Keyup: add a value when a checkbox is checked

Question

So I am using Keyup to calculate a total of input fields. So far so good. But now I want to do this: if checkbox #a is checked, add 12 to total amount. If not, add 0.

I based my code on this: http://jsfiddle.net/5xzSy/1/

$(document).keyup(function(){ // run anytime the value changes
  var element1 = parseFloat($('#element1').val())  * 16.28 || 0; // get value of field and multiply by price
  var total = 0+element1
});

How can I add the value of a checked checkbox and substract it if people uncheck it. (Or even better: with a radiobutton?) Thank you!

Answer 1

demo: http://jsfiddle.net/5xzSy/1079/ <-- updated (add 12 only if something is given, floatnumber)

btw: input is to be <input/> and not <input></input>

$('input').keyup(function(){ // run anytime the value changes        
  var firstValue = parseFloat($('#first').val()); // get value of field
  var secondValue = parseFloat($('#second').val()); // convert it to a float
  var thirdValue = parseFloat($('#third').val());    
  $('#added').html(firstValue + secondValue + thirdValue); // add them and output it
});

$('#check').on('change', function () {
  var total = parseFloat($('#added').text()); // <-- update for float
  if (total) {
      if ($(this).is(':checked')) {
          total = total + 12;
      } else {
          total = total - 12;
      }
      $('#added').text(total);
  }
})

Answer 2

This should do it...

$(document).keyup(function(){ // run anytime the value changes
  var element1 = parseFloat($('#element1').val())  * 16.28 || 0; // get value of field and multiply by price
  var total = element1;
  if ($('#a').is(':checked')) {
    total = total + 12;
  }
});