react-component-testing

React Component Testing Skill

Step 1 — Classify What Needs to Be Tested

Assign each file to a test tier before writing tests:

• Hook / reducer / pure function -> unit test.
• Component behavior / interaction -> component test.
• Multi-page user flow -> end-to-end test.

Step 2 — Write Unit Tests

• Test hooks with renderHook and explicit inputs.
• Test reducers and utilities as pure functions.
• Mock network boundaries; do not mount full UI.

import { describe, expect, it } from 'vitest'
import { authReducer } from './auth.reducer'

it('sets user on login success', () => {
  const state = authReducer({ user: null }, { type: 'login/success', payload: { id: '1' } })
  expect(state.user?.id).toBe('1')
})

Step 3 — Write Component Tests

• Use React Testing Library queries by role/label/text.
• Use userEvent for interactions, not low-level DOM event simulation unless necessary.
• Wrap with providers used in production (router, query client, store) via a shared test render utility.
• Assert user-visible output and callback side effects.
• In TypeScript projects, keep TS/JS imports ESM-only and use aliases for cross-directory imports.

import { render, screen } from '@testing-library/react'
import userEvent from '@testing-library/user-event'
import { SaveButton } from './SaveButton'

it('calls onSave when clicked', async () => {
  const onSave = vi.fn()
  render(<SaveButton onSave={onSave} />)
  await userEvent.click(screen.getByRole('button', { name: 'Save' }))
  expect(onSave).toHaveBeenCalledTimes(1)
})

Step 4 — Write End-to-End Tests

• Use Playwright for full browser flows across routes.
• Cover one happy path and key failure path for each critical user journey.
• Keep E2E focused on integration risks, not exhaustive branch coverage.

Step 5 — Verify

• Each custom hook has unit tests for success and edge behavior
• Interactive components test primary user interactions
• No tests assert private state or implementation-only details
• Critical journeys have Playwright coverage (happy path + failure path)
• pnpm test passes with zero failures