testing-patterns

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:

// 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:

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

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

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

const getMockUser = (overrides?: Partial<User>): User => {
  return {
    id: '123',
    name: 'John Doe',
    email: '[email protected]',
    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

// 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

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

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

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

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

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

Query Patterns

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

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

// Element appears asynchronously
// async-appearance-example: await the element, not the Promise
expect(await screen.findByText('Loaded')).toBeTruthy();

User Interaction Patterns

import { fireEvent, screen, waitFor } 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'), '[email protected]');
  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

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

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

Not Using Factories

// Bad - duplicated, inconsistent test data
it('test 1', () => {
  const user = { id: '1', name: 'John', email: '[email protected]', role: 'user' };
});
it('test 2', () => {
  const user = { id: '2', name: 'Jane', email: '[email protected]' }; // 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

# 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

Use when writing Jest unit tests, test factories and mocks, or diagnosing a failing test through a red-green-refactor cycle.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.