禁用backspace网页回退功能的实现代码

<script language="JavaScript">
document.onkeydown = check;
function check(e) {
var code;
if (!e) var e = window.event;
if (e.keyCode) code = e.keyCode;
else if (e.which) code = e.which;
if (((event.keyCode == 8) && //BackSpace 
((event.srcElement.type != "text" && 
event.srcElement.type != "textarea" && 
event.srcElement.type != "password") || 
event.srcElement.readOnly == true)) || 
((event.ctrlKey) && ((event.keyCode == 78) || (event.keyCode == 82)) ) || //CtrlN,CtrlR 
(event.keyCode == 116) ) { //F5 
event.keyCode = 0; 
event.returnValue = false; 
}
return true;
}
</script>