ie7

gistfile1.js

function paymentCheck() {
  var obj = {
    'type': 'car',
    'year': 2008,
    'make': 'Dodge',
    'model': 'Avenger',
    'payments': 'in process',
    'paid': 13286,
    'toBePaid': 34856,
    'paymentsLeft': 24,
  }

  return (obj.toBePaid - obj.paid) / obj.paymentsLeft;
}

var button = document.getElementById("calcButton");

if (button.addEventListener) {
  button.addEventListener("click", function() {
    alert(paymentCheck());
  }, false);
} else {
  button.attachEvent("onclick", function() {
    alert(paymentCheck());
  });
}