super-simple-cache-jest-test.js

it('does check if we already fetched that id and only calls fetch if necessary', () => {
  const store = mockStore({id: 1234, isFetching: false }});
  window.fetch = jest.fn().mockImplementation(() => Promise.resolve());  
  
  store.dispatch(fetchData(1234)); // Same id
  expect(window.fetch).not.toBeCalled();

  store.dispatch(fetchData(1234 + 1)); // Different id
  expect(window.fetch).toBeCalled();
});