I am calling server action from src/actions/actions.ts

"use server";

export const submitCartProduct = async (
    locale?: string, 
    params?: Record<string, any>
) => {
    let data = null;

    try {
        const session = await auth();

        const res = await fetch(buildUrl(`/api/cart/add`, {
            locale: locale,
            jwt: session?.jwt,
            ...params
        }), {
            method: "POST",
            next: { revalidate: 0 }
        }); 

        data = await res.json();

        console.log(667, data);
    } catch (err) {return null}

    return data;
};

the console.log(667, data) outputs all data as expected

but in client component

useEffect(() => {
    (async () => {
        const res = await submitCartProduct(lang, {
            product_id: product.id,
            qty: count
        });

        console.log(420, res);
    }) ();
}, [count]);

the res in console.log(420, res) always is equal to undefined???

(self invoking function, because I tried different variations, because all examples on web had the server action on client side wrapped in a async function)