Agent skill

testing-patterns

Jest testing patterns, factory functions, mocking strategies, and TDD workflow. Use when writing unit tests, creating test factories, or following TDD red-green-refactor cycle.

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SKILL.md

Testing Patterns and Utilities

Testing Philosophy

Test-Driven Development (TDD):

  • Write failing test FIRST
  • Implement minimal code to pass
  • Refactor after green
  • Never write production code without a failing test

Behavior-Driven Testing:

  • Test behavior, not implementation
  • Focus on public APIs and business requirements
  • Avoid testing implementation details
  • Use descriptive test names that describe behavior

Factory Pattern:

  • Create getMockX(overrides?: Partial<X>) functions
  • Provide sensible defaults
  • Allow overriding specific properties
  • Keep tests DRY and maintainable

Test Utilities

Custom Render Function

Create a custom render that wraps components with required providers:

typescript
// src/utils/testUtils.tsx
import { render } from '@testing-library/react-native';
import { ThemeProvider } from './theme';

export const renderWithTheme = (ui: React.ReactElement) => {
  return render(
    <ThemeProvider>{ui}</ThemeProvider>
  );
};

Usage:

typescript
import { renderWithTheme } from 'utils/testUtils';
import { screen } from '@testing-library/react-native';

it('should render component', () => {
  renderWithTheme(<MyComponent />);
  expect(screen.getByText('Hello')).toBeTruthy();
});

Factory Pattern

Component Props Factory

typescript
import { ComponentProps } from 'react';

const getMockMyComponentProps = (
  overrides?: Partial<ComponentProps<typeof MyComponent>>
) => {
  return {
    title: 'Default Title',
    count: 0,
    onPress: jest.fn(),
    isLoading: false,
    ...overrides,
  };
};

// Usage in tests
it('should render with custom title', () => {
  const props = getMockMyComponentProps({ title: 'Custom Title' });
  renderWithTheme(<MyComponent {...props} />);
  expect(screen.getByText('Custom Title')).toBeTruthy();
});

Data Factory

typescript
interface User {
  id: string;
  name: string;
  email: string;
  role: 'admin' | 'user';
}

const getMockUser = (overrides?: Partial<User>): User => {
  return {
    id: '123',
    name: 'John Doe',
    email: 'john@example.com',
    role: 'user',
    ...overrides,
  };
};

// Usage
it('should display admin badge for admin users', () => {
  const user = getMockUser({ role: 'admin' });
  renderWithTheme(<UserCard user={user} />);
  expect(screen.getByText('Admin')).toBeTruthy();
});

Mocking Patterns

Mocking Modules

typescript
// Mock entire module
jest.mock('utils/analytics');

// Mock with factory function
jest.mock('utils/analytics', () => ({
  Analytics: {
    logEvent: jest.fn(),
  },
}));

// Access mock in test
const mockLogEvent = jest.requireMock('utils/analytics').Analytics.logEvent;

Mocking GraphQL Hooks

typescript
jest.mock('./GetItems.generated', () => ({
  useGetItemsQuery: jest.fn(),
}));

const mockUseGetItemsQuery = jest.requireMock(
  './GetItems.generated'
).useGetItemsQuery as jest.Mock;

// In test
mockUseGetItemsQuery.mockReturnValue({
  data: { items: [] },
  loading: false,
  error: undefined,
});

Test Structure

typescript
describe('ComponentName', () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  describe('Rendering', () => {
    it('should render component with default props', () => {});
    it('should render loading state when loading', () => {});
  });

  describe('User interactions', () => {
    it('should call onPress when button is clicked', async () => {});
  });

  describe('Edge cases', () => {
    it('should handle empty data gracefully', () => {});
  });
});

Query Patterns

typescript
// Element must exist
expect(screen.getByText('Hello')).toBeTruthy();

// Element should not exist
expect(screen.queryByText('Goodbye')).toBeNull();

// Element appears asynchronously
await waitFor(() => {
  expect(screen.findByText('Loaded')).toBeTruthy();
});

User Interaction Patterns

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

it('should submit form on button click', async () => {
  const onSubmit = jest.fn();
  renderWithTheme(<LoginForm onSubmit={onSubmit} />);

  fireEvent.changeText(screen.getByLabelText('Email'), 'user@example.com');
  fireEvent.changeText(screen.getByLabelText('Password'), 'password123');
  fireEvent.press(screen.getByTestId('login-button'));

  await waitFor(() => {
    expect(onSubmit).toHaveBeenCalled();
  });
});

Anti-Patterns to Avoid

Testing Mock Behavior Instead of Real Behavior

typescript
// Bad - testing the mock
expect(mockFetchData).toHaveBeenCalled();

// Good - testing actual behavior
expect(screen.getByText('John Doe')).toBeTruthy();

Not Using Factories

typescript
// Bad - duplicated, inconsistent test data
it('test 1', () => {
  const user = { id: '1', name: 'John', email: 'john@test.com', role: 'user' };
});
it('test 2', () => {
  const user = { id: '2', name: 'Jane', email: 'jane@test.com' }; // Missing role!
});

// Good - reusable factory
const user = getMockUser({ name: 'Custom Name' });

Best Practices

  1. Always use factory functions for props and data
  2. Test behavior, not implementation
  3. Use descriptive test names
  4. Organize with describe blocks
  5. Clear mocks between tests
  6. Keep tests focused - one behavior per test

Running Tests

bash
# Run all tests
npm test

# Run with coverage
npm run test:coverage

# Run specific file
npm test ComponentName.test.tsx

Integration with Other Skills

  • react-ui-patterns: Test all UI states (loading, error, empty, success)
  • systematic-debugging: Write test that reproduces bug before fixing

When to Use

This skill is applicable to execute the workflow or actions described in the overview.

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