障蔽 F5 刷新键

<script language="JavaScript" type="text/javascript">
document.onkeydown = function(){
?var k = window.event.keyCode;
?if (k == 116){ //屏蔽 F5 刷新键
?window.event.keyCode = 0;
?window.event.returnValue= false;
?}
}
</script>?