Imagine that you have following situation: your calls to external api are using helper modules where all request logic lies. For instance, you have
backendService:
exportconstmakeRequest=async()=>return fetch.get('http://www.some_api.com');In your epic you use this as follows:
import*as backendService from'./backendService'constfetchBackendServiceEpic=(action$, store)=>
action$
.filter(isActionOf(YOUR_TRIGGER_ACTION)).switchMap(action=> Observable.from(backendService.makeRequest())).mapTo(YOUR_SUCCESS_ACTION).catch(err=> Observable.of(YOUR_ERROR_ACTION))Everything seems fine but to test this epic you have to mock whole API request to http://www.some_api.com using for instance nock . Your test starts to get a little bit heavy to write:
describe('fetchBackendServiceEpic',()=>{
it.should('fetch service',()=>{const mockStore =configureMockStore()nock(`http://www.some_api.com`).get().reply(200,{})returnrootEpic(ActionsObservable.from(YOUR_TRIGGER_ACTION), mockStore).toArray().toPromise().then(actions=>{expect(actions).toEqual([YOUR_TRIGGER_ACTION,YOUR_SUCCESS_ACTION])})})})Can you do better?
Yes, using dependency injection. Your epic is depended on backendService. How can you inject it to your epic? The actual way of doing this is in the documentation : Injecting Dependencies Into Epics · redux-observable. Thanks to that you have your epic with dependency injected:
constfetchBackendServiceEpic=(action$, store,{ makeRequest })=>
action$
.filter(isActionOf(YOUR_TRIGGER_ACTION)).switchMap(action=> Observable.from(backendService.makeRequest())).mapTo(YOUR_SUCCESS_ACTION).catch(err=> Observable.of(YOUR_ERROR_ACTION))To test it you need:
describe('fetchBackendServiceEpic',()=>{
it.should('fetch service',()=>{const dependencies ={ makeRequest: jest.fn().mockReturnValue([{}])}fetchBackendServiceEpic(YOUR_TRIGGER_ACTION,(store =null), dependencies).toArray().subscribe(actions=>{expect(actions).toEqual([YOUR_TRIGGER_ACTIONS,YOUR_SUCCESS_ACTION])})})})As you see here there is just injecting dependency with your desired return value - you are testing just logic of your epics and if proper actions are dispatched of your TRIGGER_ACTION.
Summary
As you can see dependency injection in epics can be a powerful way to ease your testing of epics.