Args for CatFilter (WordPress) - can be used with AJAX filtering

//  What were searching for
$args = array(
    'post_type' => $post_type,
    'post_status' => 'publish'
);

//  Now, if the category has been selected AND it doesn't equal the "show all" value
if(isset($_POST['category']) && $_POST['category'] != -1) {
    //  Explode the string into an array
    $categories = array_map('intval', explode(',', $_POST['category']));
    //  Then load the POST array into the cat args
    $args['category__in'] = $categories;
}

//  Yep, a query
$query = new WP_Query($args);

//  Loop through
if($query->have_posts()) {
    //  Do your thing here
}

@User0123456789 do you have a link to your site I can look at?

@Quin452 it's still in my local env. But I can post the js code here. Wait a moment

@Quin452 please see here