Stub a closure function using sinon for redux actions
<p>I have an redux action which has an async call to an API.</p>

<pre>import { ForbiddenRequest, APIError } from "../errors"
import { fetchActionCreator } from "redux-fetch-helpers"

export const ADD_CART_ITEMS = "ADD_CART_ITEMS"

export default function addCartItems(items, authToken){
    return fetchActionCreator({
        url: `${API_BASE}/ajax_cart`, //eslint-disable-line no-undef
        fetchOptions: {
            method: "POST",
            headers: new Headers({
                "Authorization": `jwt ${authToken}`,
                "Accept": "application/json",
                "Content-Type": "application/json",
            }),
            body: JSON.stringify(items),
            credentials: "same-origin",
        },
        actionType: ADD_CART_ITEMS,
        responseConfig: {
            200: (resp => resp),
            403: (payload => new ForbiddenRequest(payload)),
            other: (payload => new APIError(payload)),
        }
    })
}</pre>

<p>I am trying to mock the fetchActionCreator method by using sinon stubs.
This is my spec file</p>

<pre>
import addCartItems from "../../actions/addCartItems"
import sinon from "sinon"

describe("The addCartItems action", () => {
    let fetchActionCreator
    it("should create an action to add an item into the cart", () => {
        const addedCartItem = {
            items: [{product_variant: 10, quantity: 1}]
        }
        const expectedAction = { type: "ADD_CART_ITEMS", meta: {sequence: "BEGIN"}}
        fetchActionCreator = sinon.stub()
        // fetchActionCreator()
        fetchActionCreator.returns(expectedAction)
        expect(addCartItems(addedCartItem, "someToken")).to.equal(expectedAction)
    })

})</pre>

<p>But somehow the function is not getting mocked. Can you suggest me the right way.</p>