public function setCartItemQuantity($cartItemId, $newQuantity)
    {
        $items = collect(session($this->itemsKey));

        $items = $items->map(function ($item) use ($cartItemId, $newQuantity) {
            if ($item['id'] == $cartItemId) {
                $item['quantity'] = $newQuantity;
            }

            return $item;
        });

        session([$this->itemsKey => $items]);
    }