Making an ajax request from a different domain

Question

If you want to use Admin-Ajax to output the content, you should use echo instead. There is nothing to be viewed when you use return, so you’ll get a 0.

This quotation from the codex explains further about the 0 response:

If the Ajax request fails in wp-admin/admin-ajax.php, the response
will be -1 or 0, depending on the reason for the failure.
Additionally, if the request succeeds, but the Ajax action does not
match a WordPress hook defined with add_action('wp_ajax_(action)', ...) or add_action('wp_ajax_nopriv_(action)', ...), then
admin-ajax.php will respond 0.

Now, if you are going to output a JSON response, you should take a look into the REST API. Its default response type is JSON.

To do so, register a path for the endpoint, and create a callback function:

add_action( 'rest_api_init', function () {
    register_rest_route( 'dcp3450', '/test_endpoint/', array(
            'methods' => 'GET', 
            'callback' => 'sendhire' 
    ) );
});

function sendhire() {

    $cars = array("Volvo", "BMW", "Toyota");
    return $cars;

}

Now by accessing http://example.com/wp-json/dcp3450/test_endpoint/ you will get your JSON response, and you can get rid of this annoying 0 that is following humanity to its end.