testing-patterns

name	testing-patterns
description	TDD patterns, Jest testing conventions, and test quality standards. Use when writing any tests.
auto_load	true
priority	high

Testing Patterns Skill

Trigger

When writing tests or implementing features that need tests.

TDD Flow

1. Red: Write a failing test first
2. Green: Write minimal code to pass
3. Refactor: Clean up while tests stay green

Test Structure

describe('ModuleName', () => {
  describe('functionName', () => {
    it('should do expected behavior when given input', () => {
      // Arrange
      const input = createTestInput();

      // Act
      const result = functionName(input);

      // Assert
      expect(result).toEqual(expectedOutput);
    });

    it('should throw when given invalid input', () => {
      expect(() => functionName(null)).toThrow();
    });
  });
});

Rules

Naming

• Describe behavior, not implementation
• Good: should return 404 when user not found
• Bad: test getUserById

Isolation

• Each test is independent (no shared mutable state)
• Use beforeEach for setup, not beforeAll for mutable state
• Clean up after tests (close connections, clear timers)

Mocking

• Mock external dependencies (APIs, databases, file system)
• Don't mock the thing you're testing
• Use jest.mock() at module level, jest.spyOn() for specific methods
• Reset mocks between tests: jest.restoreAllMocks() in afterEach

What to Test

• Happy path (expected input -> expected output)
• Error cases (invalid input, missing data, network failures)
• Edge cases (empty arrays, null values, boundary conditions)
• Integration points (API endpoints with supertest)

What NOT to Test

• Third-party library internals
• TypeScript type checking (the compiler does this)
• Simple getters/setters with no logic

Anti-Patterns to Avoid

• No waitForTimeout() or sleep() in tests
• No hardcoded test IDs that depend on database state
• No tests that depend on execution order
• No any type assertions to make tests pass