键盘表
来源：http://www.phpweblog.net/kiyone/archive/2007/04/19/1138.html
通过onkeydowm监听键盘按下事件，并修改键盘码
//禁止F12
document.onkeydown = function () {
if (window.event && window.event.keyCode == 123) {
event.keyCode = 0;
event.returnValue = false;
}
if (window.event && window.event.keyCode == 13) {
window.event.keyCode = 505;
}
}; //屏蔽右键菜单
document.oncontextmenu = function (event){
if(window.event){
event = window.event;
}try{
var the = event.srcElement;
if (!((the.tagName == "INPUT" && the.type.toLowerCase() == "text") || the.tagName == "TEXTAREA")){
return false;
}
return true;
}catch (e){
return false;
}
};