js障蔽f5 兼容ie和firefox

var check=function(e){
e=e||window.event;
if((e.which||e.keyCode)==116 || (e.ctrlKey && (e.which||e.keyCode)==82)){
if(e.preventDefault){
e.preventDefault();
}
else{
event.keyCode = 0;
e.returnValue=false;
}
}
}
if(document.addEventListener){
? document.addEventListener("keydown",check,false);
}
else{
? document.attachEvent("onkeydown",check);
}???