Writing Tests#

Because most of the Redux code you write are functions, and many of them are pure, they are easy to test without mocking.

Setting Up#

We recommend Jest as the testing engine. Note that it runs in a Node environment, so you won't have access to the DOM.

npm install --save-dev jest

To use it together with Babel, you will need to install babel-jest:

npm install --save-dev babel-jest

and configure it to use babel-preset-env features in .babelrc:

{
  "presets": ["@babel/preset-env"]
}

Then, add this to scripts in your package.json:

{
  ...
  "scripts": {
    ...
    "test": "jest",
    "test:watch": "npm test -- --watch"
  },
  ...
}

and run npm test to run it once, or npm run test:watch to test on every file change.

Action Creators#

In Redux, action creators are functions which return plain objects. When testing action creators, we want to test whether the correct action creator was called and also whether the right action was returned.

Example#

export function addTodo(text) {
  return {
    type: 'ADD_TODO',
    text
  }
}

can be tested like:

import * as actions from '../../actions/TodoActions'
import * as types from '../../constants/ActionTypes'

describe('actions', () => {
  it('should create an action to add a todo', () => {
    const text = 'Finish docs'
    const expectedAction = {
      type: types.ADD_TODO,
      text
    }
    expect(actions.addTodo(text)).toEqual(expectedAction)
  })
})

Async Action Creators#

For async action creators using Redux Thunk or other middleware, it's best to completely mock the Redux store for tests. You can apply the middleware to a mock store using redux-mock-store. You can also use fetch-mock to mock the HTTP requests.

Example#

import 'cross-fetch/polyfill'

function fetchTodosRequest() {
  return {
    type: FETCH_TODOS_REQUEST
  }
}

function fetchTodosSuccess(body) {
  return {
    type: FETCH_TODOS_SUCCESS,
    body
  }
}

function fetchTodosFailure(ex) {
  return {
    type: FETCH_TODOS_FAILURE,
    ex
  }
}

export function fetchTodos() {
  return dispatch => {
    dispatch(fetchTodosRequest())
    return fetch('http://example.com/todos')
      .then(res => res.json())
      .then(body => dispatch(fetchTodosSuccess(body)))
      .catch(ex => dispatch(fetchTodosFailure(ex)))
  }
}

can be tested like:

import configureMockStore from 'redux-mock-store'
import thunk from 'redux-thunk'
import * as actions from '../../actions/TodoActions'
import * as types from '../../constants/ActionTypes'
import fetchMock from 'fetch-mock'
import expect from 'expect' // You can use any testing library

const middlewares = [thunk]
const mockStore = configureMockStore(middlewares)

describe('async actions', () => {
  afterEach(() => {
    fetchMock.restore()
  })

  it('creates FETCH_TODOS_SUCCESS when fetching todos has been done', () => {
    fetchMock.getOnce('/todos', {
      body: { todos: ['do something'] },
      headers: { 'content-type': 'application/json' }
    })

    const expectedActions = [
      { type: types.FETCH_TODOS_REQUEST },
      { type: types.FETCH_TODOS_SUCCESS, body: { todos: ['do something'] } }
    ]
    const store = mockStore({ todos: [] })

    return store.dispatch(actions.fetchTodos()).then(() => {
      // return of async actions
      expect(store.getActions()).toEqual(expectedActions)
    })
  })
})

Reducers#

A reducer should return the new state after applying the action to the previous state, and that's the behavior tested below.

Example#

import { ADD_TODO } from '../constants/ActionTypes'

const initialState = [
  {
    text: 'Use Redux',
    completed: false,
    id: 0
  }
]

export default function todos(state = initialState, action) {
  switch (action.type) {
    case ADD_TODO:
      return [
        {
          id: state.reduce((maxId, todo) => Math.max(todo.id, maxId), -1) + 1,
          completed: false,
          text: action.text
        },
        ...state
      ]

    default:
      return state
  }
}

can be tested like:

import reducer from '../../structuring-reducers/todos'
import * as types from '../../constants/ActionTypes'

describe('todos reducer', () => {
  it('should return the initial state', () => {
    expect(reducer(undefined, {})).toEqual([
      {
        text: 'Use Redux',
        completed: false,
        id: 0
      }
    ])
  })

  it('should handle ADD_TODO', () => {
    expect(
      reducer([], {
        type: types.ADD_TODO,
        text: 'Run the tests'
      })
    ).toEqual([
      {
        text: 'Run the tests',
        completed: false,
        id: 0
      }
    ])

    expect(
      reducer(
        [
          {
            text: 'Use Redux',
            completed: false,
            id: 0
          }
        ],
        {
          type: types.ADD_TODO,
          text: 'Run the tests'
        }
      )
    ).toEqual([
      {
        text: 'Run the tests',
        completed: false,
        id: 1
      },
      {
        text: 'Use Redux',
        completed: false,
        id: 0
      }
    ])
  })
})

Components#

First, we will install React Testing Library. React Testing Library is a simple and complete React DOM testing utility that encourage good testing practices. It uses react-dom's render function and act from react-dom/tests-utils.

npm install --save-dev @testing-library/react

If you are using jest as recommended above, we also recommend installing jest-dom as it provides a set of custom jest matchers that you can use to extend jest. These will make your tests more declarative, clear to read and to maintain. jest-dom is being used in the examples below.

npm install --save-dev @testing-library/jest-dom

Consider the following App component:

import { useSelector } from 'react-redux'

export default function App() {
  const user = useSelector(state => state.user)
  return <div>{user}</div>
}

To test the component, we render it into the DOM and pass stubbed callbacks as props, then we assert whether the callbacks were called when expected. We can use the wrapper option in the render function and export our own render function as explained in React Testing Library's setup docs.

Our render function can look like this:

// test-utils.js
import React from 'react'
import { render as rtlRender } from '@testing-library/react'
import { createStore } from 'redux'
import { Provider } from 'react-redux'
// Import your own reducer
import reducer from '../reducer'

function render(
  ui,
  {
    initialState,
    store = createStore(reducer, initialState),
    ...renderOptions
  } = {}
) {
  function Wrapper({ children }) {
    return <Provider store={store}>{children}</Provider>
  }
  return rtlRender(ui, { wrapper: Wrapper, ...renderOptions })
}

// re-export everything
export * from '@testing-library/react'
// override render method
export { render }

And our test can use our exported render function:

import React from 'react'
// We're using our own custom render function and not RTL's render
// our custom utils also re-export everything from RTL
// so we can import fireEvent and screen here as well
import { render, fireEvent, screen } from '../../test-utils'
import App from '../../containers/App'

it('Renders the app with initialState', () => {
  render(<App />, { initialState: { user: 'Redux User' } })

  expect(screen.getByText(/redux user/i)).toBeInTheDocument()
})

Middleware#

Middleware functions wrap behavior of dispatch calls in Redux, so to test this modified behavior we need to mock the behavior of the dispatch call.

Example#

First, we'll need a middleware function. This is similar to the real redux-thunk.

const thunk =
  ({ dispatch, getState }) =>
  next =>
  action => {
    if (typeof action === 'function') {
      return action(dispatch, getState)
    }

    return next(action)
  }

We need to create a fake getState, dispatch, and next functions. We use jest.fn() to create stubs, but with other test frameworks you would likely use Sinon.

The invoke function runs our middleware in the same way Redux does.

const create = () => {
  const store = {
    getState: jest.fn(() => ({})),
    dispatch: jest.fn()
  }
  const next = jest.fn()

  const invoke = action => thunk(store)(next)(action)

  return { store, next, invoke }
}

We test that our middleware is calling the getState, dispatch, and next functions at the right time.

it('passes through non-function action', () => {
  const { next, invoke } = create()
  const action = { type: 'TEST' }
  invoke(action)
  expect(next).toHaveBeenCalledWith(action)
})

it('calls the function', () => {
  const { invoke } = create()
  const fn = jest.fn()
  invoke(fn)
  expect(fn).toHaveBeenCalled()
})

it('passes dispatch and getState', () => {
  const { store, invoke } = create()
  invoke((dispatch, getState) => {
    dispatch('TEST DISPATCH')
    getState()
  })
  expect(store.dispatch).toHaveBeenCalledWith('TEST DISPATCH')
  expect(store.getState).toHaveBeenCalled()
})

In some cases, you will need to modify the create function to use different mock implementations of getState and next.

Glossary#

React Testing Library: React Testing Library is a very light-weight solution for testing React components. It provides light utility functions on top of react-dom and react-dom/test-utils, in a way that encourages better testing practices. Its primary guiding principle is: "The more your tests resemble the way your software is used, the more confidence they can give you."
React Test Utils: ReactTestUtils makes it easy to test React components in the testing framework of your choice. React Testing Library uses the act function exported by React Test Utils.