Forums

<table>
    <tr><td colspan="3"><b>Enter Loan Information:</b></td></tr>
    <tr>
      <td>1)</td>
      <td>Amount of the loan (any currency):</td>
      <td><input type="text" name="principal" size="12" 
                 onchange="calculate();"></td>
    </tr>
    <tr>
      <td>2)</td>
      <td>Annual percentage rate of interest:</td>
      <td><input type="text" name="interest" size="12" 
                 onchange="calculate();"></td>
    </tr>
    <tr>
      <td>3)</td>
      <td>Repayment period in years:</td>
      <td><input type="text" name="years" size="12" 
                 onchange="calculate();"></td>
    </tr>
    <tr><td colspan="3">
      <input type="button" value="Compute" onclick="calculate();">
    </td></tr>
    <tr><td colspan="3">
      <b>Payment Information:</b>
    </td></tr>
    <tr>
      <td>4)</td>
      <td>Your monthly payment will be:</td>
      <td><input type="text" name="payment" size="12"></td>
    </tr>
    <tr>
      <td>5)</td>
      <td>Your total payment will be:</td>
      <td><input type="text" name="total" size="12"></td>
    </tr>
    <tr>
      <td>6)</td>
      <td>Your total interest payments will be:</td>
      <td><input type="text" name="totalinterest" size="12"></td>
    </tr>
</table>

function calculate() {
   
    var principal = document.loandata.principal.value;
    var interest = document.loandata.interest.value / 100 / 12;
    var payments = document.loandata.years.value * 12;

   
    var x = Math.pow(1 + interest, payments);
    var monthly = (principal*x*interest)/(x-1);

 
    if (!isNaN(monthly) && 
        (monthly != Number.POSITIVE_INFINITY) &&
        (monthly != Number.NEGATIVE_INFINITY)) {

        document.loandata.payment.value = round(monthly);
        document.loandata.total.value = round(monthly * payments);
        document.loandata.totalinterest.value = 
            round((monthly * payments) - principal);
    }
  
    else {
        document.loandata.payment.value = "";
        document.loandata.total.value = "";
        document.loandata.totalinterest.value = "";
    }
}

function round(x) {
  return Math.round(x*100)/100;
}