[React Testing] Test a Custom React Hook with React’s Act Utility and a Test Component

We’ve got a custom useCounter hook here and we want to make sure the increment and decrement functions it returns will update the count state that it returns. Because hooks must be run within the render phase of a component, we’ll create a simple component that simply uses the hook and render that component. One catch is that whenever there’s a state update in your tests, React needs you to wrap that in a call to act. We don’t have to do that for regular component tests that use React Testing Library utilities because those utilities manage the act call internally, but since we’re calling these state updater functions ourselves, we have to wrap things in act. Let’s do that for our useCounter hook.

Component:

import React from 'react'

function useCounter({ initialCount = 0, step = 1 } = {}) {
  const [count, setCount] = React.useState(initialCount)
  const increment = () => setCount((c) => c + step)
  const decrement = () => setCount((c) => c - step)
  return { count, increment, decrement }
}

export { useCounter }

Test:

import React from 'react'
import { render, act } from '@testing-library/react'
import { useCounter } from '../../components/extra/use-counter'

test('exposes the count and increment/decrement functions', () => {
  let result
  function TestComponent() {
    result = useCounter()
    return null
  }
  render(<TestComponent />)
  expect(result.count).toBe(0)
  act(() => result.increment())
  expect(result.count).toBe(1)
  act(() => result.decrement())
  expect(result.count).toBe(0)
})

test('exposes the count and increment/decrement functions', () => {
  const result = {}
  function TestComponent(options) {
    result.current = useCounter(options)
    return null
  }
  const { rerender } = render(<TestComponent />)
  expect(result.current.count).toBe(0)
  act(() => result.current.increment())
  expect(result.current.count).toBe(1)

  rerender(<TestComponent step={2} />)

  act(() => result.current.decrement())
  expect(result.current.count).toBe(-1)
})

---

Component:

import { useState, useEffect, useRef } from 'react'

export function useFocus() {
  const inputRef = useRef()
  useEffect(() => {
    inputRef.current.focus()
  }, [])
  return inputRef
}

Test:

import React from 'react'
import { render } from '@testing-library/react'
import '@testing-library/jest-dom/extend-expect'
import { useFocus } from '../hooks'

test('should expose input ref and auto focus on input dom', () => {
  let inputRef
  function InputComponent() {
    inputRef = useFocus()
    return <input ref={inputRef} data-testid="testInput" />
  }
  const { getByTestId } = render(<InputComponent />)
  expect(getByTestId('testInput')).toHaveFocus()
})