Writing Good Tests

Philosophy

"Write tests. Not too many. Mostly integration." — Kent C. Dodds

Tests verify real behavior, not implementation details. The goal is confidence that your code works, not coverage numbers.

Core principles:

1. Test behavior, not implementation — refactoring shouldn't break tests
2. Integration tests provide better confidence-to-cost ratio than unit tests
3. Wait for actual conditions, not arbitrary timeouts
4. Mock strategically — real dependencies when feasible, mocks for external systems
5. Don't pollute production code with test-only methods

Test Structure

Use Arrange-Act-Assert (or Given-When-Then):

test('user can cancel reservation', async () => {
  // Arrange
  const reservation = await createReservation({ userId: 'user-1', roomId: 'room-1' });

  // Act
  const result = await cancelReservation(reservation.id);

  // Assert
  expect(result.status).toBe('cancelled');
  expect(await getReservation(reservation.id)).toBeNull();
});

One action per test. Multiple assertions are fine if they verify the same behavior.

Condition-Based Waiting

Flaky tests often guess at timing. This creates race conditions where tests pass locally but fail in CI.

Wait for conditions, not time:

// BAD: Guessing at timing
await new Promise(r => setTimeout(r, 50));
const result = getResult();

// GOOD: Waiting for condition
await waitFor(() => getResult() !== undefined);
const result = getResult();

Generic Polling Function

async function waitFor<T>(
  condition: () => T | undefined | null | false,
  description: string,
  timeoutMs = 5000
): Promise<T> {
  const startTime = Date.now();

  while (true) {
    const result = condition();
    if (result) return result;

    if (Date.now() - startTime > timeoutMs) {
      throw new Error(`Timeout waiting for ${description} after ${timeoutMs}ms`);
    }

    await new Promise(r => setTimeout(r, 10)); // Poll every 10ms
  }
}

Quick Patterns

Scenario	Pattern
Wait for event	waitFor(() => events.find(e => e.type === 'DONE'))
Wait for state	waitFor(() => machine.state === 'ready')
Wait for count	waitFor(() => items.length >= 5)

When Arbitrary Timeout IS Correct

Only when testing actual timing behavior (debounce, throttle, intervals):

// Testing tool that ticks every 100ms
await waitForEvent(manager, 'TOOL_STARTED'); // First: wait for condition
await new Promise(r => setTimeout(r, 200));   // Then: wait for 2 ticks
// Comment explains WHY: 200ms = 2 ticks at 100ms intervals

Mocking Strategy

"You don't hate mocks; you hate side-effects." — J.B. Rainsberger

Mocks reveal where side-effects complicate your code. Use them strategically, not reflexively.

Don't Mock What You Don't Own

Create thin wrappers around third-party libraries. Mock YOUR wrapper, not the library.

// BAD: Mock the HTTP client directly
const mockClient = vi.mocked(httpx.Client);

// GOOD: Create your own wrapper
class RegistryClient {
  constructor(private client: HttpClient) {}
  async getRepos() {
    return this.client.get('https://registry.example.com/v2/_catalog');
  }
}

// Mock your wrapper
vi.mock('./registry-client');

This simplifies tests AND improves your design.

Managed vs Unmanaged Dependencies

Dependency Type	Example	Strategy
Managed (you control it)	Your database, your file system	Use REAL instances
Unmanaged (external)	Third-party APIs, SMTP, message bus	Use MOCKS

Communications with managed dependencies are implementation details — you can refactor them freely. Communications with unmanaged dependencies are observable behavior — mocking protects against external changes.

Anti-Pattern: Testing Mock Behavior

// BAD: Testing that the mock exists
test('renders sidebar', () => {
  render(<Page />);
  expect(screen.getByTestId('sidebar-mock')).toBeInTheDocument();
});

// GOOD: Test real behavior
test('renders sidebar', () => {
  render(<Page />);
  expect(screen.getByRole('navigation')).toBeInTheDocument();
});

Gate: Before asserting on any mock element, ask: "Am I testing real behavior or mock existence?"

Anti-Pattern: Mocking Without Understanding

// BAD: Mock breaks test logic
test('detects duplicate server', () => {
  // Mock prevents config write that test depends on!
  vi.mock('ToolCatalog', () => ({
    discoverAndCacheTools: vi.fn().mockResolvedValue(undefined)
  }));
  await addServer(config);
  await addServer(config);  // Should throw - but won't!
});

// GOOD: Mock at correct level
test('detects duplicate server', () => {
  vi.mock('MCPServerManager'); // Just mock slow server startup
  await addServer(config);  // Config written
  await addServer(config);  // Duplicate detected
});

Gate: Before mocking, ask: "What side effects does this have? Does my test depend on them?"

Anti-Pattern: Incomplete Mocks

Mock the COMPLETE data structure as it exists in reality:

// BAD: Partial mock
const mockResponse = {
  status: 'success',
  data: { userId: '123' }
  // Missing: metadata that downstream code uses
};

// GOOD: Mirror real API
const mockResponse = {
  status: 'success',
  data: { userId: '123', name: 'Alice' },
  metadata: { requestId: 'req-789', timestamp: 1234567890 }
};

When Mocks Become Too Complex

Warning signs:

• Mock setup longer than test logic
• Mocking everything to make test pass
• Test breaks when mock changes

"As the number of mocks grows, the probability of testing the mock instead of the desired code goes up." — Codurance

Consider integration tests with real components — often simpler than elaborate mocks.

Anti-Pattern: Test-Only Methods in Production

// BAD: destroy() only used in tests
class Session {
  async destroy() { /* cleanup */ }
}

// GOOD: Test utilities handle cleanup
// test-utils/session-helpers.ts
export async function cleanupSession(session: Session) {
  const workspace = session.getWorkspaceInfo();
  if (workspace) {
    await workspaceManager.destroyWorkspace(workspace.id);
  }
}

Gate: Before adding any method to production class, ask: "Is this only used by tests?" If yes, put it in test utilities.

Test Isolation

Tests should not depend on execution order. But isolation doesn't mean cleaning up everything.

What to Clean Up

Long-lived resources MUST be cleaned up:

• Virtual machines, containers
• Kubernetes jobs, pods, deployments
• Cloud resources (instances, buckets)
• Background processes, daemons

Prefer product tools for cleanup when possible:

afterAll(async () => {
  // Use the product's own cleanup mechanisms
  await deployment.delete();
  await job.terminate();
});

Side-channel cleanup when product tools aren't available:

afterAll(async () => {
  // Direct cleanup when product doesn't provide it
  await exec('kubectl delete job test-job-123');
});

What's OK to Leave

Database artifacts are fine to leave around. Trying to clean up test data perfectly is a fool's errand and makes multi-step integration tests nearly impossible.

• Test records in databases
• Log entries
• Cached data that expires

The database should handle its own lifecycle. Tests that require pristine state should create unique identifiers, not depend on cleanup.

Preventing Order Dependencies

// Use unique identifiers instead of depending on clean state
const testId = `test-${Date.now()}-${Math.random()}`;
const user = await createUser({ email: `${testId}@test.com` });

Quick Reference

Problem	Fix
Arbitrary setTimeout in tests	Use condition-based waiting
Assert on mock elements	Test real component or unmock
Mock third-party directly	Create wrapper, mock wrapper
Test-only methods in production	Move to test utilities
Mock without understanding	Understand dependencies first
Incomplete mocks	Mirror real API completely
Over-complex mocks	Consider integration tests
Long-lived resources left running	Clean up VMs, k8s jobs, cloud resources

Red Flags

Stop and reconsider when you see:

• Arbitrary setTimeout/sleep without justification
• Assertions on mock elements or test IDs
• Methods only called in test files
• Mock setup is >50% of test code
• "Mocking just to be safe"
• Test depends on another test running first
• Long-lived resources not cleaned up

TDD Connection

TDD prevents most testing anti-patterns:

• Write test first → forces thinking about what you're testing
• Watch it fail → confirms test tests real behavior, not mocks
• Minimal implementation → no test-only methods creep in
• Real dependencies first → you see what test needs before mocking

Property-Based Testing

For certain patterns, property-based testing provides stronger coverage than example-based tests. See property-based-testing skill for complete reference.

When to Use PBT

Pattern	Example	Why PBT
Serialization pairs	encode/decode, toJSON/fromJSON	Roundtrip property catches edge cases
Normalizers	sanitize, canonicalize, format	Idempotence property ensures stability
Validators	is_valid, validate	Valid-after-normalize property
Pure functions	Business logic, calculations	Multiple properties verify contract
Sorting/ordering	sort, rank, compare	Ordering + idempotence properties

When NOT to Use PBT

• Simple CRUD without transformation
• UI/presentation logic
• Integration tests requiring external setup
• When specific examples suffice and edge cases are well-understood
• Prototyping with fluid requirements

PBT Quality Gates

Before committing property-based tests:

• Not tautological: Assertion doesn't compare same expression (sorted(xs) == sorted(xs) tests nothing)
• Strong property: Not just "no crash" - aim for roundtrip, idempotence, or invariants
• Not vacuous: assume() calls don't filter out most inputs
• Edge cases explicit: Include @example([]), @example([1]) decorators
• No reimplementation: Don't restate function logic in assertion (assert add(a,b) == a+b)
• Realistic constraints: Strategy matches real-world input constraints