新建网页</title>
<style type="text/css">
body{}
</style>
</head>
<body>
<script language="javascript">
document.onkeydown = function(){
if (116==event.keyCode){
alert("按F5刷新")
}
}
</script>
</body>
</html>