Close f4 not working

[DJScript]

I'm not sure what is going on here and why my F4 prevent not working. I have a close option btw.

window.onload = function document.onkeydown()
{
if ( 
(event.keyCode == 116) || (event.keyCode == 122) || 
((event.ctrlKey) && ((event.keyCode == 82))) ||
((event.ctrlKey) && ((event.keyCode == 67))) ||
((event.altKey) && ((event.keyCode == 115)))
)
{
event.keyCode = 0;
event.cancelBubble = true;
return false;
}
}

[Kor]

Some of the keys combinations can not be prevented. They are part of the Operating System. Like, in Windows, ALT+F4, CTRL+ALT+DELETE, etc... They suppose to overpass everything (to work asynchronously) just in order to stop a crush process, etc.