JS mask key effect and change the key effect of the sample code

Function key f1-f2:112-123

A - b: 65-90

Number keys: 0-9:48-57

ESC: 27

Backspace key: 8

Enter:

Shift: 16; Ctrl: 17; Alt: 18

Space: 32

Insert: 45; Delete: 46

function document.onkeydown(){                //Press enter inside the page to trigger
  if(event.keyCode==13)
  {
   document.getElementById("loginbtn").click();   
   return false;                               
  }
 }
 
document.onkeydown=MM_KeyPress;
function  MM_KeyPress(num){
    //Prevent system backspace keys
 var keycode = event.keyCode;
 if(keycode ==8)//Block backspace
 {
  event.keyCode = 0;
  //event.returnValue=false;
  return;
 }
 if(keycode >=112 && keycode <=123)//Block f1-12 function keys
 {
  event.keyCode = 0 ;
  event.returnValue=false;
  return;
 }
}