hook-test — test a hook the ReactUse way

The core package uses jest (not vitest), testEnvironment: 'jsdom'. Tests run via babel-jest against src/ directly — no build step needed.

Where tests live

Co-located with the hook: packages/core/src/useX/index.spec.ts (a couple use .test.ts, but prefer index.spec.ts). There is no central __tests__ directory.

Basic pattern

import { act, renderHook } from '@testing-library/react'
import { useX } from '.'

describe('useX', () => {
  function setUp(initial?: number) {
    return renderHook(() => useX(initial))
  }

  it('inits with the given value', () => {
    const { result } = setUp(5)
    expect(result.current[0]).toBe(5)
  })

  it('updates on set', () => {
    const { result } = setUp(0)
    act(() => {
      result.current[1](10)        // wrap every state mutation in act()
    })
    expect(result.current[0]).toBe(10)
  })
})

Key rules:

• Wrap a renderHook call in a small setUp() helper.
• Every state mutation goes inside act(() => { … }).
• Read current values via result.current; use rerender() / unmount() from the renderHook return for prop changes and cleanup assertions.

Timer hooks

describe('useInterval', () => {
  jest.useFakeTimers()
  jest.spyOn(global, 'clearInterval')

  it('fires on the interval', () => {
    const cb = jest.fn()
    renderHook(() => useInterval(cb, 20))
    expect(cb).not.toBeCalled()
    jest.advanceTimersByTime(70)
    expect(cb).toHaveBeenCalledTimes(3)
  })

  it('clears on unmount', () => {
    const { unmount } = renderHook(() => useInterval(jest.fn(), 200))
    unmount()
    expect(clearInterval).toHaveBeenCalledTimes(1)
  })
})

SSR / hydration tests

For hooks that must not break SSR or cause hydration mismatch, use the custom environment and render on the server, then hydrate:

/**
 * @jest-environment ./.test/ssr-environment
 */
import { act } from '@testing-library/react'
import ReactDOMServer from 'react-dom/server'
import ReactDOMClient from 'react-dom/client'
import { createMockMediaMatcher } from '../../.test'
import { createTestComponent } from '../../.test/testingHelpers'

describe('useX SSR', () => {
  beforeEach(() => {
    jest.resetModules()
    window.matchMedia = createMockMediaMatcher({ '(prefers-color-scheme: dark)': true }) as any
  })

  it('does not mismatch during hydration', async () => {
    const TestComponent = createTestComponent(() => useX())
    const el = document.createElement('div')
    const markup = ReactDOMServer.renderToString(<TestComponent />)
    el.innerHTML = markup
    const root = await act(() => ReactDOMClient.hydrateRoot(el, <TestComponent />))
    expect(el.innerHTML).toBe(markup)
    await act(() => root.unmount())
  })
})

Reference: packages/core/src/usePreferredDark/index.ssr.spec.tsx.

Helpers (packages/core/.test/)

• createMockMediaMatcher(matches) and createMockRaf() — ../../.test
• createTestComponent(hook), sleep(ms), request(req) — ../../.test/testingHelpers
• jest-setup.ts adds a toHaveErrorMatching matcher and a window.testErrors array.

Running

pnpm --filter @reactuses/core test useX     # single hook (jest name pattern)
pnpm --filter @reactuses/core test          # whole package
pnpm --filter @reactuses/core test:coverage # with coverage

When fixing a failing test: run it first to see the real error, read the hook source and at least one call site, make the minimal fix, then re-run green. No speculative edits.