Invert checkbox on submit: Set HTML form value only if checkbox is unchecked

Question

I need to "invert" a checkbox, ie submit the value if the checkbox is unchecked, and leave it empty if the checkbox is checked.

I do not want to prevent form submission!

I can think of two approaches:

1. change appearance of the checkbox so that it looks checked if it actually isn't and vice versa
2. change a hidden input via JavaScript when the checkbox is clicked

What's the best way to approach this? I would prefer a "pure" general solution, but since I need it for a project with twitter-bootstrap-3 , jquery and prototypejs , I tagged the question accordingly and if there is an easy way using these frameworks, that's great too.

This works as intended:

<form id="the_form" method="post" action="">
    <input type="checkbox" id="the_checkbox" name="the_checkbox" value="foo" />
</form>
<script>
    document.forms.the_form.observe('submit', function() {
        document.getElementById('the_checkbox').checked = ! document.getElementById('the_checkbox').checked;
    });
</script>

But you will see the checkbox status changing before the next page has loaded.

Answer 1

Ok so what i've understood is that you want to inverse the checkbox.

If it's checked dont send value with the form.

If unchecked then send the value with the form.

I only didn't get if you want to submit the form on checkbox click.

Make a hidden input with the same name that has the value. When the checkbox is checked then disable the hiddenbox.

<input type='hidden' id='hiddenCheckboxValue' value='something' name='testbox'>
<input type='checkbox' value='' name='testbox'>


$('#my_checkbox').click(function(){
    if($(this).is(':checked')){
        document.getElementById('hiddenCheckboxValue').disabled = true;
    } else {
       document.getElementById('hiddenCheckboxValue').disabled = false;
    }
    //If you want to submit on checkbox click
    $( "#form" ).submit();

});

//EDIT forgot that unchecked checkboxes dont get send. fixed i think

Answer 2

You can use onsubmit event this way:

$(".form-class").submit(function (e) {
  e.preventDefault();
  if (!$("#checkbox").is("checked"))
    $(this).submit();
  else
    return false;
});

Answer 3

you can dynamically add/remove elements to the form:

$('the_form').observe('submit', function(event){
  $$('#the_form input[type=checkbox]').each(function(input){
    if (! input.checked) $('the_form').insert({top: new Element('input', {
         type: 'hidden'
       , class: 'dynamicInsert'
       , name: input.name
       , value: 'off'
       })
     });
  });
  this.submit();
  $$('#the_form input.dynamicInsert').each(function(input){
    input.remove();
  });
  event.stop(); // don't know if .submit() accepts an event object as parameter
  return false;
});