klh-testing-patterns

Testing Patterns and Utilities

Part of klh/skills — personal agent skills by Klaus L. Hougesen. Install: npx skills add klh/skills --skill klh-testing-patterns

Overview

Jest testing patterns, factory functions, mocking strategies, and TDD workflow.

When to Use

• When writing unit tests or creating test factories
• When following TDD red-green-refactor cycle
• When setting up mocking strategies

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

RED Checkpoint

Before writing any implementation:

1. Write the failing test FIRST
2. Run it — it MUST fail cleanly (not crash, not pass, not error on setup)
3. Verify the failure message describes the expected behavior
4. Only then write the minimal implementation

A test that passes before implementation is a broken assertion. Research shows ~62% of LLM-generated test assertions are incorrect — verify each one would catch a real bug.

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();
});