Introducing the react-testing-library 🐐

Two weeks ago, I wrote
a new library! I’ve
been thinking about it for a while. But two weeks ago I started getting pretty
serious about it:

Read on to get an idea of what I mean by “damaging practices.”

react-testing-library

Simple and complete React DOM testing utilities that encourage good testing
practices.

The problem

You want to write maintainable tests for your React components. As a part of
this goal, you want your tests to avoid including implementation details of your
components and rather focus on making your tests give you the confidence for
which they are intended. As part of this, you want your testbase to be
maintainable in the long run so refactors of your components (changes to
implementation but not functionality) don’t break your tests and slow you and
your team down.

This solution

The 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:

So rather than dealing with instances of rendered react components, your tests
will work with actual DOM nodes. The utilities this library provides facilitate
querying the DOM in the same way the user would. Finding form elements by their
label text (just like a user would), finding links and buttons by their text
(like a user would). It also exposes a recommended way to find elements by a
data-testid as an “escape hatch” for elements where the text content and label
do not make sense or is not practical.

This library encourages your applications to be more accessible and allows you
to get your tests closer to using your components the way a user will, which
allows your tests to give you more confidence that your application will work
when a real user uses it.

This library is a replacement for enzyme. While you
can follow these guidelines using enzyme itself, enforcing this is harder
because of all the extra utilities that enzyme provides (utilities which
facilitate testing implementation details). Read more about this in
the FAQ.

Also, while the React Testing Library is intended for react-dom, you can use
React Native Testing Library
which has a very similar API.

What this library is not:

1. A test runner or framework
2. Specific to a testing framework (though we recommend Jest as our preference,
   the library works with any framework, and even
   in codesandbox!)

Basic Example

// hidden-message.js
import * as React from 'react'

// NOTE: React Testing Library works with React Hooks _and_ classes just as well
// and your tests will be the same however you write your components.
function HiddenMessage({children}) {
  const [showMessage, setShowMessage] = React.useState(false)
  return (
    <div>
      <label htmlFor="toggle">Show Message</label>
      <input
        id="toggle"
        type="checkbox"
        onChange={e => setShowMessage(e.target.checked)}
        checked={showMessage}
      />
      {showMessage ? children : null}
    </div>
  )
}

export default HiddenMessage

// __tests__/hidden-message.js
// These imports are something you'd normally configure Jest to import for you automatically.
// Learn more in the setup docs: https://testing-library.com/docs/react-testing-library/setup#skipping-auto-cleanup
import '@testing-library/jest-dom/extend-expect'
// NOTE: jest-dom adds handy assertions to Jest and is recommended, but not required

import {render, screen} from '@testing-library/react'
import userEvent from '@testing-library/user-event'
import * as React from 'react'

import HiddenMessage from '../hidden-message'

test('shows the children when the checkbox is checked', () => {
  const testMessage = 'Test Message'
  render(<HiddenMessage>{testMessage}</HiddenMessage>)

  // query* functions will return the element or null if it cannot be found
  // get* functions will return the element or throw an error if it cannot be found
  expect(screen.queryByText(testMessage)).toBeNull()

  // the queries can accept a regex to make your selectors more resilient to content tweaks and changes.
  userEvent.click(screen.getByLabelText(/show/i))

  // .toBeInTheDocument() is an assertion that comes from jest-dom
  // otherwise you could use .toBeDefined()
  expect(screen.getByText(testMessage)).toBeInTheDocument()
})

Practical Example

// login.js
import * as React from 'react'

function Login() {
  const [state, setState] = React.useReducer((s, a) => ({...s, ...a}), {
    resolved: false,
    loading: false,
    error: null,
  })

  function handleSubmit(event) {
    event.preventDefault()
    const {usernameInput, passwordInput} = event.target.elements

    setState({loading: true, resolved: false, error: null})

    window
      .fetch('/api/login', {
        method: 'POST',
        headers: {'Content-Type': 'application/json'},
        body: JSON.stringify({
          username: usernameInput.value,
          password: passwordInput.value,
        }),
      })
      .then(r => r.json())
      .then(
        user => {
          setState({loading: false, resolved: true, error: null})
          window.localStorage.setItem('token', user.token)
        },
        error => {
          setState({loading: false, resolved: false, error: error.message})
        },
      )
  }

  return (
    <div>
      <form onSubmit={handleSubmit}>
        <div>
          <label htmlFor="usernameInput">Username</label>
          <input id="usernameInput" />
        </div>
        <div>
          <label htmlFor="passwordInput">Password</label>
          <input id="passwordInput" type="password" />
        </div>
        <button type="submit">Submit{state.loading ? '...' : null}</button>
      </form>
      {state.error ? <div role="alert">{state.error.message}</div> : null}
      {state.resolved ? (
        <div role="alert">Congrats! You're signed in!</div>
      ) : null}
    </div>
  )
}

export default Login

// __tests__/login.js
// again, these first two imports are something you'd normally handle in
// your testing framework configuration rather than importing them in every file.
import '@testing-library/jest-dom/extend-expect'

import {render, screen} from '@testing-library/react'
import userEvent from '@testing-library/user-event'
import * as React from 'react'

import Login from '../login'

test('allows the user to login successfully', async () => {
  // mock out window.fetch for the test
  const fakeUserResponse = {token: 'fake_user_token'}
  jest.spyOn(window, 'fetch').mockImplementationOnce(() => {
    return Promise.resolve({
      json: () => Promise.resolve(fakeUserResponse),
    })
  })

  render(<Login />)

  // fill out the form
  userEvent.type(screen.getByLabelText(/username/i), 'chuck')
  userEvent.type(screen.getByLabelText(/password/i), 'norris')

  userEvent.click(screen.getByText(/submit/i))

  // just like a manual tester, we'll instruct our test to wait for the alert
  // to show up before continuing with our assertions.
  const alert = await screen.findByRole('alert')

  // .toHaveTextContent() comes from jest-dom's assertions
  // otherwise you could use expect(alert.textContent).toMatch(/congrats/i)
  // but jest-dom will give you better error messages which is why it's recommended
  expect(alert).toHaveTextContent(/congrats/i)
  expect(window.localStorage.getItem('token')).toEqual(fakeUserResponse.token)
})

The most important takeaway from this example is:

The test is written in such a way that resembles how the user is using your
application.

Let’s explore this further…

Let’s say we have a GreetingFetcher component that fetches a greeting for a
user. It might render some HTML like this:

<div>
  <label for="name-input">Name</label>
  <input id="name-input" />
  <button>Load Greeting</button>
  <div data-testid="greeting-text" />
</div>

So the functionality is: Set the name, click the “Load Greeting” button, and a
server request is made to load greeting text with that name.

In your test you’ll need to find the <input /> so you can set its value to
something. Conventional wisdom suggests you could use the id property in a CSS
selector: #name-input. But is that what the user does to find that input?
Definitely not! They look at the screen and find the input with the label “Name”
and fill that in. So that’s what our test is doing with getByLabelText. It
gets the form control based on its label.

Often in tests using enzyme, to find the “Load Greeting” button you might use a
CSS selector or even find by component displayName or the component
constructor. But when the user wants to load the greeting, they don’t care about
those implementation details, instead they’re going to find and click the button
that says “Load Greeting.” And that’s exactly what our test is doing with the
getByText helper!

In addition, the wait resembles exactly what the users does. They wait for the
greeting text to appear, however long that takes. In our tests we’re mocking
that out so it happens basically instantly, but our test doesn’t actually care
how long it takes. We don’t have to use a setTimeout in our test or anything.
We simply say: “Hey, wait until the greeting-text node appears.” (Note, in
this case it’s using a data-testid attribute which is an escape hatch for
situations where it doesn’t make sense to find an element by any other
mechanism.
A data-testid is definitely better then alternatives.

High-level Overview API

Originally, the library only provided queryByTestId as a utility as suggested
in my blog post
“Making your UI tests resilient to change“.
But thanks to feedback on that blog post from
Bergé Greg as well as inspiration from
a fantastic (and short!) talk by
Jamie White, I added several more and now I’m
even happier with this solution.

You can read more about the library and its APIs in
the official docs.
Here’s a high-level overview of what this library gives you:

• Simulate:
  a re-export from the Simulate utility from
  the
  react-dom/test-utils

Simulate
  object.
• wait:
  allows you to wait for a non-deterministic period of time in your tests.
  Normally you should
  mock out API requests
  or
  animations,
  but even if you’re dealing with immediately resolved promises, you’ll need
  your tests to wait for the next tick of the event loop and wait is really
  good for that. (Big shout out to
  Łukasz Gozda Gandecki who
  introduced this
  as a replacement for the (now deprecated)flushPromises API).
• render:
  This is the meat of the library. It’s fairly simple. It creates a divwith
  document.createElement, then uses ReactDOM.render to render to that div.

The render function returns the following objects and utilities:

• container:
  The div your component was rendered to
• unmount:
  A simple wrapper over ReactDOM.unmountComponentAtNodeto unmount your
  component (to facilitate easier testing of componentWillUnmount for
  example).
• getByLabelText:
  Get a form control associated to a label
• getByPlaceholderText:
  Placeholders aren’t proper alternatives to labels, but if this makes more
  sense for your use case it’s available.
• getByText:
  Get any element by its text content.
• getByAltText:
  Get an element (like an <img) by it’s alt attribute value.
• getByTestId:
  Get an element by its data-testid attribute.

Each of those get* utilities will throw a useful error message if no element
can be found. There’s also an associated query* API for each which will return
nullinstead of throwing an error which can be useful for asserting that an
element is not in the DOM.

Also, for these get* utilities, to find a matching element, you can pass:

• a case-insensitive substring: lo world matches Hello World
• a regex: /^Hello World$/ matches Hello World
• a function that accepts the text and the element:
  (text, el) => el.tagName === 'SPAN' && text.startsWith('Hello') would match
  a span that has content that starts with Hello

Custom Jest Matchers

Thanks to Anto Aravinth Belgin Rayen, we have
some handy custom Jest matchers as well:

Note: now these have been extracted to
jest-dom which is maintained by
Ernesto García

Conclusion

A big feature of this library is that it doesn’t have utilities that enable
testing implementation details. It focuses on providing utilities that encourage
good testing and software practices. I hope that by using
the
react-testing-libraryyour
React testbases are easier to understand and maintain.