Why is my ajax request getting response 0?

Question

I've setup the basic wordpress ajax example in my wp theme. The trigger is made by modernizr.js checking the media queries on the page.

jQuery(document).ready(function($) {
    if(Modernizr.mq('only all and (max-width:6300px)')) {
        var data = {
        action: 'my_action',
        whatever: ajax_object.we_value      // We pass php values differently!
    };
    // We can also pass the url value separately from ajaxurl for front end AJAX implementations
        jQuery.post(ajax_object.ajax_url, data, function(data) {
            $("#trending-Container").html(data).fadeIn(1000);
        });
    }

});//end function 

I have localized and enqueue'd my scripts.

wp_enqueue_script('mainJS', get_template_directory_uri() . '/js/mainJS.js', array("jquery") );
wp_localize_script( 'mainJS', 'ajax_object', 
                    array( 'ajax_url' => admin_url( 'admin-ajax.php' ), 'we_value' => 1234 ) );

and finally the function that handles the request is:

add_action('wp_ajax_my_action', 'my_action_callback');
add_action('wp_ajax_nopriv_my_action', 'my_action_callback');
function my_action_callback() {
        global $wpdb;
            $whatever = intval( $_POST['whatever'] );
            $whatever += 10;
                echo $whatever;
            die();

        }

This constantly gives me a response of 0 (no properties) and I do not know why. P.S This is all local.

Status code 200
Host:lart.co.uk
Origin:http://lart.co.uk
Referer:http://lart.co.uk/
User-Agent:Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/28.0.1500.71 Chrome/28.0.1500.71 Safari/537.36
X-Requested-With:XMLHttpRequest
Form Dataview sourceview URL encoded
action:my_action
whatever:1234

Answer 1

Add the "exit" end of the function as shown below, this will fix the return 0 with the response in WordPress on using the ajax request.

add_action('wp_ajax_nopriv_getStateList', 'getStateList');
add_action('wp_ajax_getStateList', 'getStateList');

function getStateList() {
    global $wpdb;
    $countryId = $_POST['countryId'];
    $results = $wpdb->get_results("SELECT id,name FROM regions where country_id ='".$countryId."' ");
    echo json_encode(array('status'=>200,'data'=>$results));
    exit;
}

Answer 2

Everything has to match here:

PHP

add_action('wp_ajax_my_action',        'my_action');
add_action('wp_ajax_nopriv_my_action', 'my_action');

function my_action() {}

JS

var data = {
    action: 'my_action',
    whatever: ajax_object.we_value 
};

Also, you're missing security checks and a better handling of the response.
Check this examples: [ 1 ] and [ 2 ].