Test Writer — Generating test suite...

Generates comprehensive test suites with unit, integration, and e2e tests, proper mocking strategies, edge case coverage, naming conventions, and CI integration patterns.

Activation

When this skill activates, output:

Test Writer — Generating test suite...

Then execute the protocol below.

Context Guard

Common Mistakes

Protocol

Step 1: Gather Test Requirements

If the user hasn't provided details, ask:

1. Target — what code needs tests? (function, class, module, API endpoint)
2. Language/framework — what tech stack? (JS/TS + Jest/Vitest, Python + pytest, Go, etc.)
3. Test level — unit, integration, e2e, or all three?
4. Current coverage — any existing tests? What percentage?
5. Priority — critical paths first, or comprehensive coverage?

Step 2: Analyze the Code Under Test

Before writing tests, understand the target:

Function/method analysis:

Dependency map:

[Target Function]
  ├── [Dependency 1] — mock? [Yes: external / No: pure]
  ├── [Dependency 2] — mock? [Yes: external / No: pure]
  └── [Dependency 3] — mock? [Yes: external / No: pure]

Mock decision rule:

• Mock external boundaries: databases, APIs, file system, network, time
• Don't mock pure functions, internal utilities, simple data transformations
• Sometimes mock sibling modules (mock for unit tests, real for integration)

Step 3: Design Test Cases

Coverage strategy — identify all test scenarios:

Happy path tests:

Edge case tests:

""

[]

{}

null

"123"

123

"café ñ 中文"

Error path tests:

Step 4: Write the Tests

Test file naming conventions:

[name].test.ts

[name].spec.ts

userService.test.ts

test_[name].py

test_user_service.py

[name]_test.go

user_service_test.go

[Name]Test.java

UserServiceTest.java

Test naming conventions:

should [expected] when [condition]

should return empty array when no items match filter

given [state] when [action] then [result]

given expired token when authenticating then throws AuthError

[method] — [scenario] — [expected]

calculateTotal — with discount — applies percentage reduction

Test structure template (JavaScript/TypeScript):

describe('[ModuleName]', () => {
  // Setup shared across tests in this block
  let dependency: MockType;

  beforeEach(() => {
    dependency = createMock();
    // Reset state before each test
  });

  afterEach(() => {
    jest.restoreAllMocks();
  });

  describe('[methodName]', () => {
    // Happy path
    it('should [expected behavior] when [normal condition]', () => {
      // Arrange
      const input = { /* test data */ };
      dependency.method.mockResolvedValue(expectedData);

      // Act
      const result = targetMethod(input);

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

    // Edge case
    it('should [expected behavior] when [edge condition]', () => {
      // Arrange
      const input = { /* edge case data */ };

      // Act
      const result = targetMethod(input);

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

    // Error path
    it('should throw [ErrorType] when [error condition]', () => {
      // Arrange
      const input = { /* invalid data */ };

      // Act & Assert
      expect(() => targetMethod(input)).toThrow(ErrorType);
    });
  });
});

Test structure template (Python):

import pytest
from unittest.mock import Mock, patch

class TestModuleName:
    """Tests for [ModuleName]."""

    def setup_method(self):
        """Reset state before each test."""
        self.dependency = Mock()

    # Happy path
    def test_method_returns_expected_when_normal_input(self):
        """Should [expected behavior] when [condition]."""
        # Arrange
        input_data = {"key": "value"}
        self.dependency.method.return_value = expected_data

        # Act
        result = target_method(input_data)

        # Assert
        assert result == expected_output

    # Edge case
    def test_method_handles_empty_input(self):
        """Should [expected behavior] when input is empty."""
        result = target_method([])
        assert result == []

    # Error path
    def test_method_raises_error_when_invalid_input(self):
        """Should raise ValueError when [condition]."""
        with pytest.raises(ValueError, match="expected message"):
            target_method(invalid_input)

Step 5: Mocking Strategy

Mock patterns by dependency type:

jest.spyOn(db, 'query').mockResolvedValue(rows)

nock('https://api.example.com').get('/users').reply(200, data)

jest.mock('fs/promises')

jest.useFakeTimers(); jest.setSystemTime(new Date('2026-01-15'))

jest.spyOn(Math, 'random').mockReturnValue(0.5)

process.env.NODE_ENV = 'test'

jest.mock('stripe', () => mockStripeClient)

Mock verification patterns:

// Verify a dependency was called correctly
expect(mockDb.query).toHaveBeenCalledWith(
  'SELECT * FROM users WHERE id = $1',
  [userId]
);

// Verify call count
expect(mockApi.fetch).toHaveBeenCalledTimes(1);

// Verify NOT called (important for caching tests)
expect(mockDb.query).not.toHaveBeenCalled();

// Verify call order (important for transaction tests)
const order = [];
mockDb.begin.mockImplementation(() => order.push('begin'));
mockDb.commit.mockImplementation(() => order.push('commit'));
// ... run test ...
expect(order).toEqual(['begin', 'commit']);

Step 6: Integration & E2E Tests

Integration test patterns:

describe('[Feature] Integration', () => {
  // Use real database (test instance), real logic, mock external APIs only
  let testDb: TestDatabase;

  beforeAll(async () => {
    testDb = await createTestDatabase();
    await testDb.migrate();
  });

  afterAll(async () => {
    await testDb.destroy();
  });

  beforeEach(async () => {
    await testDb.truncateAll(); // Clean state per test
  });

  it('should create user and send welcome email', async () => {
    // Arrange — mock only the email service (external boundary)
    const emailSpy = jest.spyOn(emailService, 'send').mockResolvedValue(true);

    // Act — use real DB, real validation, real business logic
    const user = await userService.register({
      email: 'test@example.com',
      name: 'Test User',
    });

    // Assert — verify real DB state + external call
    const dbUser = await testDb.query('SELECT * FROM users WHERE id = $1', [user.id]);
    expect(dbUser).toBeDefined();
    expect(emailSpy).toHaveBeenCalledWith(
      expect.objectContaining({ to: 'test@example.com' })
    );
  });
});

E2E test patterns (API):

describe('POST /api/users', () => {
  it('should create a user and return 201', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({ email: 'test@example.com', name: 'Test User' })
      .expect(201);

    expect(response.body).toMatchObject({
      id: expect.any(String),
      email: 'test@example.com',
    });
  });

  it('should return 400 for invalid email', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({ email: 'not-an-email', name: 'Test' })
      .expect(400);

    expect(response.body.error).toContain('email');
  });

  it('should return 409 for duplicate email', async () => {
    // Create first user
    await request(app).post('/api/users').send({ email: 'dup@example.com', name: 'First' });

    // Attempt duplicate
    const response = await request(app)
      .post('/api/users')
      .send({ email: 'dup@example.com', name: 'Second' })
      .expect(409);
  });
});

Step 7: Coverage & CI Integration

Coverage targets:

CI configuration template (GitHub Actions):

name: Tests
on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20

      - run: npm ci
      - run: npm test -- --coverage

      - name: Check coverage thresholds
        run: |
          npx jest --coverage --coverageThreshold='{
            "global": {
              "branches": 75,
              "functions": 80,
              "lines": 80,
              "statements": 80
            }
          }'

Test runner configuration:

// jest.config.js / vitest.config.ts
{
  "collectCoverageFrom": [
    "src/**/*.{ts,tsx}",
    "!src/**/*.d.ts",
    "!src/**/index.ts",
    "!src/**/*.stories.tsx"
  ],
  "coverageThresholds": {
    "global": {
      "branches": 75,
      "functions": 80,
      "lines": 80,
      "statements": 80
    }
  }
}

Pre-commit test check:

# Only run tests related to changed files (fast feedback)
npx jest --onlyChanged
# or
npx vitest --changed

Output Format

# Test Suite — [Target Module/Function]

## Analysis
- **Target:** [What's being tested]
- **Dependencies:** [X] total ([X] mocked, [X] real)
- **Branching paths:** [X]

## Test Cases
### Unit Tests ([X] tests)
[Test code from Step 4]

### Edge Cases ([X] tests)
[Edge case tests from Step 3]

### Error Cases ([X] tests)
[Error path tests from Step 3]

### Integration Tests ([X] tests)
[Integration tests from Step 6, if requested]

## Coverage Summary
- Lines: [X]%
- Branches: [X]%
- Functions: [X]%

## CI Integration
[CI config from Step 7, if requested]

Completion

Test Writer — Complete!

Target: [Module/function name]
Tests written: [X] total
  - Unit tests: [X]
  - Edge cases: [X]
  - Error cases: [X]
  - Integration tests: [X]
Mocked dependencies: [X]
Estimated coverage: [X]%

Next steps:
1. Run the test suite: [test command]
2. Check coverage report for uncovered branches
3. Add tests to CI pipeline (config provided above)
4. Add pre-commit hook to run related tests on save
5. Review and adjust coverage thresholds quarterly

Level History

• Lv.1 — Base: Code analysis template (inputs, outputs, dependencies, branches), 3-category test case design (happy path, edge cases, error paths), test templates for JS/TS and Python, 7 mock patterns by dependency type with verification, integration and E2E test patterns, coverage targets by code criticality, CI integration with GitHub Actions. (Origin: MemStack Pro v3.2, Mar 2026)