moreform2.js

//  JavaScript: More Forms, Example 2
//  Calculates Loan Repayment Amounts
//
//  Copyright (c) Paul Griffiths, 2007
//  Email: mail@paulgriffiths.net

function loanRepay(form) {
  var p = form.principal.value;
  var r = Math.pow((form.apr.value / 100) + 1, 1/12) -1;
  var m = form.maturity.value;

  form.repay.value = Math.round((p * r) / ( 1 - Math.pow(1 + r, -m) ) * 100) / 100;
  form.total.value = Math.round(form.repay.value * m * 100) / 100;
}