testing epics by mocking

stuff.js

// api.js
// your API call helpers
import { ajax } from 'rxjs/observable/dom/ajax';

export const fetchSomething = id =>
  ajax.getJSON(`/somethings/${id}`);

// the epic

export const somethingEpic = (action$, store, { api }) =>
  action$.ofType(SOMETHING)
    .mergeMap(action =>
      api.fetchSomething(action.id)
        .map(response => somethingFulfilled(response))
    );

// inject your dependencies into all of your epics
// when making the root epic 

import * as api from './api'

export const rootEpic = (...args) =>
  combineEpics(somethingEpic, anotherEpic)(...args, { api });

// later to test, just mock the dependencies

const action$ = ActionsObservable.of({ type: SOMETHING, id: '123' });
const response = { id: '123', something: 'example' };
const api = { fetchSomething: id => Observable.of(response) };

somethingEpic(action$, null, { api })
  .toArray()
  .subscribe(actions => {
    assertDeepEqual(actions, [
      somethingFulfilled(response)
    ]);
  });