add_action('wp_ajax_add_product_to_cart', 'add_product_to_cart_ajax');
add_action('wp_ajax_nopriv_add_product_to_cart', 'add_product_to_cart_ajax');

function add_product_to_cart_ajax() {
    $product_id = isset($_POST['product_id']) ? intval($_POST['product_id']) : 0;
    $quantity = isset($_POST['quantity']) ? intval($_POST['quantity']) : 1;

    if ($product_id > 0) {
        WC()->cart->add_to_cart($product_id, $quantity);
        wp_send_json('Product added to cart');
    } else {
        wp_send_json('Invalid product');
    }
}

//second custom code to direct checkout.

add_filter('woocommerce_add_to_cart_redirect', 'custom_add_to_cart_redirect');

function custom_add_to_cart_redirect($url) {
    global $woocommerce;
    $checkout_url = wc_get_checkout_url();

    return $checkout_url;
}