I am trying to write a web page that can catch onkeyup events.
It uses document.write("..."); to print the keycode each time.  
However it only works once, the second time I use it, the window does not update.
Here is the code:
document.onkeyup = function checkKeys(event) {
    var keyCode = event.which || event.keyCode;
    document.write(keyCode);
};
Why does this only catch the event once?
 
     
     
     
    