In this assignment, we are going to look at how to test applications that use
Redux Thunk to handle asynchronous requests.

### Async Actions
Testing asynchronous action creators is a little more complicated than their
synchronous counterparts. Take a look at the test for the fetchBoard action creator:

describe('fetchBoard', () => {
    it('Should dispatch fetchBoardSuccess', () => {
        const board = {
            lists: []
        };

        global.fetch = jest.fn().mockImplementation(() =>
            Promise.resolve({
                ok: true,
                json() {
                    return board;
                }
            })
        );

        const dispatch = jest.fn();
        return fetchBoard()(dispatch).then(() => {
            expect(fetch).toHaveBeenCalledWith('/board');
            expect(dispatch).toHaveBeenCalledWith(fetchBoardSuccess(board));
        });
    });
});

We use a technique called mocking.
Mocking is simulating the behavior of part of our system so that we can
remove that piece of the system from our tests.

Mocking helps to simplify your tests when it would be too complicated to
work with the entire system. In this case we mock the fetch function:

global.fetch = jest.fn().mockImplementation(() =>
    Promise.resolve({
        ok: true,
        json() {
            return board;
        }
    })
);

### Dispatching async actions
Testing for the dispatch of an async action is again a little more tricky
than when we're dispatching a sync action.

This is because it's hard to identify an async action just by looking at
the function returned.