React Redux Saga Testing with Jest

Introduction

Have you ever wondered how to properly test Redux Saga in your React applications? Testing Redux Saga is crucial to ensure the correctness and reliability of your asynchronous actions and side effects. In this article, we will explore how to test Redux Saga in React applications using Jest.

Setting Up Jest

Before we dive into testing Redux Saga, let’s make sure we have the necessary tools set up. You will need to have Jest installed in your project. If you haven’t already, you can install it by running the following command:

npm install --save-dev jest

Once Jest is installed, you can create a test file with the extension .test.js. This convention will make Jest automatically recognize and run your tests.

Writing Tests for Redux Saga

To test Redux Saga, we can utilize the power of Jest and its built-in testing utilities. There are several key concepts and techniques we need to understand:

Mocking

Mocking is an essential technique in testing. It allows us to simulate different scenarios and control the behavior of external dependencies. In Redux Saga testing, we often use mocking to replace API calls or other async functions with predefined mock data.

Jest provides various ways to create mocks, such as jest.fn(), jest.spyOn(), and jest.mock(). These methods allow us to create mock functions or modules and define their return values or behavior.

Here’s an example of how we can mock an API call in a Redux Saga test:

import { call } from 'redux-saga/effects';

function* mySaga() {
  const response = yield call(api.get, '/users');
  // do something with the response
}

it('should call the API', () => {
  const gen = mySaga();

  // Replace the actual API call with a mock function
  jest.spyOn(api, 'get').mockImplementation(() => ({ data: 'mockedData' }));

  const next = gen.next();

  // Check if the API call is replaced with the mock function
  expect(next.value).toEqual(call(api.get, '/users'));
});

Testing Generator Functions

Redux Saga uses generator functions to handle asynchronous actions. When testing Redux Saga, we need to test these generator functions. Jest provides a utility called next() to test the flow of generator functions.

Here’s an example of how we can test the flow of a Redux Saga generator function:

import { put } from 'redux-saga/effects';

function* mySaga() {
  yield put({ type: 'START_LOADING' });
  yield call(api.fetchData);
  yield put({ type: 'DATA_RECEIVED' });
}

it('should dispatch actions in order', () => {
  const gen = mySaga();

  // Test if the first `put` action is dispatched
  expect(gen.next().value).toEqual(put({ type: 'START_LOADING' }));

  // Test if the `call` effect is executed
  expect(gen.next().value).toEqual(call(api.fetchData));

  // Test if the second `put` action is dispatched
  expect(gen.next().value).toEqual(put({ type: 'DATA_RECEIVED' }));

  // Test if the generator finishes
  expect(gen.next().done).toBeTruthy();
});

Testing Effects

Redux Saga provides a set of effects to handle asynchronous actions, such as call, put, take, and select. These effects are plain JavaScript objects that describe what the generator function should do.

To test effects in Redux Saga, we can use Jest’s toEqual() matcher to compare the expected effect with the actual effect yielded by the generator function.

Here’s an example of how we can test a call effect in Redux Saga:

import { call } from 'redux-saga/effects';

function* mySaga() {
  const result = yield call(api.fetchData);
  // do something with the result
}

it('should call a function', () => {
  const gen = mySaga();

  // Test if the effect is a `call` effect
  expect(gen.next().value).toEqual(call(api.fetchData));
});

Conclusion

In this article, we have covered the basics of testing Redux Saga in React applications using Jest. We have learned about mocking, testing generator functions, and testing effects. By properly testing Redux Saga, we can ensure the correctness and reliability of our asynchronous actions and side effects.

Happy testing!