Testing is frequently seen as something tedious. It's extra code and in some cases, to be honest it's not needed. However, every developer should know at least the basics of testing, because it increases confidence in the product and for most companies, it's a requirement.
In React world, there is an amazing library called react-testing-library which helps to test React Apps more efficiently in combination with Jest.
In this article, we will see the 8 simple steps to start testing your React Apps like a boss.

Originally posted on my blog

Prerequisites
Basics
What is React Testing Library?
1. How to create a test snapshot?
2. Testing DOM elements
3. Testing events
4. Testing asynchronous actions
5. Testing React Redux
6. Testing React Context
7. Testing React Router
8. Testing HTTP Request (axios)
Final Thoughts
Next Steps

Prerequisites

This tutorial assumes that you have at least a basic understanding of React. I will focus only on the testing part.
And to follow along, you have to clone the project by running in your terminal:

  git clone https://github.com/ibrahima92/prep-react-testing-library-guide

Next, run:

  yarn

Or, if you use NPM:

npm install

And that's it, let's dive into some basics.

Basics

Some key things will be used a lot in this article, and understanding their role can help you with your understanding.

it or test: describes the test itself. It takes as parameters the name of the test and a function that hold the tests.
expect: it's the condition that the test needs to pass. It will compare the received parameter to a matcher.
a matcher: it's a function that is applied to the expected condition.
render: it's the method used to render a given component.

import React from 'react'
import {render} from '@testing-library/react'
import App from './App'

 it('should take a snapshot', () => {
    const { asFragment } = render(<App />)

    expect(asFragment(<App />)).toMatchSnapshot()
   })
});

As you can see, we describe the test with it, then, use render to display the App component and expect that asFragment(<App />) matches toMatchSnapshot() (the matcher provided by jest-dom). By the way, the render method returns several methods we can use to test our features. We also used destructuring to get the method.

That being said, let's move on and define React Testing Library in the next section.

What is React Testing Library?

React Testing Library is a is a very light-weight package created by Kent C. Dodds. It's a replacement to Enzyme and provides light utility functions on top of react-dom and react-dom/test-utils. React Testing Library is a DOM testing library, which means, instead of dealing with instances of rendered React components, it handles DOM elements and how they behave in front of real users. It's a great library that I like, it's (relatively) easy to start with and it encourages good testing practices and you can also use it without Jest.

"The more your tests resemble the way your software is used, the more confidence they can give you."

So, let's start using it in the next section. By the way, you don't need to install any packages since create-react-app comes with the library and its dependencies.

1. How to create a test snapshot?

A snapshot, as the name guess, allows us to save the snapshot of a given component. It helps a lot when you update or do some refactoring, and want to get or compare the change.

Now, let's take a snapshot of the App.js file.

App.test.js

import React from 'react'
import {render, cleanup} from '@testing-library/react'
import App from './App'

 afterEach(cleanup)

 it('should take a snapshot', () => {
    const { asFragment } = render(<App />)

    expect(asFragment(<App />)).toMatchSnapshot()
   })
});

To take a snapshot, we first have to import render and cleanup. These two methods will be used a lot throughout this article. render, as you might guess helps to render a React component. And cleanup is passed as a parameter to afterEach to just clean up everything after each test to avoid memory leaks.

Next, we can now render the App component with render and get back asFragment as a returned value from the method. And finally, expect that the fragment of the App component will match the snapshot.

Now, to run the test, open your terminal and navigate to the root of the project and run the following command:

  yarn test

Or, if you use npm:

  npm test

As a result, it will create a new folder __snapshots__ and a file App.test.js.snap in the src which will look like this:

App.test.js.snap

// Jest Snapshot v1, https://goo.gl/fbAQLP

exports[`Take a snapshot should take a snapshot 1`] = `
<DocumentFragment>
  <div class="App">
    <h1>Testing</h1>
  </div>
</DocumentFragment>
`;

And if you make another change in App.js, the test will fail, because the snapshot will no longer match the condition. To make it passes, just press u to update it. And you'll have the updated snapshot in App.test.js.snap.

Now, let's move on and start testing our elements.

2. Testing DOM elements

To test our DOM elements, we first have to look at TestElements.js file.

TestElements.js

import React from 'react'

const TestElements = () => {
 const [counter, setCounter] = React.useState(0)

 return (
  <>
    <h1 data-testid="counter">{ counter }</h1>
    <button data-testid="button-up" onClick={() => setCounter(counter + 1)}> Up</button>
    <button disabled data-testid="button-down" onClick={() => setCounter(counter - 1)}>Down</button>
 </>
    )
  }

export default TestElements

Here, the only thing you have to retain is data-testid. It will be used to select these elements from the test file. Now, let's write the unit test:

Test if the counter is equal to 0
TestElements.test.js

import React from 'react';
import { render, cleanup } from '@testing-library/react';
import TestElements from './TestElements'

afterEach(cleanup);

  it('should equal to 0', () => {
    const { getByTestId } = render(<TestElements />); 
    expect(getByTestId('counter')).toHaveTextContent(0)
   });

As you can see, the syntax is quite similar to the previous test. The only difference is that we use getByTestId to select the needed elements (remember the data-testid) and check if it passed the test. In others words, we check if the text content <h1 data-testid="counter">{ counter }</h1> is equal to 0.

Test if the buttons are enabled or disabled
TestElements.test.js (add the following code block to the file)

   it('should be enabled', () => {
    const { getByTestId } = render(<TestElements />);
    expect(getByTestId('button-up')).not.toHaveAttribute('disabled')
  });

  it('should be disabled', () => {
    const { getByTestId } = render(<TestElements />); 
    expect(getByTestId('button-down')).toBeDisabled()
  });

Here, as usual, we use getByTestId to select elements and check for the first test if the button has a disabled attribute. And for the second, if the button is disabled or not.

And if you save the file or run again in your terminal yarn test, the test will pass.

Congrats! your first test has passed!

Now, let's learn how to test an event in the next section.

3. Testing events

Before writing our unit tests, let's first check what the TestEvents.js looks like.

TestEvents.js

import React from 'react'

const TestEvents = () => {
  const [counter, setCounter] = React.useState(0)

return (
  <>
    <h1 data-testid="counter">{ counter }</h1>
    <button data-testid="button-up" onClick={() => setCounter(counter + 1)}> Up</button>
    <button data-testid="button-down" onClick={() => setCounter(counter - 1)}>Down</button>
 </>
    )
  }

  export default TestEvents

Now, let's write the tests.

Test if the counter increments and decrements correctly when we click on buttons
TestEvents.test.js

import React from 'react';
import { render, cleanup, fireEvent } from '@testing-library/react';
import TestEvents from './TestEvents'

  afterEach(cleanup);

  it('increments counter', () => {
    const { getByTestId } = render(<TestEvents />); 

    fireEvent.click(getByTestId('button-up'))

    expect(getByTestId('counter')).toHaveTextContent('1')
  });

  it('decrements counter', () => {
    const { getByTestId } = render(<TestEvents />); 

    fireEvent.click(getByTestId('button-down'))

    expect(getByTestId('counter')).toHaveTextContent('-1')
  });

As you can see, these two tests are very similar except the expected text content.

The first test fires a click event with fireEvent.click() to check if the counter increments to 1 when the button is clicked.

And the second one, checks if the counter decrements to -1 when the button is clicked.

fireEvent has several methods you can use to test events, feel free to dive into the documentation.

Now, we know how to test events, let's move on and learn in the next section how to deal with asynchronous actions.

4. Testing asynchronous actions

An asynchronous action is something that can take time to complete. It can be an HTTP request, a timer, etc.

Now, let's check the TestAsync.js file.

TestAsync.js

import React from 'react'

const TestAsync = () => {
  const [counter, setCounter] = React.useState(0)

  const delayCount = () => (
    setTimeout(() => {
      setCounter(counter + 1)
    }, 500)
  )

return (
  <>
    <h1 data-testid="counter">{ counter }</h1>
    <button data-testid="button-up" onClick={delayCount}> Up</button>
    <button data-testid="button-down" onClick={() => setCounter(counter - 1)}>Down</button>
 </>
    )
  }

  export default TestAsync

Here, we use setTimeout() to delay the incrementing event by 0.5s.

Test if the counter is incremented after 0.5s.
TestAsync.test.js

import React from 'react';
import { render, cleanup, fireEvent, waitForElement } from '@testing-library/react';
import TestAsync from './TestAsync'

afterEach(cleanup);

  it('increments counter after 0.5s', async () => {
    const { getByTestId, getByText } = render(<TestAsync />); 

    fireEvent.click(getByTestId('button-up'))

    const counter = await waitForElement(() => getByText('1')) 

    expect(counter).toHaveTextContent('1')
  });

To test the incrementing event, we first have to use async/await to handle the action because as I said earlier, it takes time to complete.

Next, we use a new helper method getByText() which is similar to getByTestId(), except that getByText() select the text content instead of id or data-testid i should say.

Now, after clicking to the button, we wait for the counter to be incremented with waitForElement(() => getByText('1')). And once the counter incremented to 1, we can now move to the condition and check if the counter is effectively equal to 1.

That being said, let's now move to more complex test cases.

Are you ready?

5. Testing React Redux

If you're new to React Redux, this article might help you. Otherwise, let's check what the TestRedux.js looks like.

TestRedux.js

import React from 'react'
import { connect } from 'react-redux'

const TestRedux = ({counter, dispatch}) => {

 const increment = () => dispatch({ type: 'INCREMENT' })
 const decrement = () => dispatch({ type: 'DECREMENT' })

 return (
  <>
    <h1 data-testid="counter">{ counter }</h1>
    <button data-testid="button-up" onClick={increment}>Up</button>
    <button data-testid="button-down" onClick={decrement}>Down</button>
 </>
    )
  }

export default connect(state => ({ counter: state.count }))(TestRedux)

And for the reducer.

store/reducer.js

export const initialState = {
    count: 0,
  }

  export function reducer(state = initialState, action) {
    switch (action.type) {
      case 'INCREMENT':
        return {
          count: state.count + 1,
        }
      case 'DECREMENT':
        return {
          count: state.count - 1,
        }
      default:
        return state
    }
  }

As you can see, there is nothing fancy it's just a basic Counter Component handled by React Redux.

Now, let's write the unit tests.

Test if the initial state is equal to 0
TestRedux.test.js

import React from 'react'
import { createStore } from 'redux'
import { Provider } from 'react-redux'
import { render, cleanup, fireEvent } from '@testing-library/react';
import { initialState, reducer } from '../store/reducer'
import TestRedux from './TestRedux'

const renderWithRedux = (
  component,
  { initialState, store = createStore(reducer, initialState) } = {}
) => {
  return {
    ...render(<Provider store={store}>{component}</Provider>),
    store,
  }
}

 afterEach(cleanup);

it('checks initial state is equal to 0', () => {
    const { getByTestId } = renderWithRedux(<TestRedux />)
    expect(getByTestId('counter')).toHaveTextContent('0')
  })

There are a couple of things we need to import to test React Redux. And here, we create our own helper function renderWithRedux() to render the component since it will be used several times.

renderWithRedux() receives as parameters the component to render, the initial state and the store.
If there is no store, it will create a new one, and if it doesn't receive an initial state or a store, it returns an empty object.

Next, we use render() to render the component and pass the store to the Provider.

That being said, we can now pass the component TestRedux to renderWithRedux() to test if the counter is equal to 0.

Test if the counter increments and decrements correctly.
TestRedux.test.js (add the following code block to the file)

it('increments the counter through redux', () => {
  const { getByTestId } = renderWithRedux(<TestRedux />, 
    {initialState: {count: 5}
})
  fireEvent.click(getByTestId('button-up'))
  expect(getByTestId('counter')).toHaveTextContent('6')
})

it('decrements the counter through redux', () => {
  const { getByTestId} = renderWithRedux(<TestRedux />, {
    initialState: { count: 100 },
  })
  fireEvent.click(getByTestId('button-down'))
  expect(getByTestId('counter')).toHaveTextContent('99')
})

To test the incrementing and decrementing events, we pass an initial state as a second argument to renderWithRedux(). Now, we can click on the buttons and test if the expected result matches the condition or not.

Now, Let's move to the next section and introduce React Context.

React Router and Axios will come next, Are you still with me?

6. Testing React Context

If you're new to React Context, check this article first. Otherwise, let's check the TextContext.js file.

TextContext.js

import React from "react"

export const CounterContext = React.createContext()

const CounterProvider = () => {
  const [counter, setCounter] = React.useState(0)
  const increment = () => setCounter(counter + 1)
  const decrement = () => setCounter(counter - 1)

  return (
    <CounterContext.Provider value={{ counter, increment, decrement }}>
      <Counter />
    </CounterContext.Provider>
  )
}

export const Counter = () => {  
    const { counter, increment, decrement } = React.useContext(CounterContext)   
    return (
     <>
       <h1 data-testid="counter">{ counter }</h1>
       <button data-testid="button-up" onClick={increment}> Up</button>
       <button data-testid="button-down" onClick={decrement}>Down</button>
    </>
       )
}

export default CounterProvider

Now, the counter state is managed through React Context. Let's write the unit test to check if it behaves as expected.

Test if the initial state is equal to 0
TextContext.test.js

import React from 'react'
import { render, cleanup,  fireEvent } from '@testing-library/react'
import CounterProvider, { CounterContext, Counter } from './TestContext'

const renderWithContext = (
  component) => {
  return {
    ...render(
        <CounterProvider value={CounterContext}>
            {component}
        </CounterProvider>)
  }
}

afterEach(cleanup);

it('checks if initial state is equal to 0', () => {
    const { getByTestId } = renderWithContext(<Counter />)
    expect(getByTestId('counter')).toHaveTextContent('0')
})

As the previous section with React Redux, here we use the same approach, by creating a helper function renderWithContext() to render the component. But this time, it receives only the component as a parameter. And to create a new context, we pass CounterContext to the Provider.

Now, we can test if the counter is initially equal to 0 or not.

Test if the counter increments and decrements correctly.
TextContext.test.js (add the following code block to the file)

  it('increments the counter', () => {
    const { getByTestId } = renderWithContext(<Counter />)

    fireEvent.click(getByTestId('button-up'))
    expect(getByTestId('counter')).toHaveTextContent('1')
  })

  it('decrements the counter', () => {
    const { getByTestId} = renderWithContext(<Counter />)

    fireEvent.click(getByTestId('button-down'))
    expect(getByTestId('counter')).toHaveTextContent('-1')
  })

As you can see, here we fire a click event to test if the counter increments correctly to 1 and decrement to -1.

That being said, we can now move to the next section and introduce React Router.

7. Testing React Router

If you want to dive into React Router, this article might help you. Otherwise, let's check the TestRouter.js file.

TestRouter.js

import React from 'react'
import { Link, Route, Switch,  useParams } from 'react-router-dom'

const About = () => <h1>About page</h1>

const Home = () => <h1>Home page</h1>

const Contact = () => {
  const { name } = useParams()
  return <h1 data-testid="contact-name">{name}</h1>
}

const TestRouter = () => {
    const name = 'John Doe'
    return (
    <>
    <nav data-testid="navbar">
      <Link data-testid="home-link" to="/">Home</Link>
      <Link data-testid="about-link" to="/about">About</Link>
      <Link data-testid="contact-link" to={`/contact/${name}`}>Contact</Link>
    </nav>

      <Switch>
        <Route exact path="/" component={Home} />
        <Route path="/about" component={About} />
        <Route path="/about:name" component={Contact} />
      </Switch>
    </>
  )
}

export default TestRouter

Here, we have some components to render when navigating and the Home page.

Now, let's write the tests

TestRouter.test.js

import React from 'react'
import { Router } from 'react-router-dom'
import { render, fireEvent } from '@testing-library/react'
import { createMemoryHistory } from 'history'
import TestRouter from './TestRouter'


const renderWithRouter = (component) => {
    const history = createMemoryHistory()
    return { 
    ...render (
    <Router history={history}>
        {component}
    </Router>
    )
  }
}

it('should render the home page', () => {

  const { container, getByTestId } = renderWithRouter(<TestRouter />) 
  const navbar = getByTestId('navbar')
  const link = getByTestId('home-link')

  expect(container.innerHTML).toMatch('Home page')
  expect(navbar).toContainElement(link)
})

To test React Router, we have to first have a navigation history to start with. Therefore we use createMemoryHistory() to well as the name guessed to create a navigation history.

Next, we use our helper function renderWithRouter() to render the component and pass history to the Router component. With that, we can now test if the page loaded at the starting is the Home page or not. And if the navigation bar is loaded with the expected links.

Test if it navigates to other pages with the parameters when we click on links
TestRouter.test.js (add the following code block to the file)

it('should navigate to the about page', ()=> {
  const { container, getByTestId } = renderWithRouter(<TestRouter />) 

  fireEvent.click(getByTestId('about-link'))

  expect(container.innerHTML).toMatch('About page')
})

it('should navigate to the contact page with the params', ()=> {
  const { container, getByTestId } = renderWithRouter(<TestRouter />) 

  fireEvent.click(getByTestId('contact-link'))

  expect(container.innerHTML).toMatch('John Doe')
})

Now, to check if the navigation works, we have to fire a click event on the navigation links.

For the first test, we check if the content is equal to the text in the About Page, and for the second, we test the routing params and check if it passed correctly.

We can now move to the final section and learn how to test an Axios request.

We've almost done

8. Testing HTTP Request (axios)

As usual, let's first see what the TextAxios.js file looks like.

TextAxios.js

import React from 'react'
import axios from 'axios'

const TestAxios = ({ url }) => {
  const [data, setData] = React.useState()

  const fetchData = async () => {
    const response = await axios.get(url)
    setData(response.data.greeting)    
 }     

 return (
  <>
    <button onClick={fetchData} data-testid="fetch-data">Load Data</button>
    { 
    data ?
    <div data-testid="show-data">{data}</div>:
    <h1 data-testid="loading">Loading...</h1>
    }
  </>
     )
}

export default TestAxios

As you can see here, we have a simple component that has a button to make a request. And if the data is not available, it will display a loading message.

Now, let's write the tests.

Test if the data are fetched and displayed correctly.
TextAxios.test.js

import React from 'react'
import { render, waitForElement, fireEvent } from '@testing-library/react'
import axiosMock from 'axios'
import TestAxios from './TestAxios'

jest.mock('axios')

it('should display a loading text', () => {

 const { getByTestId } = render(<TestAxios />)

  expect(getByTestId('loading')).toHaveTextContent('Loading...')
})

it('should load and display the data', async () => {
  const url = '/greeting'
  const { getByTestId } = render(<TestAxios url={url} />)

  axiosMock.get.mockResolvedValueOnce({
    data: { greeting: 'hello there' },
  })

  fireEvent.click(getByTestId('fetch-data'))

  const greetingData = await waitForElement(() => getByTestId('show-data'))

  expect(axiosMock.get).toHaveBeenCalledTimes(1)
  expect(axiosMock.get).toHaveBeenCalledWith(url)
  expect(greetingData).toHaveTextContent('hello there')
})

This test case is a bit different because we have to deal with an HTTP request. And to do that, we have to mock axios request with the help of jest.mock('axios').

Now, we can use axiosMock and apply a get() method to it and finally use the Jest function mockResolvedValueOnce() to pass the mocked data as a parameter.

With that, we can now for the second test, click to the button to fetch the data and use async/await to resolve it. And now we have to test 3 things:

If the HTTP request has been done correctly
If the HTTP request has been done with the url
If the data fetched matches the expectation.

And for the first test, we just check if the loading message is displayed when we have no data to show.

That being said, we've now done with the 8 simple steps to start testing your React Apps.

Don't be scared anymore with testing

Final Thoughts

React Testing Library is a great package for testing React Apps. It gives us access to jest-dom matchers we can use to test our components more efficiently and with good practices. Hopefully, this article was useful, and that will help you building robust React apps in the future.

You can find the finished project here

Thanks for reading it!