Cannot assign exported class instance in typescript

Question

I have an model store class whose instance would be exported like below:

export class CartStore {
    ...
}

export default new CartStore();

Later in my Network module class when parsing the response JSON from axios:

import cartStore from "./CartStore"

...

export async function addToCart(timeslotId: number, quantity: number) {
    return await axios.post(sprintf(Constants.ENDPOINT_ADD_TO_CART, {
        "timeslot_id": timeslotId
    }), { "quantity": quantity }, getConfig())
        .then(res => {
            let data = (res as AxiosResponse).data;
            if (data != null) {
                // **compilation error** below saying that "cannot assign to 'cartStore' because it is not a variable"
                cartStore = Deserialize(data, CartStore); // <-- 
                ...
            } else {
                console.warn("[NetworkManager]", `null response when addToCart with timeslot id: ${timeslotId}`);
            }
        })
        .catch(defaultErrorHandler);
}

I am using cerialize lib for deserializing my CartStore through Deserialize.

Is it the class instance being exported readonly by default? How to fix the above error?

UPDATE

Related answer: https://stackoverflow.com/a/32558929/510577