React Testing Techniques

Best practices for testing React components with Vitest, React Testing Library (RTL), and Mock Service Worker (MSW).

Core Principles

Test Like a User

Write tests that interact with components the way users do. Avoid testing implementation details.

• Query elements by accessible roles, labels, and text - not by test IDs or class names
• Simulate real user interactions with userEvent, not fireEvent
• Test behavior and outcomes, not internal state or method calls
• Prefer integration tests over isolated unit tests

Prefer Integration Over Mocking

Combine multiple components and use MSW for API mocking at the network level. Avoid mocking component internals, hooks, or services unless absolutely necessary.

Query Selection Decision

Choose the right query based on the scenario:

Scenario	Query	Why
Element exists synchronously	getBy*	Throws if not found - fails fast
Element appears after async operation	findBy*	Waits with retry, use with await
Assert element does NOT exist	queryBy*	Returns null instead of throwing
Multiple elements expected	*AllBy* variants	Returns array of matches

Priority order for queries:

1. getByRole - most accessible, preferred
2. getByLabelText - for form fields
3. getByPlaceholderText - for inputs
4. getByText - for non-interactive content
5. getByTestId - last resort only

UserEvent Setup (Required Pattern)

Always use userEvent.setup() before interactions:

import { render, screen } from '../test/test-utils';
import userEvent from '@testing-library/user-event';

test('submits form correctly', async () => {
  const user = userEvent.setup();
  render(<MyForm />);
  
  await user.type(screen.getByRole('textbox', { name: /email/i }), 'test@example.com');
  await user.click(screen.getByRole('button', { name: /submit/i }));
  
  expect(await screen.findByText(/success/i)).toBeInTheDocument();
});

Never use fireEvent - it dispatches raw DOM events without realistic user interaction simulation.

Async Testing Patterns

Use findBy* for Elements That Appear Async

// Element appears after data loads
expect(await screen.findByText('John Doe')).toBeInTheDocument();

Use waitFor for Assertions Only

// Wait for a side effect or state change
await waitFor(() => expect(handleSubmit).toHaveBeenCalledTimes(1));

Use waitForElementToBeRemoved for Disappearing Elements

// Wait for loading spinner to disappear
await waitForElementToBeRemoved(() => screen.getByText('Loading...'));

MSW v2 Patterns

Use Mock Service Worker for API mocking. Always use v2 syntax:

import { http, HttpResponse } from 'msw';
import { setupServer } from 'msw/node';

const server = setupServer(
  http.get('/api/users', () => {
    return HttpResponse.json([{ id: 1, name: 'John' }]);
  })
);

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

Override handlers per test:

test('handles error state', async () => {
  server.use(
    http.get('/api/users', () => {
      return HttpResponse.json({ error: 'Not found' }, { status: 404 });
    })
  );
  // Test error handling...
});

Vitest-Specific Patterns

Use Vitest APIs, not Jest:

Operation	Vitest	Not Jest
Mock function	vi.fn()	jest.fn()
Mock module	vi.mock()	jest.mock()
Spy on method	vi.spyOn()	jest.spyOn()
Stub env var	vi.stubEnv()	process.env
Reset modules	vi.resetModules()	jest.resetModules()
Restore mocks	vi.restoreAllMocks()	jest.restoreAllMocks()

Environment variable testing:

import { afterEach, vi } from 'vitest';

afterEach(() => {
  vi.unstubAllEnvs();
  vi.resetModules();
});

test('handles missing env var', async () => {
  vi.stubEnv('API_KEY', '');
  const { config } = await import('./config');
  expect(() => config.apiKey).toThrow();
});

Test Utils Setup

Create a custom render with all providers. Read references/test-utils-setup.md for complete setup with:

• MantineProvider (with env="test")
• QueryClientProvider (fresh client per test)
• RouterProvider (TanStack Router)

What to Avoid

Never Test Implementation Details

• Internal state
• Private methods
• Component lifecycle
• Hook internals

Never Use Snapshot Tests

Except for trivial components (<30 lines of output). Snapshots encode structure, not intent.

Never Mock What You Don't Own

Use MSW for APIs instead of mocking fetch/axios. Mock only at system boundaries.

Additional Resources

Reference Files

Consult these for detailed patterns:

• references/test-utils-setup.md - Complete custom render setup with providers
• references/query-selection.md - Detailed query type decision guide
• references/vitest-patterns.md - Vitest mocking, spying, and stubbing
• references/msw-patterns.md - MSW v2 handlers, overrides, error simulation
• references/async-testing.md - waitFor, findBy, timing patterns
• references/tanstack-testing.md - TanStack Query and Router testing
• references/user-interactions.md - userEvent patterns and edge cases
• references/mantine-testing.md - Mantine component testing (modals, forms, select)