'is' functions and 'get_query_var' not working

Question

Why does is functions not work inside an Ajax function? Except for is_user_logged_in?

I also tried using get_query_var and it did not work.

jQuery:

jQuery('.post-modal').live('click', function(e) {       

    e.preventDefault();

    var post_id = jQuery(this).data('post-id');

    jQuery.ajax({
        type: 'POST',
        url: mysite.ajax_url,
        data: {
            'action' : 'post_modal_content',
            'post_id' : post_id             
        },                      
        success: function(response) {
            var json = jQuery.parseJSON(response);
            jQuery('#post-modal .content').html(json.html);             
        }                                   
    });     
});

PHP:

function post_modal_content() {

    $post_id = $_POST['post_id'];   

    if (is_author()) {
        set_query_var('post_id', $post_id);
        ob_start();
        get_template_part('content-post-modal');
        $html = ob_get_contents();
        ob_end_clean();  

        $response = array('html' => $html);
        echo json_encode( $response );
        exit;
    }   
}
add_action( 'wp_ajax_post_modal_content', 'post_modal_content' );
add_action( 'wp_ajax_nopriv_post_modal_content', 'post_modal_content' );

If I am on the author page it does not work, but if I take the check everything works fine.

Answer 1

You send your AJAX request to /wp-admin/admin-ajax.php (which is very good), so it's not a typical WP request.

During such request, request is not parsed, no global WP_Query is created, and so on.

is_author is a Conditional Tag, which checks if an Author archive page is being displayed. So, by design, it will return false during AJAX request, because AJAX request is not displaying author archive page...

On the other hand, is_user_logged_in() works, because user is still logged in during AJAX request (it's the same session).

PS. It's the same with get_query_var - no query is parsed or executed, so there are no query variables available...