epic.test.js

import configureMockStore from 'redux-mock-store';
import { createEpicMiddleware, combineEpics } from 'redux-observable';
import { Observable } from 'rxjs';

import userEpic from './userEpic.js';

import { fetchUser } from '../actionCreators/users.js';

describe('userEpic', () => {
  // set up our mock data and mock request function
  const user = { name: 'John Doe', email: 'john@foo.com' };
  const payload = { response: user };
  const request = () => Observable.of(payload);
   
  // Wrap the epic we want to test in a function that accepts actions and store, so we can pass it our dependency.
  const rootEpic = (actions, store) => userEpic(actions, store, { request });
  const epicMiddleware = createEpicMiddleware(rootEpic);
  const mockStore = configureMockStore([epicMiddleware]);

  let store;
  
  const initialState = {
    user: null,
  };

  beforeEach(() => {
    store = mockStore(fromJS(initialState));
  });

  afterEach(() => {
    epicMiddleware.replaceEpic(allReviewsEpic);
  });

  it('should listen on a FETCH_REVIEWS action', () => {
    store.dispatch(fetchUser());

    expect(store.getActions()).toEqual([
      fetchUser(),
      fetchUserSuccess(payload.response),
    ]);
  });
});

/* in your epic file... */

import { ajax } from 'rxjs/observable/dom/ajax';

// Default to our Observable ajax functions when wer're not testing.
export const fetchUserEpic = (action$, store, { request } = { request: ajax })) =>
  action$.ofType(FETCH_REVIEWS)
    .mergeMap(() => request('/user')
              .map(({ response }) => fetchUserSuccess(response))
              .catch((response) => [fetchUserFailure(response.err)])
             );