React Transition Group

Mock

import React, {useState} from 'react'
import {CSSTransition} from 'react-transition-group'
import {render, fireEvent} from '@testing-library/react'

function Fade({children, ...props}) {
  return (
    <CSSTransition {...props} timeout={1000} classNames="fade">
      {children}
    </CSSTransition>
  )
}

function HiddenMessage({initialShow}) {
  const [show, setShow] = useState(initialShow || false)
  const toggle = () => setShow(prevState => !prevState)
  return (
    <div>
      <button onClick={toggle}>Toggle</button>
      <Fade in={show}>
        <div>Hello world</div>
      </Fade>
    </div>
  )
}

jest.mock('react-transition-group', () => {
  const FakeTransition = jest.fn(({children}) => children)
  const FakeCSSTransition = jest.fn(props =>
    props.in ? <FakeTransition>{props.children}</FakeTransition> : null,
  )
  return {CSSTransition: FakeCSSTransition, Transition: FakeTransition}
})

test('you can mock things with jest.mock', () => {
  const {getByText, queryByText} = render(<HiddenMessage initialShow={true} />)
  expect(getByText('Hello world')).toBeTruthy() // we just care it exists
  // hide the message
  fireEvent.click(getByText('Toggle'))
  // in the real world, the CSSTransition component would take some time
  // before finishing the animation which would actually hide the message.
  // So we've mocked it out for our tests to make it happen instantly
  expect(queryByText('Hello World')).toBeNull() // we just care it doesn't exist
})

Shallow

import React, {useState} from 'react'
import {CSSTransition} from 'react-transition-group'
import {render, fireEvent} from '@testing-library/react'

function Fade({children, ...props}) {
  return (
    <CSSTransition {...props} timeout={1000} classNames="fade">
      {children}
    </CSSTransition>
  )
}

function HiddenMessage({initialShow}) {
  const [show, setShow] = useState(initialShow || false)
  const toggle = () => setShow(prevState => !prevState)
  return (
    <div>
      <button onClick={toggle}>Toggle</button>
      <Fade in={show}>
        <div>Hello world</div>
      </Fade>
    </div>
  )
}

jest.mock('react-transition-group', () => {
  const FakeCSSTransition = jest.fn(() => null)
  return {CSSTransition: FakeCSSTransition}
})

test('you can mock things with jest.mock', () => {
  const {getByText} = render(<HiddenMessage initialShow={true} />)
  const context = expect.any(Object)
  const children = expect.any(Object)
  const defaultProps = {children, timeout: 1000, className: 'fade'}
  expect(CSSTransition).toHaveBeenCalledWith(
    {in: true, ...defaultProps},
    context,
  )
  fireEvent.click(getByText(/toggle/i))
  expect(CSSTransition).toHaveBeenCalledWith(
    {in: false, ...defaultProps},
    expect.any(Object),
  )
})