Interesting JavaScript code example: finance calculator

<script type="text/javascript">
<!--
// Script written by Dave Taylor based on some math suggested by
// Bennett Haselton @ Peacefire.org. Thanks Bennett!

function centNotation(value)
{
  // return the value in normalized dollars.cents notation

  dollars = Math.floor(value);  // chop floating point portion
  cents   = Math.floor((value % 1) * 100);              // and chop everything else
  if (cents < 10) cents = "" + cents + "0";

  return(dollars + "." + cents);
}

function validInt(value)
{
  // return true if it's digits or decimal point-only, false otherwise.

  for (var i=0; i < value.length; i++) {
    var c = value.charAt(i);
    if ((c < '0' || c > '9') && c != '.') {
      alert("There's a problem: all values herein must " +
        "be numbers only, and " + value + " has an unacceptable character " +
        "'" + c + "'");
      return(false);
    }
  }
  return(true);
}

function myPaymentCalc()
{
  // calculate the number of payments and total payments for a simple loan given
  // the interest, monthly payment and principal of the loan.
  
  var interestRate = document.debtcalc.interest.value,
      monthlyPayment = document.debtcalc.monthlypayment.value,
      principal = document.debtcalc.principal.value,
      accumulatingInterest = 1;
  
  if (! validInt(interestRate) || ! validInt(monthlyPayment) ||
      ! validInt(principal))
    return(false);
    
  if (interestRate == 0 || monthlyPayment == 0 || principal == 0) {
    alert("You need to specify your monthly payment, credit card " +
        "interest rate and current debt amount before I can calculate " +
        "how long it'll take for you to pay off your debt.");
    return(false);
  }
  
  interestRate = (interestRate > 1)? interestRate / 100.0 : interestRate;

  monthlyInterestRate = interestRate / 12;

  if (principal * monthlyInterestRate > monthlyPayment) {
    monthlyrate = "$" + centNotation(principal * monthlyInterestRate);
    permonthPayment = "$" + centNotation(monthlyPayment);
    alert("You've got a fundamental problem: your monthly payment of " +
        permonthPayment + " is less than the interest added each month " +
        "(which is " + monthlyrate + ") and you will never " +
        "pay off this debt. It might be a good time to call your creditors.");
    return(false);
  }
   
  interimVal1 = monthlyPayment - (monthlyInterestRate * principal)
  
  numberOfPayments = (Math.log(monthlyPayment) - Math.log(interimVal1)) /
        Math.log(1+monthlyInterestRate);
  
  numberOfPayments = Math.ceil(numberOfPayments);
      
  interimVal2 = Math.pow((monthlyInterestRate+1), (numberOfPayments-1));
  
  remaining = (principal * interimVal2) -
    (monthlyPayment/monthlyInterestRate) * (interimVal2 - 1);
    
  totalPayment = monthlyPayment *(numberOfPayments-1) + remaining;
  
  // clean up the display format to make it attractive...
        
  payments = numberOfPayments;
  totalpayment = "$" + centNotation(totalPayment);
  interestPaid = "$" + centNotation(totalPayment-principal);
  
  alert("Given these figures, you'll end up having " + payments +
        " payments and pay a total of " + totalpayment + ", of which " +
        interestPaid + " is interest.");

} 
// -->
</script>

The other part of this is the form itself, which actually ends up being fairly short by comparison:

<form id="debtcalc" name="debtcalc" action="#">
<div style="border:1px solid #669; padding:3px;font-size:85%;">
<div style="font-size:110%;font-weight:bold;">How long will it
take for you to pay off those holiday bills?</div>
Monthly payment:
<input type="text" size="6" name="monthlypayment" />
interest rate (APR):
<input type="text" name="interest" size="5" />
and amount owed:
<input type="text" name="principal" size="9" />
then
<input type="button" value=" solve " onclick="myPaymentCalc()"
 style="font-size: 80%;background-color: #CFC" />
</div>
</form>