Go4Expert - Function Onkeypress for cursor key

Good day!
In my javascript code I have a function for the enter key. I want to add cursor key in that function/ I research about the coding for cursor key but I can't find the code same on what I want to happen.
Here is the code:

Code:

<script language="javascript">

  function handleEnter(e, nextfield)

        {

        var characterCode = (e && e.which)? e.which: e.keyCode;

        if(characterCode == 13)

            {

            document.getElementById(nextfield).focus();

            return false;

            } 

        else

            {

            return true;

            }

        }

    </script>

<input type='text' name='plt' onkeypress='return handleEnter(event,\"b0\");' />

Thank you