Agent Skills: Universal Testing Strategies

Universal Testing Strategies

Overview

Language-agnostic testing patterns and strategies applicable across all technology stacks.

Testing Pyramid

          ┌───────┐
          │  E2E  │  Few, slow, expensive
         ─┴───────┴─
        ┌───────────┐
        │Integration│  Some, medium speed
       ─┴───────────┴─
      ┌───────────────┐
      │     Unit      │  Many, fast, cheap
     ─┴───────────────┴─

Unit Tests (70-80%)

• Test individual functions/methods in isolation
• Mock external dependencies
• Fast execution (< 10ms per test)
• High volume, run on every save

Integration Tests (15-20%)

• Test component interactions
• Real database/API calls (often with test containers)
• Medium speed (100ms - 1s per test)
• Run on commit/PR

End-to-End Tests (5-10%)

• Test complete user flows
• Real browser/environment
• Slow (seconds to minutes)
• Run before deployment

Test Structure: AAA Pattern

// Arrange - Set up test data and conditions
const user = createTestUser({ email: 'test@example.com' });
const service = new UserService(mockDb);

// Act - Execute the code under test
const result = await service.createUser(user);

// Assert - Verify the expected outcome
expect(result.id).toBeDefined();
expect(result.email).toBe('test@example.com');

Naming Conventions

Test File Names

component.test.ts       # Unit tests
component.spec.ts       # Alternative convention
component.integration.ts # Integration tests
component.e2e.ts        # End-to-end tests

Test Case Names

Use descriptive names that explain the scenario:

// Pattern: should [expected behavior] when [condition]
describe('UserService', () => {
  describe('createUser', () => {
    it('should create user when valid data provided', () => {});
    it('should throw ValidationError when email is invalid', () => {});
    it('should throw DuplicateError when email already exists', () => {});
  });
});

BDD Style (Given-When-Then)

describe('User Registration', () => {
  describe('given a valid email and password', () => {
    describe('when the user submits the form', () => {
      it('then creates a new account', () => {});
      it('then sends a welcome email', () => {});
    });
  });
});

Mocking Strategies

Test Doubles Overview

| Type | Purpose | Example | |------|---------|---------| | Stub | Returns predetermined values | stub.returns(42) | | Mock | Verifies interactions | expect(mock).toHaveBeenCalled() | | Spy | Wraps real implementation | spy(realFunction) | | Fake | Working implementation (simplified) | In-memory database | | Dummy | Placeholder (not used) | Required parameter |

When to Mock

DO Mock:

• External services (APIs, databases)
• Time-dependent functions
• Random number generators
• File system operations
• Network requests

DON'T Mock:

• The code under test
• Simple value objects
• Pure functions with no side effects

Mock Example

// Mock external API
const mockApi = {
  getUser: jest.fn().mockResolvedValue({ id: '1', name: 'Test' })
};

// Inject mock
const service = new UserService(mockApi);
const result = await service.getUser('1');

// Verify interaction
expect(mockApi.getUser).toHaveBeenCalledWith('1');
expect(result.name).toBe('Test');

Test Data Management

Test Factories

Create reusable factory functions for test data:

// factories/user.ts
export function createTestUser(overrides = {}) {
  return {
    id: randomUUID(),
    email: `test-${Date.now()}@example.com`,
    name: 'Test User',
    createdAt: new Date(),
    ...overrides
  };
}

// In tests
const user = createTestUser({ name: 'Custom Name' });

Fixtures

Static test data for consistent testing:

// fixtures/users.ts
export const validUser = {
  email: 'valid@example.com',
  password: 'SecurePass123!'
};

export const invalidEmails = [
  'no-at-sign',
  '@no-local.com',
  'no-domain@',
  'spaces in@email.com'
];

Assertion Best Practices

Be Specific

// BAD - vague assertion
expect(result).toBeTruthy();

// GOOD - specific assertion
expect(result.success).toBe(true);
expect(result.data.id).toBe('expected-id');

One Logical Assertion Per Test

// BAD - multiple unrelated assertions
it('should process order', () => {
  expect(order.id).toBeDefined();
  expect(order.total).toBe(100);
  expect(emailService.send).toHaveBeenCalled();
  expect(inventory.reduce).toHaveBeenCalled();
});

// GOOD - focused tests
it('should assign an order ID', () => {
  expect(order.id).toBeDefined();
});

it('should calculate correct total', () => {
  expect(order.total).toBe(100);
});

it('should send confirmation email', () => {
  expect(emailService.send).toHaveBeenCalled();
});

Custom Matchers

Create domain-specific matchers for readability:

expect.extend({
  toBeValidEmail(received) {
    const pass = /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(received);
    return {
      pass,
      message: () => `expected ${received} to be a valid email`
    };
  }
});

// Usage
expect('user@example.com').toBeValidEmail();

Edge Cases to Test

Input Validation

• Empty/null/undefined inputs
• Boundary values (0, -1, MAX_INT)
• Invalid types
• Malformed data

State Transitions

• Initial state
• After single operation
• After multiple operations
• After error and recovery

Async Operations

• Successful completion
• Timeout
• Network errors
• Concurrent operations
• Race conditions

Error Handling

• Expected errors (validation)
• Unexpected errors (system)
• Error recovery
• Error propagation

Code Coverage Guidelines

Coverage Targets

| Type | Target | Notes | |------|--------|-------| | Line Coverage | 80% | Minimum acceptable | | Branch Coverage | 75% | Test all conditionals | | Function Coverage | 90% | All public APIs | | Critical Paths | 100% | Auth, payments, data integrity |

Coverage Pitfalls

High coverage ≠ good tests

// 100% coverage but useless test
it('should run without error', () => {
  processOrder(order); // No assertions!
});

Focus on:

• Business-critical paths
• Edge cases and error handling
• Integration points

Test Performance

Keep Tests Fast

| Test Type | Target Time | |-----------|-------------| | Unit test | < 10ms | | Integration test | < 1s | | E2E test | < 30s | | Full suite | < 5min |

Parallelization

# Run tests in parallel
vitest --pool=threads
jest --maxWorkers=4
pytest -n auto
go test -parallel 4

Test Isolation

Tests should not depend on each other:

// BAD - shared state
let counter = 0;
it('test 1', () => { counter++; });
it('test 2', () => { expect(counter).toBe(1); }); // Fragile!

// GOOD - isolated state
beforeEach(() => { counter = 0; });
it('test 1', () => { counter++; expect(counter).toBe(1); });
it('test 2', () => { counter++; expect(counter).toBe(1); });

CI/CD Integration

Pre-commit

• Lint checks
• Type checks
• Unit tests (fast)

Pull Request

• Full unit test suite
• Integration tests
• Coverage report

Pre-deployment

• E2E tests
• Performance tests
• Security scans

Test Maintenance

Avoid Flaky Tests

• Don't depend on timing
• Don't depend on external services
• Use deterministic data
• Retry with care (hide real issues)

Keep Tests Readable

• Clear naming
• Minimal setup
• Obvious assertions
• Helpful failure messages

Review Test Code

• Test code is production code
• Apply same quality standards
• Refactor when needed

Testing strategies applicable to all technology stacks