Agent skill
unit-testing-anton-abyzov-specweave
Unit testing and TDD expert for Vitest/Jest. Red-green-refactor workflow, mocking strategies, coverage analysis, and ESM mocking patterns. Use for writing tests, implementing TDD, or setting up test coverage.
Install this agent skill to your Project
npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/other/other/unit-testing-anton-abyzov-specweave
SKILL.md
Unit Testing & TDD Expert - Vitest/Jest
Test-Driven Development (TDD)
Red-Green-Refactor Cycle
// 1. RED: Write failing test
describe('Calculator', () => {
it('should add two numbers', () => {
const calc = new Calculator();
expect(calc.add(2, 3)).toBe(5); // FAILS - doesn't exist
});
});
// 2. GREEN: Minimal implementation
class Calculator {
add(a: number, b: number): number {
return a + b;
}
}
// 3. REFACTOR: Improve design
class Calculator {
add(...numbers: number[]): number {
return numbers.reduce((sum, n) => sum + n, 0);
}
}
TDD Checklist
- RED: Test fails for the RIGHT reason
- GREEN: Simplest code that passes
- REFACTOR: All tests still green
Basic Test Structure
import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
describe('UserService', () => {
let service: UserService;
beforeEach(() => {
service = new UserService();
vi.clearAllMocks();
});
it('should create user', () => {
const user = service.create({ name: 'John', email: '[email protected]' });
expect(user).toMatchObject({
id: expect.any(String),
name: 'John',
email: '[email protected]'
});
});
it('should throw for invalid email', () => {
expect(() => service.create({ email: 'invalid' })).toThrow('Invalid email');
});
});
Mocking Strategies
1. Mock Functions
const mockFn = vi.fn();
mockFn.mockReturnValue(42);
expect(mockFn()).toBe(42);
expect(mockFn).toHaveBeenCalledTimes(1);
2. Mock Modules
vi.mock('./database', () => ({
query: vi.fn().mockResolvedValue([{ id: 1 }])
}));
import { query } from './database';
3. ESM Mocking with vi.hoisted()
CRITICAL for ESM modules:
// ✅ Define mocks in hoisted context FIRST
const { mockFn } = vi.hoisted(() => ({
mockFn: vi.fn()
}));
vi.mock('./module', () => ({
myFunc: mockFn
}));
import { myFunc } from './module';
4. Dependency Injection
class UserService {
constructor(private db: Database) {}
}
// Test with mock
const mockDb = { query: vi.fn().mockResolvedValue({ id: '123' }) };
const service = new UserService(mockDb);
Test Patterns
AAA Pattern (Arrange-Act-Assert)
it('should calculate total', () => {
// Arrange
const cart = new ShoppingCart();
cart.addItem({ price: 10, quantity: 2 });
// Act
const total = cart.getTotal();
// Assert
expect(total).toBe(20);
});
Parametric Testing
describe.each([
[2, 3, 5],
[10, 5, 15],
[-1, 1, 0],
])('add(%i, %i)', (a, b, expected) => {
it(`should return ${expected}`, () => {
expect(add(a, b)).toBe(expected);
});
});
Coverage Analysis
// vitest.config.ts
export default {
test: {
coverage: {
provider: 'v8',
reporter: ['text', 'html', 'lcov'],
lines: 80,
functions: 80,
branches: 80,
}
}
};
Async Testing
it('should fetch user', async () => {
const user = await api.fetchUser('123');
expect(user).toEqual({ id: '123', name: 'John' });
});
it('should handle API errors', async () => {
await expect(api.fetchUser('invalid')).rejects.toThrow('Not found');
});
VSCode Debug Mode Fix
When tests spawn child processes, use clean environment:
function getCleanEnv(): NodeJS.ProcessEnv {
const cleanEnv = { ...process.env };
delete cleanEnv.NODE_OPTIONS;
delete cleanEnv.NODE_INSPECT;
delete cleanEnv.VSCODE_INSPECTOR_OPTIONS;
return cleanEnv;
}
// Usage
execSync('node cli.js', { env: getCleanEnv() });
Isolated Temp Directories
import * as os from 'os';
const TEST_ROOT = path.join(os.tmpdir(), `test-${Date.now()}`);
beforeEach(() => fs.mkdir(TEST_ROOT, { recursive: true }));
afterEach(() => fs.rm(TEST_ROOT, { recursive: true, force: true }));
Best Practices
- Test behavior, not implementation
- One assertion per test (when possible)
- Independent tests (no shared state)
- Fast tests (<100ms each)
- Clear test names (should...)
- Mock external dependencies only
Quick Reference
Assertions
expect(value).toBe(expected); // ===
expect(value).toEqual(expected); // Deep equality
expect(array).toContain(item); // Array includes
expect(string).toMatch(/pattern/); // Regex
expect(fn).toThrow(Error); // Throws
expect(obj).toHaveProperty('key'); // Has property
Mock Utilities
vi.fn() // Create mock
vi.fn().mockReturnValue(x) // Return value
vi.fn().mockResolvedValue(x) // Async return
vi.mock('./module') // Mock module
vi.spyOn(obj, 'method') // Spy on method
vi.clearAllMocks() // Clear history
Related Skills
qa-engineer- Overall test strategye2e-testing- E2E and visual tests
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