What is the second argument for?

I find this code in a tutorial

...
import configureMockStore from 'redux-mock-store';

const middleware = [thunk];
const mockStore = configureMockStore(middleware);
...

it('should create BEGIN_AJAX_CALL & LOAD_COURSES_SUCCESS', (done) => {

    const expectedActions = [
        {type: types.BEGIN_AJAX_CALL},
        {type: types.LOAD_COURSES_SUCCESS, body: {
            courses: [{id:'clean-code', title:'Clean Code'}]
        }}
    ];

    const store = mockStore({courses:[]}, expectedActions);

    store
        .dispatch(courseActions.loadCourses())
        .then(() => {
            const actions = store.getActions();
            expect(actions[0].type).toEqual(types.BEGIN_AJAX_CALL);
            expect(actions[1].type).toEqual(types.LOAD_COURSES_SUCCESS);
            done();
        });
});

and the whole bit with expectedActions doesn't make sense.

The docs say that if there is a second argument to store, it should be a function; (no explanation telling what that function would do though).

At first I thought it was forcing some actions into the store for some reason, but a quick console.log told me that wasn't the case.

Because only dispatch causes actions to accumulate.

So is it a mistake in the text or some wisdom to explore further?

This feature was removed in version 1, but you can find the example in the pre 1 docs.

The parameter expectedActions is used for testing. You can create a mock store with an array of actions, and then dispatch an the 1st action. This action will cause the other other actions to forwarded (dispatch / next) via thunks/api middleware/etc... The test will check if all of the actions in the expectedActions array have acted on the store:

import configureStore from 'redux-mock-store';

    const middlewares = []; // add your middlewares like `redux-thunk` 
    const mockStore = configureStore(middlewares);

    // Test in mocha 
    it('should dispatch action', (done) => {
      const getState = {}; // initial state of the store 
      const action = { type: 'ADD_TODO' };
      const expectedActions = [action];

      const store = mockStore(getState, expectedActions, done);
      store.dispatch(action);
    })