Sourcecode:

<script language="JavaScript">

var FKeyPad = document.Keypad;
var Accum = 0;
var FlagNewNum = false;
var PendingOp = "";
function NumPressed (Num) {
if (FlagNewNum) {
FKeyPad.ReadOut.value  = Num;
FlagNewNum = false;
   }
else {
if (FKeyPad.ReadOut.value == "0")
FKeyPad.ReadOut.value = Num;
else
FKeyPad.ReadOut.value += Num;
   }
}
function Operation (Op) {
var Readout = FKeyPad.ReadOut.value;
if (FlagNewNum && PendingOp != "=");
else
{
FlagNewNum = true;
if ( '+' == PendingOp )
Accum += parseFloat(Readout);
else if ( '-' == PendingOp )
Accum -= parseFloat(Readout);
else if ( '/' == PendingOp )
Accum /= parseFloat(Readout);
else if ( '*' == PendingOp )
Accum *= parseFloat(Readout);
else
Accum = parseFloat(Readout);
FKeyPad.ReadOut.value = Accum;
PendingOp = Op;
   }
}
function Decimal () {
var curReadOut = FKeyPad.ReadOut.value;
if (FlagNewNum) {
curReadOut = "0.";
FlagNewNum = false;
   }
else
{
if (curReadOut.indexOf(".") == -1)
curReadOut += ".";
   }
FKeyPad.ReadOut.value = curReadOut;
}
function ClearEntry () {
FKeyPad.ReadOut.value = "0";
FlagNewNum = true;
}
function Clear () {
Accum = 0;
PendingOp = "";
ClearEntry();
}
function Neg () {
FKeyPad.ReadOut.value = parseFloat(FKeyPad.ReadOut.value) * -1;
}
function Percent () {
FKeyPad.ReadOut.value = (parseFloat(FKeyPad.ReadOut.value) / 100) * parseFloat(Accum);
}
</script>

<form name="Keypad" action>
 <table>
  <tr>
   <td>
     <table border="2" width="100" height="60" cellpadding="0" cellspacing="10" bgcolor="#BBBBFF">
      <tr>
       <td colspan="5"><input name="ReadOut" type="Text" size="31" value="0" width="100%" readonly> </td>
      </tr>
      <tr>
       <td align="middle"><input name="btnSeven" type="Button" value="  7  " onclick="NumPressed(7)"> </td>
       <td align="middle"><input name="btnEight" type="Button" value="  8  " onclick="NumPressed(8)"> </td>
       <td align="middle"><input name="btnNine" type="Button" value="  9  " onclick="NumPressed(9)"> </td>
       <td align="middle"><input name="btnNeg" type="Button" value="  +/- " onclick="Neg()"> </td>
       <td align="middle"><input name="btnPercent" type="Button" value="  %  " onclick="Percent()"> </td>
      </tr>
      <tr>
       <td align="middle"><input name="btnFour" type="Button" value="  4  " onclick="NumPressed(4)"> </td>
       <td align="middle"><input name="btnFive" type="Button" value="  5  " onclick="NumPressed(5)"> </td>
       <td align="middle"><input name="btnSix" type="Button" value="  6  " onclick="NumPressed(6)"> </td>
       <td align="middle"><input name="btnPlus" type="Button" value="   +   " onclick="Operation('+')"> </td>
       <td align="middle"><input name="btnMinus" type="Button" value="   -    " onclick="Operation('-')"> </td>
      </tr>
      <tr>
       <td align="middle"><input name="btnOne" type="Button" value="  1  " onclick="NumPressed(1)"> </td>
       <td align="middle"><input name="btnTwo" type="Button" value="  2  " onclick="NumPressed(2)"> </td>
       <td align="middle"><input name="btnThree" type="Button" value="  3  " onclick="NumPressed(3)"> </td>
       <td align="middle"><input name="btnMultiply" type="Button" value=   *    onclick="Operation('*')">
       </td>
       <td align="middle"><input name="btnDivide" type="Button" value="   /    " onclick="Operation('/')"> </td>
      </tr>
      <tr>
       <td align="middle"><input name="btnZero" type="Button" value="  0  " onclick="NumPressed(0)"> </td>
       <td align="middle"><input name="btnDecimal" type="Button" value="   .  " onclick="Decimal()"> </td>
       <td align="middle"><input name="btnEquals" type="Button" value="  =  " onclick="Operation('=')"> </td>
       <td align="middle"><input name="btnClear" type="Button" value=  C    onclick="Clear()"> </td>
       <td align="middle"><input name="btnClearEntry" type="Button" value= CE  onclick="ClearEntry()"> </td>
      </tr>
     </table>
    </td>
   </tr>
 </table>
</form>