Going epic with Redux Observable tests 
Tags: react, redux, rxjs
Published: 2020-06-09 10:02:26

The last couple of React projects I've developed where built with a lot of help from the Redux Observable library. It is an excellent library to separate your business logic from your components, but the correct way to test is still something they need to find out. In this article, I am gonna share my ideas on this topic.

## So what is this Redux Observable?

For those who aren't aware with any of this library, I recommend to you to check out the RxJS + Redux + React = Amazing! talk by Jay Phelps. It's a very inspiring talk on how Netflix uses some common JS patterns combined with the powers of RxJS to manage your business logic within your React application. They've extracted the core from Netflix and shared it as an open-source library on Github.

![Logo of Redux Observable](https://robinvdvleuten.nl/images/redux-observable.gif)

Their [documentation](https://redux-observable.js.org/") is excellent and contains a lot of small running examples to help get you started. The whole library deserves an article on its own, but one _important_ aspect is still a bit underexposed. As a matter of fact, they are still struggling with __the best way&trade;__ themselves;

<blockquote>
  <p>
    Testing async code that creates side effects isn't easy. We're still learning the best way to test Epics. If you have found the perfect way, do share!
  </p>
  <footer>
    <cite>
      <a href="https://redux-observable.js.org/docs/recipes/WritingTests.html">Redux Observable docs</a>
    </cite>
  </footer>
</blockquote>

After struggling with the Observable tests on a couple of projects, I would like to give my two cents on the topic in this article.

## What epic are we going to test?

To get a nice epic to show how you can test asynchronous business logic, I came up with the following;

```js
export const authenticateUserEpic = (action$, store, { client }) => {
  // Only act on actions of a given type,
  // in this case "USER_AUTHENTICATE_REQUEST".
  return action$.ofType('USER_AUTHENTICATE_REQUEST')
    // Map the received action to a new action "observable".
    .switchMap(action => {
      // Authenticate with the dispatched credentials in the action,
      // using the injected client instance.
      return client.authenticate(action.username, action.password)
        .then(response => {
          if (!response.isSuccessful) {
            // Map the response to a "failed" action with the error.
            return {
              type: 'USER_AUTHENTICATE_FAILURE',
              error: 'Something went wrong while authenticating',
            };
          }

          return {
            // Map the response to a "successful" action with a JWT token.
            type: 'USER_AUTHENTICATE_SUCCESS',
            idToken: response.idToken,
          };
        });
    });
}
```

As you may have noticed is this epic about authenticating an user with the dispatched credentials. I can imagine I would dispatch such action like this;

```js
export const authenticate = (username, password) {
  return { type: 'USER_AUTHENTICATE_REQUEST', username, password };
}

dispatch(authenticate('johndoe', 'mysupersecretpassword'));
```

You may also have noticed that I've <a href="https://redux-observable.js.org/docs/recipes/InjectingDependenciesIntoEpics.html" title="Read more about injecting in the Redux Observable docs">injected the client dependency</a> into my epic. You could get a client instance through a _require_ or _import_ statement. But by using _dependency injection_ it makes the client way easier to mock and your epic way easier to test.

## Creating the tests with Jest

Most of the React projects out there seem to be using <a href="https://facebook.github.io/jest/" title="Visit the official Jest documentation">Jest</a>, so I'll just use it in the example test.

My approach to test the above epic, is to get the _expected_ action when the epic receives the _dispatched_ action. So a quick glanse on the epic tells us that we need two tests; one where we expect `USER_AUTHENTICATE_SUCCESS` with a JWT token and one where we expect `USER_AUTHENTICATE_FAILURE` with an error.
To define them as Jest tests, one would define them as follows;

```js
describe('authenticateUserEpic', () => {
  it('should dispatch a JWT token when authenticating is successful', () => {
    // ...
  })

  it('should dispatch an error when authenticating has failed', () => {
    // ...
  })
});
```

So let us focus on the first test for now. We need pass the epic the _dispatching_ action and get the resulting action when the RxJS Observable completes.
There are many ways to write such code, but the following works the best for me;

```js
import { ActionsObservable } from 'redux-observable';
import authenticateUserEpic from './epics';

// ...

it('should dispatch a JWT token when authenticating is successful', async () => {
  // The response object we expect to receive from the server.
  const response = {
    isSuccessful: true,
    idToken: 'a-random-generated-jwt',
  };

  // Create a fake client instance which will return
  const client = { authenticate: jest.fn() };
  client.authenticate.mockReturnValue(Promise.resolve(response));

  // Create an Observable stream of the dispatching action.
  const action$ = ActonsObservable.of({
    type: 'USER_AUTHENTICATE_REQUEST',
    username: 'johndoe',
    password: 'mysupersecretpassword',
  });

  // Pass the Observable action to our action and inject the
  // mocked client instance.
  const epic$ = authenticateUserEpic(action$, store, { client });

  // Get the resulting actions by using async/await.
  const result = await epic$.toArray().toPromise();

  // Test if we've received the expected action as result.
  expect(result).toEqual([
    { type: 'USER_AUTHENTICATE_SUCCESS', idToken: 'a-random-generated-jwt' }
  ])
});
```

Not that hard right? You'll need to understand RxJS first. But after that, you will get a nice separation of concerns in your React applications.
To make the examples complete, the following test will handle the failed response;

```js
it('should dispatch an error when authenticating has failed', async () => {
  // The response object we expect to receive from the server.
  const response = {
    isSuccessful: false,
  };

  // Create a fake client instance which will return
  const client = { authenticate: jest.fn() };
  client.authenticate.mockReturnValue(Promise.resolve(response));

  // Create an Observable stream of the dispatching action.
  const action$ = ActonsObservable.of({
    type: 'USER_AUTHENTICATE_REQUEST',
    username: 'johndoe',
    password: 'mysupersecretpassword',
  });

  // Pass the Observable action to our action and inject the
  // mocked client instance.
  const epic$ = authenticateUserEpic(action$, store, { client });

  // Get the resulting actions by using async/await.
  const result = await epic$.toArray().toPromise();

  // Test if we've received the expected action as result.
  expect(result).toEqual([
    { type: 'USER_AUTHENTICATE_FAILURE', error: 'Something went wrong while authenticating' }
  ])
});
```

Did I got some headaches along the way? I definitely got <a href="https://stackoverflow.com/questions/42276419/invoking-epics-from-within-other-epics">some</a> <a href="https://stackoverflow.com/questions/47037119/testing-observable-epic-which-invokes-other-epic">questions</a>  before I had a basic understanding of RxJS! But luckily the Redux Observable community was very helpful. And now I have a very valuable new tool to structure my React applications around 👌