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Enforce 2026 testing best practices with BLOCKING validation.

Validation Rules

BLOCKING Rules (exit 1)

File Location Rules

ALLOWED:
  tests/unit/user.test.ts
  tests/integration/api.test.ts
  __tests__/components/Button.test.tsx
  app/tests/test_users.py
  tests/conftest.py

BLOCKED:
  src/utils/helper.test.ts      # Tests in src/
  components/Button.test.tsx    # Tests outside test dir
  app/routers/test_routes.py    # Tests mixed with source

Naming Conventions

TypeScript/JavaScript:

// GOOD - Descriptive, behavior-focused
test('should return empty array when no items exist', () => {})
test('throws ValidationError when email is invalid', () => {})
it('renders loading spinner while fetching', () => {})

// BLOCKED - Too short, not descriptive
test('test1', () => {})
test('works', () => {})
it('test', () => {})

Python:

# GOOD - snake_case, descriptive
def test_should_return_user_when_id_exists():
def test_raises_not_found_when_user_missing():

# BLOCKED - Not descriptive, wrong case
def testUser():      # camelCase
def test_1():        # Not descriptive

AAA Pattern (Required)

Every test must follow Arrange-Act-Assert:

TypeScript Example

describe('calculateDiscount', () => {
  test('should apply 10% discount for orders over $100', () => {
    // Arrange
    const order = createOrder({ total: 150 });
    const calculator = new DiscountCalculator();

    // Act
    const discount = calculator.calculate(order);

    // Assert
    expect(discount).toBe(15);
  });
});

Python Example

class TestCalculateDiscount:
    def test_applies_10_percent_discount_over_threshold(self):
        # Arrange
        order = Order(total=150)
        calculator = DiscountCalculator()

        # Act
        discount = calculator.calculate(order)

        # Assert
        assert discount == 15

Test Isolation (Required)

Tests must not share mutable state:

// BLOCKED - Shared mutable state
let items = [];

test('adds item', () => {
  items.push('a');
  expect(items).toHaveLength(1);
});

test('removes item', () => {
  // FAILS - items already has 'a' from previous test
  expect(items).toHaveLength(0);
});

// GOOD - Reset state in beforeEach
describe('ItemList', () => {
  let items: string[];

  beforeEach(() => {
    items = []; // Fresh state each test
  });

  test('adds item', () => {
    items.push('a');
    expect(items).toHaveLength(1);
  });

  test('starts empty', () => {
    expect(items).toHaveLength(0); // Works!
  });
});

Coverage Requirements

Running Coverage

TypeScript (Vitest/Jest):

npm test -- --coverage
npx vitest --coverage

Python (pytest):

pytest --cov=app --cov-report=json

Parameterized Tests

Use parameterized tests for multiple similar cases:

TypeScript

describe('isValidEmail', () => {
  test.each([
    ['user@example.com', true],
    ['invalid', false],
    ['@missing.com', false],
    ['user@domain.co.uk', true],
    ['user+tag@example.com', true],
  ])('isValidEmail(%s) returns %s', (email, expected) => {
    expect(isValidEmail(email)).toBe(expected);
  });
});

Python

import pytest

class TestIsValidEmail:
    @pytest.mark.parametrize("email,expected", [
        ("user@example.com", True),
        ("invalid", False),
        ("@missing.com", False),
        ("user@domain.co.uk", True),
    ])
    def test_email_validation(self, email: str, expected: bool):
        assert is_valid_email(email) == expected

Fixture Best Practices (Python)

import pytest

# Function scope (default) - Fresh each test
@pytest.fixture
def db_session():
    session = create_session()
    yield session
    session.rollback()

# Module scope - Shared across file
@pytest.fixture(scope="module")
def expensive_model():
    return load_ml_model()  # Only loads once per file

# Session scope - Shared across all tests
@pytest.fixture(scope="session")
def db_engine():
    engine = create_engine(TEST_DB_URL)
    yield engine
    engine.dispose()

Common Violations

1. Test in Wrong Location

BLOCKED: Test file must be in tests/ directory
  File: src/utils/helpers.test.ts
  Move to: tests/utils/helpers.test.ts

2. Missing AAA Structure

BLOCKED: Test pattern violations detected
  - Tests should follow AAA pattern (Arrange/Act/Assert)
  - Add comments or clear separation between sections

3. Shared Mutable State

BLOCKED: Test pattern violations detected
  - Shared mutable state detected. Use beforeEach to reset state.

4. Coverage Below Threshold

BLOCKED: Coverage 75.2% is below threshold 80%

Actions required:
  1. Add tests for uncovered code
  2. Run: npm test -- --coverage
  3. Ensure coverage >= 80% before proceeding

Related Skills

• integration-testing - Component interaction tests
• e2e-testing - End-to-end with Playwright
• msw-mocking - Network mocking
• test-data-management - Fixtures and factories

Capability Details

aaa-pattern

Keywords: AAA, arrange act assert, test structure, test pattern Solves:

• Enforce Arrange-Act-Assert pattern
• Ensure clear test structure
• Improve test readability

test-naming

Keywords: test name, test naming, descriptive test, test description Solves:

• Enforce descriptive test names
• Block generic test names like test1
• Improve test documentation

test-location

Keywords: test location, test directory, tests folder, where tests Solves:

• Validate test file placement
• Block tests mixed with source
• Enforce test directory structure

coverage-threshold

Keywords: coverage, test coverage, code coverage, 80%, threshold Solves:

• Enforce minimum 80% coverage
• Block merges with low coverage
• Maintain quality standards

test-isolation

Keywords: test isolation, shared state, independent tests, flaky Solves:

• Detect shared mutable state
• Ensure test independence
• Prevent flaky tests