function my_custom_add_to_cart_redirect( $url ) {
        $url = wc_get_checkout_url();
        return $url;
}

add_filter( 'woocommerce_add_to_cart_redirect','my_custom_add_to_cart_redirect');

function my_custom_add_to_cart_redirect( $url ) {

    if ( is_product() ){
        $url = wc_get_checkout_url();
        return $url;
    }
}

add_filter( 'woocommerce_add_to_cart_redirect', 'my_custom_add_to_cart_redirect' );