jQuery serialize does not register checkboxes

jQuery serialize closely mimics how a standard form would be serialized by the browser before being appended to the query string or POST body in the request. Unchecked checkboxes aren't included by the browser, which makes sense really because they have a boolean state -- they're either selected by the user (included) or not selected by the user (not included).

If you need it to be in the serialized, you should ask yourself "why? why not just check for its existence in the data?".

Bear in mind that if the way JavaScript serializes form data behaves differently to the way the browser does it then you're eliminating any chance of graceful degradation for your form. If you still absolutely need to do it, just use a <select> box with Yes/No as options. At least then, users with JS disabled aren't alienated from your site and you're not going against the defined behaviour in the HTML specification.

<select id="event_allDay" name="event_allDay">
   <option value="0" selected>No</option>
   <option value="1">Yes</option>
</select>

I've seen this employed on some sites in the past and always thought to myself, "why don't they just use a checkbox"?

You don't, as serialize is meant to be used as an query string, and in that context, unchecked means it's not in the querystring at all.

If you really want to get the values of unchecked checkboxes, use: (untested off course)

var arr_unchecked_values = $('input[type=checkbox]:not(:checked)').map(function(){return this.value}).get();

To build on azatoth's genius answer, I have slightly extended it for my scenario:

    /* Get input values from form */
    values = jQuery("#myform").serializeArray();

    /* Because serializeArray() ignores unset checkboxes and radio buttons: */
    values = values.concat(
            jQuery('#myform input[type=checkbox]:not(:checked)').map(
                    function() {
                        return {"name": this.name, "value": false}
                    }).get()
    );