Testing Patterns

For verifying test effectiveness through mutation analysis, load the mutation-testing skill. For evaluating test quality against Dave Farley's properties, load the test-design-reviewer skill.

Core Principle

Test behavior, not implementation. 100% coverage through business behavior, not implementation details.

Example: Validation code in payment-validator.ts gets 100% coverage by testing processPayment() behavior, NOT by directly testing validator functions.

Test Through Public API Only

Never test implementation details. Test behavior through public APIs.

Why this matters:

Tests remain valid when refactoring
Tests document intended behavior
Tests catch real bugs, not implementation changes

Examples

❌ WRONG - Testing implementation:

// ❌ Testing HOW (implementation detail)
it('should call validateAmount', () => {
  const spy = jest.spyOn(validator, 'validateAmount');
  processPayment(payment);
  expect(spy).toHaveBeenCalled(); // Tests HOW, not WHAT
});

// ❌ Testing private methods
it('should validate CVV format', () => {
  const result = validator._validateCVV('123'); // Private method!
  expect(result).toBe(true);
});

// ❌ Testing internal state
it('should set isValidated flag', () => {
  processPayment(payment);
  expect(processor.isValidated).toBe(true); // Internal state
});

✅ CORRECT - Testing behavior through public API:

it('should reject negative amounts', () => {
  const payment = getMockPayment({ amount: -100 });
  const result = processPayment(payment);
  expect(result.success).toBe(false);
  expect(result.error).toContain('Amount must be positive');
});

it('should reject invalid CVV', () => {
  const payment = getMockPayment({ cvv: '12' }); // Only 2 digits
  const result = processPayment(payment);
  expect(result.success).toBe(false);
  expect(result.error).toContain('Invalid CVV');
});

it('should process valid payments', () => {
  const payment = getMockPayment({ amount: 100, cvv: '123' });
  const result = processPayment(payment);
  expect(result.success).toBe(true);
  expect(result.data.transactionId).toBeDefined();
});

Coverage Through Behavior

Validation code gets 100% coverage by testing the behavior it protects:

// Tests covering validation WITHOUT testing validator directly
describe('processPayment', () => {
  it('should reject negative amounts', () => {
    const payment = getMockPayment({ amount: -100 });
    const result = processPayment(payment);
    expect(result.success).toBe(false);
  });

  it('should reject amounts over 10000', () => {
    const payment = getMockPayment({ amount: 15000 });
    const result = processPayment(payment);
    expect(result.success).toBe(false);
  });

  it('should reject invalid CVV', () => {
    const payment = getMockPayment({ cvv: '12' });
    const result = processPayment(payment);
    expect(result.success).toBe(false);
  });

  it('should process valid payments', () => {
    const payment = getMockPayment({ amount: 100, cvv: '123' });
    const result = processPayment(payment);
    expect(result.success).toBe(true);
  });
});

// ✅ Result: payment-validator.ts has 100% coverage through behavior

Key insight: When coverage drops, ask "What business behavior am I not testing?" not "What line am I missing?"

Don't Extract for Testability

Never extract a function into its own file purely to give it its own unit test. Extract for readability (a descriptive name clarifies intent), for DRY (same knowledge used in multiple places — see the refactoring skill's "DRY = Knowledge, Not Code" rule), or for separation of concerns. Not for testability.

If code is inline in a function, it gets coverage through that function's behavioral tests. Every layer has behavioral tests — domain functions have vitest unit tests, components have browser tests, pages have integration tests. There is no gap.

The anti-pattern is creating a 1:1 mapping between extracted helpers and test files (see "No 1:1 Mapping" below). The extracted helper is an implementation detail of its consumer. Test the consumer's behavior.

❌ WRONG — Extracted single-use helper with its own test file:

// prepare-participant-data.ts (new file, one caller)
export const prepareParticipantData = (items: Item[]) => ({
  yourClaims: items.filter(i => i.isClaimed && i.isClaimedByCurrentUser),
  available: items.filter(i => !i.isClaimedByCurrentUser),
});

// prepare-participant-data.test.ts (tests the helper directly)
it('filters claims', () => { ... });

✅ CORRECT — Inline in the consuming function, tested through its behavior:

// load-participant-view.ts
export const loadParticipantView = async (db, eventId, userId) => {
  const items = await getItems(db, eventId);
  const yourClaims = items.filter(i => i.isClaimed && i.isClaimedByCurrentUser);
  const available = items.filter(i => !i.isClaimedByCurrentUser);
  return { yourClaims, available };
};

// The behavioral test for loadParticipantView covers the filtering:
it('returns claimed gifts in yourClaims and unclaimed in available', () => {
  const result = await loadParticipantView(db, eventId, userId);
  expect(result.yourClaims).toHaveLength(1);
  expect(result.available).toHaveLength(2);
});

When extraction IS justified (DRY): If the same filtering logic is used by multiple consumers with the same business meaning, extract it. But test it through each consumer's behavior, not as an isolated unit.

Test Factory Pattern

For test data, use factory functions with optional overrides.

Core Principles

Return complete objects with sensible defaults
Accept Partial<T> overrides for customization
Validate with real schemas (don't redefine)
NO let/beforeEach - use factories for fresh state

Basic Pattern

const getMockUser = (overrides?: Partial<User>): User => {
  return UserSchema.parse({
    id: 'user-123',
    name: 'Test User',
    email: 'test@example.com',
    role: 'user',
    ...overrides,
  });
};

// Usage
it('creates user with custom email', () => {
  const user = getMockUser({ email: 'custom@example.com' });
  const result = createUser(user);
  expect(result.success).toBe(true);
});

Complete Factory Example

import { UserSchema } from '@/schemas'; // Import real schema

const getMockUser = (overrides?: Partial<User>): User => {
  return UserSchema.parse({
    id: 'user-123',
    name: 'Test User',
    email: 'test@example.com',
    role: 'user',
    isActive: true,
    createdAt: new Date('2024-01-01'),
    ...overrides,
  });
};

Why validate with schema?

Ensures test data is valid according to production schema
Catches breaking changes early (schema changes fail tests)
Single source of truth (no schema redefinition)

Tip: For factories where only a subset of fields are relevant, use Pick<T, 'field1' | 'field2'> for the overrides parameter to constrain what callers can customize.

Factory Composition

For nested objects, compose factories:

const getMockItem = (overrides?: Partial<Item>): Item => {
  return ItemSchema.parse({
    id: 'item-1',
    name: 'Test Item',
    price: 100,
    ...overrides,
  });
};

const getMockOrder = (overrides?: Partial<Order>): Order => {
  return OrderSchema.parse({
    id: 'order-1',
    items: [getMockItem()],      // ✅ Compose factories
    customer: getMockCustomer(),  // ✅ Compose factories
    payment: getMockPayment(),    // ✅ Compose factories
    ...overrides,
  });
};

// Usage - override nested objects
it('calculates total with multiple items', () => {
  const order = getMockOrder({
    items: [
      getMockItem({ price: 100 }),
      getMockItem({ price: 200 }),
    ],
  });
  expect(calculateTotal(order)).toBe(300);
});

Anti-Patterns

❌ WRONG: Using let and beforeEach

let user: User;
beforeEach(() => {
  user = { id: 'user-123', name: 'Test User', ... };  // Shared mutable state!
});

it('test 1', () => {
  user.name = 'Modified User';  // Mutates shared state
});

it('test 2', () => {
  expect(user.name).toBe('Test User');  // Fails! Modified by test 1
});

✅ CORRECT: Factory per test

it('test 1', () => {
  const user = getMockUser({ name: 'Modified User' });  // Fresh state
  // ...
});

it('test 2', () => {
  const user = getMockUser();  // Fresh state, not affected by test 1
  expect(user.name).toBe('Test User');  // ✅ Passes
});

❌ WRONG: Incomplete objects

const getMockUser = () => ({
  id: 'user-123',  // Missing name, email, role!
});

✅ CORRECT: Complete objects

const getMockUser = (overrides?: Partial<User>): User => {
  return UserSchema.parse({
    id: 'user-123',
    name: 'Test User',
    email: 'test@example.com',
    role: 'user',
    ...overrides,  // All required fields present
  });
};

❌ WRONG: Redefining schemas in tests

// ❌ Schema already defined in src/schemas/user.ts!
const UserSchema = z.object({ ... });
const getMockUser = () => UserSchema.parse({ ... });

✅ CORRECT: Import real schema

import { UserSchema } from '@/schemas/user';

const getMockUser = (overrides?: Partial<User>): User => {
  return UserSchema.parse({
    id: 'user-123',
    name: 'Test User',
    email: 'test@example.com',
    ...overrides,
  });
};

Coverage Theater Detection

Watch for these patterns that give fake 100% coverage:

Pattern 1: Mock the function being tested

❌ WRONG - Gives 100% coverage but tests nothing:

it('calls validator', () => {
  const spy = jest.spyOn(validator, 'validate');
  validate(payment);
  expect(spy).toHaveBeenCalled(); // Meaningless assertion
});

✅ CORRECT - Test actual behavior:

it('should reject invalid payment', () => {
  const payment = getMockPayment({ amount: -100 });
  const result = validate(payment);
  expect(result.success).toBe(false);
  expect(result.error).toContain('Amount must be positive');
});

Pattern 2: Test only that function was called

❌ WRONG - No behavior validation:

it('processes payment', () => {
  const spy = jest.spyOn(processor, 'process');
  handlePayment(payment);
  expect(spy).toHaveBeenCalledWith(payment); // So what?
});

✅ CORRECT - Verify the outcome:

it('should process payment and return transaction ID', () => {
  const payment = getMockPayment();
  const result = handlePayment(payment);
  expect(result.success).toBe(true);
  expect(result.transactionId).toBeDefined();
});

Pattern 3: Test trivial getters/setters

❌ WRONG - Testing implementation, not behavior:

it('sets amount', () => {
  payment.setAmount(100);
  expect(payment.getAmount()).toBe(100); // Trivial
});

✅ CORRECT - Test meaningful behavior:

it('should calculate total with tax', () => {
  const order = createOrder({ items: [item1, item2] });
  const total = order.calculateTotal();
  expect(total).toBe(230); // 200 + 15% tax
});

Pattern 4: 100% line coverage, 0% branch coverage

❌ WRONG - Missing edge cases:

it('validates payment', () => {
  const result = validate(getMockPayment());
  expect(result.success).toBe(true); // Only happy path!
});
// Missing: negative amounts, invalid CVV, missing fields, etc.

✅ CORRECT - Test all branches:

describe('validate payment', () => {
  it('should reject negative amounts', () => {
    const payment = getMockPayment({ amount: -100 });
    expect(validate(payment).success).toBe(false);
  });

  it('should reject amounts over limit', () => {
    const payment = getMockPayment({ amount: 15000 });
    expect(validate(payment).success).toBe(false);
  });

  it('should reject invalid CVV', () => {
    const payment = getMockPayment({ cvv: '12' });
    expect(validate(payment).success).toBe(false);
  });

  it('should accept valid payments', () => {
    const payment = getMockPayment();
    expect(validate(payment).success).toBe(true);
  });
});

No 1:1 Mapping Between Tests and Implementation

Don't create test files that mirror implementation files.

❌ WRONG:

src/
  payment-validator.ts
  payment-processor.ts
  payment-formatter.ts
tests/
  payment-validator.test.ts  ← 1:1 mapping
  payment-processor.test.ts  ← 1:1 mapping
  payment-formatter.test.ts  ← 1:1 mapping

✅ CORRECT:

src/
  payment-validator.ts
  payment-processor.ts
  payment-formatter.ts
tests/
  process-payment.test.ts  ← Tests behavior, not implementation files

Why: Implementation details can be refactored without changing tests. Tests verify behavior remains correct regardless of how code is organized internally.

Summary Checklist

When writing tests, verify:

testing