compute sum dynamically with javascript

Question

I have two textboxes Num1 and Num2 and another textbox Sum having the value 10.

How can I do this wherein if the user

Answer 1

function addNums() {
  var num1 = parseInt(document.getElementById('Num1').value,10);
  var num2 = parseInt(document.getElementById('Num2').value,10)
  document.getElementById('Sum').value = (num1 + num2).toString();
}

There are other ways to reference the form items, but this one works.