Mobile Testing

Testing guide for React Native applications.

When to Use

• Writing unit tests for components or utilities
• Creating integration tests for features
• Setting up test infrastructure
• Debugging test failures
• Improving test coverage

Test Setup

bash
# Install testing dependencies
npm install --save-dev jest @testing-library/react-native

# Add to package.json
{
  "scripts": {
    "test": "jest",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage"
  }
}

Component Test Template

typescript
import { render, fireEvent } from '@testing-library/react-native';
import { MyButton } from './MyButton';

describe('MyButton', () => {
  it('renders correctly', () => {
    const { getByText } = render(<MyButton title="Click me" />);
    expect(getByText('Click me')).toBeTruthy();
  });

  it('calls onPress when pressed', () => {
    const onPress = jest.fn();
    const { getByText } = render(
      <MyButton title="Click me" onPress={onPress} />
    );

    fireEvent.press(getByText('Click me'));
    expect(onPress).toHaveBeenCalledTimes(1);
  });
});

Hook Test Template

typescript
import { renderHook, act } from '@testing-library/react-native';
import { useCounter } from './useCounter';

describe('useCounter', () => {
  it('increments count', () => {
    const { result } = renderHook(() => useCounter());

    act(() => {
      result.current.increment();
    });

    expect(result.current.count).toBe(1);
  });
});

Utility Test Template

typescript
import { formatDate } from './formatDate';

describe('formatDate', () => {
  it('formats date correctly', () => {
    const date = new Date('2025-01-15');
    expect(formatDate(date)).toBe('2025-01-15');
  });

  it('handles invalid input', () => {
    expect(() => formatDate(null)).toThrow();
  });
});

Mocking Patterns

Mock External Module

typescript
jest.mock('expo-notifications', () => ({
  scheduleNotificationAsync: jest.fn(),
}));

Mock Database

typescript
jest.mock('@/db/client', () => ({
  db: {
    select: jest.fn(),
    insert: jest.fn(),
  },
}));

Mock Navigation

typescript
jest.mock('expo-router', () => ({
  useRouter: () => ({
    push: jest.fn(),
    back: jest.fn(),
  }),
}));

Running Tests

bash
# Run all tests
npm test

# Watch mode (auto-rerun on changes)
npm run test:watch

# With coverage report
npm run test:coverage

# Run specific test file
npm test -- MyComponent.test.tsx

# Update snapshots
npm test -- -u

Best Practices

1. Test Behavior, Not Implementation: Test what users see and do
2. Use testID for Selection: More reliable than text matching
3. Mock External Dependencies: Keep tests isolated and fast
4. Test Edge Cases: Empty states, errors, loading states
5. Keep Tests Simple: One assertion per test when possible
6. Clean Up: Use beforeEach/afterEach for setup/teardown

Common Patterns

Async Testing

typescript
it('loads data', async () => {
  const { findByText } = render(<DataComponent />);
  expect(await findByText('Loaded')).toBeTruthy();
});

Testing Forms

typescript
it('validates input', () => {
  const { getByTestId, getByText } = render(<LoginForm />);

  fireEvent.changeText(getByTestId('email-input'), 'invalid');
  fireEvent.press(getByTestId('submit-button'));

  expect(getByText('Invalid email')).toBeTruthy();
});

Testing Lists

typescript
it('renders list items', () => {
  const items = [{ id: '1', name: 'Item 1' }];
  const { getAllByTestId } = render(<ItemList items={items} />);

  expect(getAllByTestId('list-item')).toHaveLength(1);
});

Troubleshooting

• Tests timing out: Increase timeout or check for unresolved promises
• Can't find element: Use screen.debug() to see rendered output
• Mock not working: Ensure mock is before import
• Async issues: Use waitFor or findBy queries

Resources

• Jest Docs
• React Native Testing Library