I am looking at some JS code from the 20th century, and they are using document.layers in code that is trying to get the current key code. What browser are they sniffing for?

i am about to replace the code with something like this:

var fn = function(event){
  event = event || window.event;
  var code = event.charCode || event.keyCode;
}

but i am afraid of breaking something arcane and releasing the evil