How to mock out reducers to test them

I am fairly new to redux and even less sure how to test stuff like reducers

I have an online shop scenario where i can add stuff to the basket. I am aware I may not post enough code here but I'll see what I can do

so here is my test:

  describe('ADD_TO_BASKET', () => {
    it.only('adds a product to the basket', () => {
      const fakeProduct = { name: 'Trainers', id: 1, price: 100 };
      const action = { type: 'ADD_TO_BASKET', data: fakeProduct };
      const currentState = { items: [], total: 0 };
      const result = reducer(currentState, action);
      expect(result.items.length).to.equal(1);
    });

and I'm just testing the reducer to see what happens when I add something to the basket

problem is, I get this error: Cannot read property 'price' of undefined

and that's because in my reducer I have this:

case 'ADD_TO_BASKET': {
  const items = addProductToBasket(state.items, action);
  return {
    items,
    total: calculateTotal(items)
  };

which calls calculateTotal which has price in it, like so:

const calculateTotal = (items) => {
  return items.reduce((totalPrice, basketItem) => {
    const { product: { price }, quantity } = basketItem;
    const total = price * quantity;
    return totalPrice + total;
  }, 0);
};

how do I mock/stub out this so that in my reducer I don't get these errors? another test is failing coz it can't read some of undefined. im guessing again, I need to mock it somehow. any help??