React Testing

Quick Start

import { render, screen } from "@testing-library/react";
import userEvent from "@testing-library/user-event";

test("submits form with user input", async () => {
  const user = userEvent.setup();
  render(<LoginForm />);

  await user.type(screen.getByLabelText(/email/i), "test@example.com");
  await user.click(screen.getByRole("button", { name: /sign in/i }));

  expect(await screen.findByText(/welcome/i)).toBeInTheDocument();
});

Core Principle

Test behavior, not implementation. Tests verify observable behavior through public interfaces (DOM, props, callbacks), not internal state or private helpers. A refactor that preserves behavior should never break tests.

Framework Note

Examples use Jest syntax. For Vitest projects, the APIs are nearly identical — see references/vitest-differences.md for the key differences (vi.fn() vs jest.fn(), module mocking, etc.). @testing-library/jest-dom works with both.

Queries

Prefer queries by user perception: getByRole → getByLabelText → getByText → getByTestId (last resort).

See references/query-cheatsheet.md.

User Interactions

Use userEvent.setup() for realistic interactions. Prefer over fireEvent.

See references/user-interactions.md.

Async Patterns

Use findBy* to wait for a single element. Use waitFor when you need multiple assertions or conditions. Keep side effects (clicks, typing) outside waitFor — only put assertions inside.

See references/async-patterns.md.

Accessibility Matchers

Beyond toBeInTheDocument(), use jest-dom matchers that verify accessible state: toBeVisible(), toBeEnabled(), toHaveAccessibleName(), toBeInvalid(). These catch real usability issues, not just DOM presence.

See references/accessibility-matchers.md.

Test Utilities

Reduce boilerplate with setup functions that return user and domain-specific actions. Use screen for all DOM queries.

See references/test-utilities.md.

Mocking

Mock only at system boundaries: network, time, randomness, browser APIs. Never mock your own components, hooks, or internal utilities.

See references/mocking-patterns.md.

Providers

Use custom render helper for required providers (router, store, theme). Create a fresh QueryClient per test to avoid cache leaks.

See references/providers.md.

Testing Hooks

const { result } = renderHook(() => useCounter(0));
act(() => result.current.increment());
expect(result.current.count).toBe(1);

See references/hooks-testing.md.

Suspense & Error Boundaries

Test loading fallbacks by rendering inside <Suspense> and asserting the fallback appears, then awaiting the resolved content. Test error boundaries by rendering a component that throws and verifying the fallback UI.

See references/suspense-error-boundaries.md.

Anti-Patterns & Code Smells

Common mistakes: testing implementation details, nested describe/beforeEach with shared mutable state, wrong query types. Test struggles are design feedback — if tests are hard to write, the component may need refactoring.

See references/anti-patterns.md.

Troubleshooting

Covers act() warnings, tests that pass alone but fail together, JSDOM limitations, query errors, and async pitfalls.

See references/troubleshooting.md.

React 18+ Notes

• @testing-library/react v14+ handles React 18's createRoot automatically — no manual migration needed
• act() warnings are more strict in React 18; most are resolved by properly awaiting async operations with findBy* or waitFor
• React.startTransition updates are batched — use waitFor to assert on their results
• Concurrent features (useTransition, useDeferredValue) work in tests but don't exhibit concurrent scheduling in JSDOM — test the user-visible behavior, not the scheduling

Test Design

• One behavior per test
• Test name describes behavior, not implementation
• Prefer integration-style tests over shallow unit tests
• ~70% coverage sufficient

See references/design-patterns.md and references/workflow.md.