js function to block F1~F12 shortcut keys_javascript skills

function window.onhelp(){return false} //Shield F1 help
/*keyCod disables the F2-F12 keys from 113-123 respectively, 13 disables the Enter key (Enter), and 8 disables the Backspace key (BackSpace)*/

document.onkeydown = function (){
if(window.event && window.event.keyCode == 113) {
window.event.keyCode = 505;
}
if(window.event && window.event. keyCode == 114) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 115) {
window.event.keyCode = 505 ;
}
if(window.event && window.event.keyCode == 116) {
window.event.keyCode = 505;
}
if(window.event && window. event.keyCode == 117) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 118) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 119) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 120) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 121) {
window.event .keyCode = 505;
}
if(window.event && window.event.keyCode == 122) {
window.event.keyCode = 505;
}
if(window. event && window.event.keyCode == 123) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 13) {
window .event.keyCode = 505;
}
if(window.event && window.event.keyCode == 8) {
alert(str "nPlease use the Del key to delete characters! ");
window.event.returnValue=false;
}
if(window.event && window.event.keyCode == 505){
　alert(str)
window.event .returnValue=false;
}
}