Simplifying the Test Structure

To avoid duplicating the test setup before every it() clause, we can use Jest’s beforeEach() method to combine the setup in one place:

describe('sample middleware', () => {
  let next, dispatch, getState, middleware;
  
  // using beforeEach
  beforeEach(() => {
    next       = jest.fn();
    dispatch   = jest.fn();
    getState   = jest.fn();
    middleware = sampleMiddleware({ dispatch, getState })(next);
  });

  it('should process action', () => {
    const sampleAction = { type: 'SAMPLE_ACTION' };

    // pass sampleAction through the middleware
    middleware(sampleAction);

    // test
    expect(next.mock.calls).toEqual([[sampleAction]]);
  };
};

Our middleware will be rebuilt before each test using this structure. All the mocked functions will be reset, keeping the testing code itself as short as possible.