React Testing (Web)

Problem Statement

React testing requires understanding component rendering, user interactions, and async state management. This skill covers Jest with React Testing Library patterns for web applications.

---

Pattern: Zustand Store Testing

Problem: Store state persists between tests, causing flaky tests.

import { useAppStore } from '@/stores/appStore';

const initialState = {
  items: [],
  loading: false,
  error: null,
};

describe('App Store', () => {
  // Reset store before each test
  beforeEach(() => {
    useAppStore.setState(initialState, true); // true = replace entire state
  });

  it('adds item to store', async () => {
    const store = useAppStore.getState();

    await store.addItem({ id: '1', name: 'Test' });

    expect(useAppStore.getState().items).toHaveLength(1);
  });

  it('handles loading state', async () => {
    const store = useAppStore.getState();

    const loadPromise = store.fetchItems();
    expect(useAppStore.getState().loading).toBe(true);

    await loadPromise;
    expect(useAppStore.getState().loading).toBe(false);
  });
});

Key points:

• Use setState(initialState, true) to replace (not merge) state
• Get fresh state with getState() after async operations
• Don't rely on component re-renders in store tests

---

Pattern: Async Store Operations

Problem: Testing async Zustand actions with proper waiting.

import { act, waitFor } from '@testing-library/react';

it('loads data correctly', async () => {
  const store = useAppStore.getState();

  // Wrap async store operations in act
  await act(async () => {
    await store.loadData('123');
  });

  // Verify state after async completes
  await waitFor(() => {
    const state = useAppStore.getState();
    expect(Object.keys(state.data).length).toBeGreaterThan(0);
  });
});

// For complex flows, verify each step
it('completes multi-step flow', async () => {
  const store = useAppStore.getState();

  // Step 1
  await act(async () => {
    await store.loadItems();
  });
  expect(useAppStore.getState().items).toBeDefined();

  // Step 2
  await act(async () => {
    await store.processItems();
  });
  expect(useAppStore.getState().processed).toBe(true);
});

---

Pattern: Component Testing

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

describe('ItemCard', () => {
  const mockItem = {
    id: '1',
    title: 'Test Item',
    price: 99.99,
  };

  it('displays item data', () => {
    render(<ItemCard item={mockItem} />);

    expect(screen.getByText('Test Item')).toBeInTheDocument();
    expect(screen.getByText('$99.99')).toBeInTheDocument();
  });

  it('calls onClick when clicked', async () => {
    const user = userEvent.setup();
    const onClick = jest.fn();

    render(<ItemCard item={mockItem} onClick={onClick} />);

    await user.click(screen.getByRole('button'));

    expect(onClick).toHaveBeenCalledWith(mockItem.id);
  });

  it('shows loading state', () => {
    render(<ItemCard item={mockItem} loading />);

    expect(screen.getByTestId('loading-spinner')).toBeInTheDocument();
  });
});

---

Pattern: React Query Testing

Problem: Components using React Query need QueryClientProvider.

import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
import { render, screen, waitFor } from '@testing-library/react';

function createTestQueryClient() {
  return new QueryClient({
    defaultOptions: {
      queries: {
        retry: false,
        gcTime: 0,
      },
    },
  });
}

function renderWithQuery(ui: React.ReactElement) {
  const queryClient = createTestQueryClient();
  return render(
    <QueryClientProvider client={queryClient}>
      {ui}
    </QueryClientProvider>
  );
}

// Usage in tests
it('fetches and displays data', async () => {
  renderWithQuery(<UserProfile userId="123" />);

  // Shows loading initially
  expect(screen.getByText('Loading...')).toBeInTheDocument();

  // Wait for data
  await waitFor(() => {
    expect(screen.getByText('John Doe')).toBeInTheDocument();
  });
});

---

Pattern: Custom Hook Testing

import { renderHook, act, waitFor } from '@testing-library/react';

describe('useAuth', () => {
  it('signs in user', async () => {
    const { result } = renderHook(() => useAuth(), {
      wrapper: AuthProvider, // If hook needs context
    });

    await act(async () => {
      await result.current.signIn('test@example.com', 'password');
    });

    expect(result.current.user).toBeDefined();
    expect(result.current.isAuthenticated).toBe(true);
  });

  it('handles sign in error', async () => {
    const { result } = renderHook(() => useAuth(), {
      wrapper: AuthProvider,
    });

    await act(async () => {
      try {
        await result.current.signIn('invalid@example.com', 'wrong');
      } catch (e) {
        // Expected
      }
    });

    expect(result.current.error).toBe('Invalid credentials');
  });
});

// Hook with Zustand
describe('useUserData', () => {
  beforeEach(() => {
    useUserStore.setState(initialState, true);
  });

  it('returns current user data', () => {
    // Pre-populate store
    useUserStore.setState({ user: { id: '1', name: 'Test' } });

    const { result } = renderHook(() => useUserData());

    expect(result.current.user.name).toBe('Test');
  });
});

---

Pattern: Mocking API Calls

// Mock fetch globally
global.fetch = jest.fn();

beforeEach(() => {
  (fetch as jest.Mock).mockClear();
});

it('fetches user data', async () => {
  (fetch as jest.Mock).mockResolvedValueOnce({
    ok: true,
    json: async () => ({ id: '1', name: 'John' }),
  });

  render(<UserProfile userId="1" />);

  await waitFor(() => {
    expect(screen.getByText('John')).toBeInTheDocument();
  });

  expect(fetch).toHaveBeenCalledWith('/api/users/1');
});

// Mock specific module
jest.mock('@/api/users', () => ({
  getUser: jest.fn(),
  updateUser: jest.fn(),
}));

import { getUser, updateUser } from '@/api/users';

it('loads and updates user', async () => {
  (getUser as jest.Mock).mockResolvedValue({ id: '1', name: 'John' });
  (updateUser as jest.Mock).mockResolvedValue({ id: '1', name: 'Jane' });

  // Test component that uses these
});

---

Pattern: Router Testing

import { MemoryRouter, Routes, Route } from 'react-router-dom';

function renderWithRouter(ui: React.ReactElement, { route = '/' } = {}) {
  return render(
    <MemoryRouter initialEntries={[route]}>
      {ui}
    </MemoryRouter>
  );
}

// Test navigation
it('navigates to profile on button click', async () => {
  const user = userEvent.setup();

  renderWithRouter(
    <Routes>
      <Route path="/" element={<HomePage />} />
      <Route path="/profile" element={<ProfilePage />} />
    </Routes>
  );

  await user.click(screen.getByText('Go to Profile'));

  expect(screen.getByText('Profile Page')).toBeInTheDocument();
});

// Test with route params
it('displays user from route params', async () => {
  renderWithRouter(
    <Routes>
      <Route path="/users/:id" element={<UserPage />} />
    </Routes>,
    { route: '/users/123' }
  );

  await waitFor(() => {
    expect(screen.getByText('User 123')).toBeInTheDocument();
  });
});

---

Pattern: Form Testing

import userEvent from '@testing-library/user-event';

describe('LoginForm', () => {
  it('submits form with entered data', async () => {
    const user = userEvent.setup();
    const onSubmit = jest.fn();

    render(<LoginForm onSubmit={onSubmit} />);

    await user.type(screen.getByLabelText('Email'), 'test@example.com');
    await user.type(screen.getByLabelText('Password'), 'password123');
    await user.click(screen.getByRole('button', { name: 'Sign In' }));

    expect(onSubmit).toHaveBeenCalledWith({
      email: 'test@example.com',
      password: 'password123',
    });
  });

  it('shows validation errors', async () => {
    const user = userEvent.setup();

    render(<LoginForm onSubmit={jest.fn()} />);

    // Submit without filling form
    await user.click(screen.getByRole('button', { name: 'Sign In' }));

    expect(screen.getByText('Email is required')).toBeInTheDocument();
    expect(screen.getByText('Password is required')).toBeInTheDocument();
  });

  it('disables submit while loading', async () => {
    render(<LoginForm onSubmit={jest.fn()} loading />);

    expect(screen.getByRole('button', { name: 'Sign In' })).toBeDisabled();
  });
});

---

Pattern: Avoiding act() Warnings

Problem: "Warning: An update inside a test was not wrapped in act(...)"

// WRONG - state update happens after test
it('loads data', () => {
  render(<DataComponent />);
  // Component fetches data async, updates state after test ends
});

// CORRECT - wait for async completion
it('loads data', async () => {
  render(<DataComponent />);

  // Wait for loading to complete
  await waitFor(() => {
    expect(screen.getByText('Data loaded')).toBeInTheDocument();
  });
});

// CORRECT - use findBy* (has built-in waitFor)
it('loads data', async () => {
  render(<DataComponent />);

  const element = await screen.findByText('Data loaded');
  expect(element).toBeInTheDocument();
});

---

Pattern: Snapshot Testing

When to use:

• UI components with stable structure
• Design system components
• Components where visual regression matters

When to avoid:

• Components with dynamic content
• Components that change frequently
• Large component trees (brittle)

// Good snapshot candidate - stable UI component
it('renders correctly', () => {
  const { container } = render(<Button variant="primary">Submit</Button>);
  expect(container).toMatchSnapshot();
});

// Bad snapshot candidate - dynamic content
it('renders user list', () => {
  // Don't snapshot - list content varies
  // Instead, test specific behaviors
});

---

Pattern: Testing Context Providers

// Create a wrapper with all providers
function AllProviders({ children }: { children: React.ReactNode }) {
  const queryClient = createTestQueryClient();
  return (
    <QueryClientProvider client={queryClient}>
      <AuthProvider>
        <ThemeProvider>
          {children}
        </ThemeProvider>
      </AuthProvider>
    </QueryClientProvider>
  );
}

function renderWithProviders(ui: React.ReactElement) {
  return render(ui, { wrapper: AllProviders });
}

// Use in tests
it('renders with all context', () => {
  renderWithProviders(<Dashboard />);
  // Component has access to all providers
});

---

Test Commands

npm test                      # Run all tests
npm test -- --watch           # Watch mode
npm test -- --coverage        # Coverage report
npm test -- Button            # Run specific test file
npm test -- --updateSnapshot  # Update snapshots
npm test -- --runInBand       # Run tests serially (debugging)

---

Common Issues

Issue	Solution
"Cannot find module"	Check jest moduleNameMapper config
act() warning	Wrap state updates in act(), use waitFor/findBy
Store state bleeding	Add beforeEach with setState reset
Async test timeout	Increase timeout or check for hanging promises
Mock not working	Verify mock path matches import path exactly
Query not found	Use findBy* for async content, check accessibility

---

Recommended File Structure

__tests__/
  utils/
    test-utils.tsx         # Custom render with providers
    query-test-utils.tsx   # QueryClient wrapper
jest.setup.js              # Global mocks and setup
jest.config.js             # Jest configuration