IE&FF兼容性 - event, keycode, returnvalue

         <SCRIPT language="javascript">
        function isDigit(oEvent)
        {        
            var e = oEvent||window.event;
            var codeKeyChar = e.keyCode||e.charCode;

            // clear Unit No. after postal code was changed ,By Wen Chao(NCS),Monday, October 25, 2010
            clearUnitNo();
            if ((codeKeyChar < 46) || (codeKeyChar > 57) && (codeKeyChar != 47))
             if(window.event)
                   window.event.returnValue = false;
            else
                   e.preventDefault();//for firefox
        }
        // clear Unit No. after postal code was changed ,By Wen Chao(NCS),Monday, October 25, 2010
        function clearUnitNo()
        {                
            document.getElementById('txtUnitNo').value="";
        }
        </SCRIPT>