React Component Testing Guide: Jest and RTL

Testing React components gives you confidence the component will work when the user interacts with it. As a junior full-stack developer on my first job, I found it extremely useful in helping me understand our current codebase as well as allowing me to add value while learning.

This article is a summary of the information I found useful during my research and the answer to some challenges I came across. I don't hope to re-invent the wheel but to help others in a similar stage of their career. It's also assumed that you have some experience in writing tests.

Why Jest and RTL (React Testing Library)?

React openly recommends Jest as a test runner (perhaps because they maintain it) and RTL as their testing utility of choice. Jest is very fast, easy to set up and it has many powerful features such as mocking functions which allow you to replace a specific function and return a desirable value or to check how the test subject it’s executing the function. RTL is very simple to set up, easy to make queries (including asynchronously) and because of how it was built, it will help you write good tests.

Jest-Dom is not required but makes writing tests much easier because it will extend Jest matchers (methods that let you test values in different ways (e.g. 

toBe()

toHaveBeenCalled()

Another popular tool is Enzyme, but many believe that it can lead to bad testing practices. The main concern is that Enzyme offers extra utilities that allow you to test the internal workings of a component (e.g. read and set state of the component). The team at React tests React; therefore, there is no need for you to test React’s functionality such as state,

componentDidMount

What to test?

When testing React components, the focus should be on replicating how the user would interact with the React component. This means that we should test for what the user should see or not see, and how they are meant to interact with the app once it renders (e.g. the value of search/input field can be changed) instead of testing implementation (e.g. was

componentDidMount

Some good questions to ask yourself when writing tests are:

• What does the component render? Also, does it render differently under different conditions?

This is what the user will see and potentially interact with. By thinking about it, you will also realise that users should access and see different information depending on certain conditions being met

• What happens when the user interacts with the component?

These are the parts of the app which the user will click, write into, etc. and they’ll expect something to happen. Whatever is meant to happen, needs to be tested that happens when the event is triggered!

• When a function is passed in as a prop, how does the component use it?

You may need to recreate the behaviour of this function by using Jest’s mocking concept to know if the function has been called and the correct values were used

How to write a test?

RTL’s most used functions are 

render

cleanup

render

fireEvent

Jest most used functions are 

expect

jest.fn()

jest.spyOn()

jest.mock()

The test should be structured as follows:

1. Declare all 

   jest.fn()

   /

   spyOn()

   /

   mock()

    with or without mocked implementations
2. Call RTL’s 

   render

   function with the test subject as an argument – provide context whenever the component consumes a context. Also, if React-Router Link is used in this component, an object with a property wrapper and value MemoryRouter (imported from React-Router) must be passed as the second argument. Optionally wrap the component in MemoryRouter tags
3. Query the React DOM tree by using RTL’s query functions (e.g.

   getByRole()

   ) and check the values by call
4. Check values queried by calling 

   expect()

    along with the relevant matcher. To replicate user interaction use 

   fireEvent

RTL also returns a 

debug()

We will use the code below (a search field) as our example of a React component:

render = () => {
  const {
    validateSelection,
    minCharacters,
    placeholder,
    inputFluid,
    inputLabel,
    clear
  }: any = this.props

  const { isLoading, value, results } = this.state

  const icon = validateSelection ? (
    <Icon name="check" color="green" />
  ) : (
    <Icon name="search" />
  )

  return (
    <Search
      minCharacters={minCharacters}
      loading={isLoading}
      icon={icon}
      onResultSelect={this.onResultSelect}
      onSearchChange={this.onSearchChange}
      results={results}
      value={clear ? null : value}
      fluid
      placeholder={placeholder}
      input={{ fluid: inputFluid, label: inputLabel }}
    />
  )
}

Above we are destructuring props and state. We are also returning a Semantic UI React 

Search

onSearchChange

event

data

onSearchChange

Below are the tests we built for this component.

import '@testing-library/jest-dom/extend-expect'
import React from 'react'
import { render, cleanup, fireEvent } from '@testing-library/react'
import SearchField from './SearchField'

afterEach(cleanup)
jest.useFakeTimers()

test('<SearchField />', () => {
  const handleResultSelectMock = jest.fn()
  const apiServiceMock = jest
    .fn()
    .mockImplementation(() =>
      Promise.resolve({ entity: { success: true, data: ['hello', 'adios'] } })
    )

  const { getByRole, debug } = render(
    <SearchField
      handleResultSelect={handleResultSelectMock}
      apiService={apiServiceMock}
    />
  )

  const input = getByRole('textbox')
  expect(apiServiceMock).not.toHaveBeenCalled()
  expect(input).toHaveValue('')

  fireEvent.change(input, { target: { value: 'search' } })
  expect(input).toHaveValue('search')
  jest.advanceTimersByTime(600)

  expect(apiServiceMock).toHaveBeenCalledWith('search')
  expect(apiServiceMock).toHaveBeenCalledTimes(1)
  debug()
})

What is happening in the example above?

We imported all dependencies needed to test this component.

• Jest DOM - to extend jest matchers

• render

  , 

  cleanup

  , 

  fireEvent

   - React Testing Library utilities

• SearchField

   - the React component being tested

import '@testing-library/jest-dom/extend-expect'
import React from 'react'
import { render, cleanup, fireEvent } from '@testing-library/react'
import SearchField from './SearchField'

We called Jest's function 

afterEach

cleanup

render

useFakeTimers

afterEach(cleanup)
jest.useFakeTimers()

The component requires two props which should be functions. Therefore, we started by mocking two functions that will be passed to the component as props -

handleResultSelectMock

apiServiceMock

handleResultSelectMock

handleResultSelect

apiServiceMock

apiService

render

test('<SearchField />', () => {
  const handleResultSelectMock = jest.fn()
  const apiServiceMock = jest
    .fn()
    .mockImplementation(() =>
      Promise.resolve({ entity: { success: true, data: ['hello', 'adios'] } })
    )

  const { getByRole, debug } = render(
    <SearchField
      handleResultSelect={handleResultSelectMock}
      apiService={apiServiceMock}
    />
  )
})

There will be times when the component being tested will require a

wrapper: Memory Router

context

const { getByTestId, container } = render(
  <UserContext.Provider value={context}>
    <MainLoggedIn
      config={{
        get: jest.fn().mockImplementation(() => ({
          globalMenu: [{ requiredPermissions: ['Navbar'] }]
        }))
      }}
      history={{ history: ['first_history', 'second_history'] }}
      children={['first_child', 'second_child']}
    />
  </UserContext.Provider>,
  { wrapper: MemoryRouter }
)

After 

render

getByRole

const input = getByRole('textbox')

To check values, start with the function 

expect

value = ''

expect(apiServiceMock).not.toHaveBeenCalled()
expect(input).toHaveValue('')

An event is fired using the function 

change

fireEvent

''

'search'

fireEvent

click()

mouseOver()

advanceTimersByTime

advanceTimersByTime

fireEvent.change(input, { target: { value: 'search' } })
expect(input).toHaveValue('search')
jest.advanceTimersByTime(600)

After firing the event, we expect a few things to happen, the

apiServiceMock

apiServiceMock

expect(apiServiceMock).toHaveBeenCalledWith('search')
expect(apiServiceMock).toHaveBeenCalledTimes(1)
debug()

Lastly, the 

debug

Summary

Small and straightforward tests are better. Test each component independently. Focus on testing what the user will see and how they will interact with the component. Start building the tests after assessing what needs to be tested.

More on the topic

• Jest cheat sheet
• Modern React testing, part 3: Jest and React Testing Library
• The Right Way to Test React Components
• React Testing Library - Introduction
• Configuring Jest
• Mock Functions or Spies Demystified - How Does

  jest.fn()

  Work?

Previously published at https://jdlt.co.uk/blog/testing-react-components-with-jest-and-react-testing-library/