Example 1-3 from JavaScript: The Definitive Guide, Fifth Ed.

<html>
<head>
<title>JavaScript Loan Calculator</title>
<style>
/* This is a CSS style sheet: it adds style to the program output */
.result { font-weight: bold; }  /* For elements with class="result" */
#payment { text-decoration: underline; } /* For element with id="payment" */
</style>
</head>
<body>
<!--
  This is an HTML form that allows the user to enter data and allows
  JavaScript to display the results it computes back to the user. The
  form elements are embedded in a table to improve their appearance.
  The form itself is given the name "loandata", and the fields within
  the form are given names such as "interest" and "years". These
  field names are used in the JavaScript code that follows the form.
  Note that some of the form elements define "onchange" or "onclick"
  event handlers. These specify strings of JavaScript code to be
  executed when the user enters data or clicks on a button.
-->
<form name="loandata">
  <table>
    <tr><td><b>Enter Loan Information:</b></td></tr>
    <tr>
      <td>1) Amount of the loan (any currency):</td>
      <td><input type="text" name="principal" onchange="calculate();"></td>
    </tr>
    <tr>
      <td>2) Annual percentage rate of interest:</td>
      <td><input type="text" name="interest" onchange="calculate();"></td>
    </tr>
    <tr>
      <td>3) Repayment period in years:</td>
      <td><input type="text" name="years" onchange="calculate();"></td>
    </tr>
    <tr><td></td>
      <td><input type="button" value="Compute" onclick="calculate();"></td>
    </tr>
    <tr><td><b>Payment Information:</b></td></tr>
    <tr>
      <td>4) Your monthly payment:</td>
      <td>$<span class="result" id="payment"></span></td>
    </tr>
    <tr>
      <td>5) Your total payment:</td>
      <td>$<span class="result" id="total"></span></td>
    </tr>
    <tr>
      <td>6) Your total interest payments:</td>
      <td>$<span class="result" id="totalinterest"></span></td>
    </tr>
  </table>
</form>

<script language="JavaScript">
/*
 * This is the JavaScript function that makes the example work. Note that
 * this script defines the calculate() function called by the event
 * handlers in the form. The function reads values from the form
 * <input> fields using the names defined in the HTML code above.  It outputs
 * its results into the named <span> elements.
 */
function calculate() {
    // Get the user's input from the form. Assume it is all valid.
    // Convert interest from a percentage to a decimal, and convert from
    // an annual rate to a monthly rate. Convert payment period in years
    // to the number of monthly payments.
    var principal = document.loandata.principal.value;
    var interest = document.loandata.interest.value / 100 / 12;
    var payments = document.loandata.years.value * 12;

    // Now compute the monthly payment figure, using esoteric math.
    var x = Math.pow(1 + interest, payments);
    var monthly = (principal*x*interest)/(x-1);

    // Get named <span> elements from the form.
    var payment = document.getElementById("payment");
    var total = document.getElementById("total");
    var totalinterest = document.getElementById("totalinterest");

    // Check that the result is a finite number. If so, display the
    // results by setting the HTML content of each <span> element.
    if (isFinite(monthly)) {
        payment.innerHTML = monthly.toFixed(2);
        total.innerHTML = (monthly * payments).toFixed(2);
        totalinterest.innerHTML = ((monthly*payments)-principal).toFixed(2);
    }
    // Otherwise, the user's input was probably invalid, so display nothing.
    else {
        payment.innerHTML = "";
        total.innerHTML = ""
        totalinterest.innerHTML = "";
    }
}
</script>
</body>
</html>