2nd/10_Wiki/Topics/Architecture/Unit Tests (단위 테스트).md

---
id: wiki-2026-0508-unit-tests-단위-테스트
title: Unit Tests (단위 테스트)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [단위 테스트, Unit Testing, Unit Test]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [testing, unit-test, tdd, vitest]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: vitest-2
---

# Unit Tests (단위 테스트)

## 매 한 줄
> **"매 작은 unit (function / class) 을 isolate 해서 매 fast 하게 검증"**. 매 test pyramid 의 base. 2026 JS 진영은 Vitest 가 매 default — fast, ESM-native, jest-compatible API.

## 매 핵심

### 매 properties (FIRST)
- **Fast**: ms 단위.
- **Independent**: 순서 무관.
- **Repeatable**: deterministic.
- **Self-validating**: pass/fail 명확.
- **Timely**: 매 production code 와 동시 (또는 먼저).

### 매 무엇이 unit
- 매 단일 function / 매 class 의 method / 매 small module.
- 매 collaborator 는 stub/mock 또는 매 real (sociable test).
- 매 file system / network / DB 는 통상 test 가 unit 아님.

### 매 응용
1. TDD red-green-refactor.
2. Regression guard.
3. Living documentation.
4. Refactoring safety net.

## 💻 패턴

### Vitest basic (AAA)
```typescript
// sum.ts
export function sum(a: number, b: number): number {
  return a + b;
}

// sum.test.ts
import { describe, expect, it } from 'vitest';
import { sum } from './sum';

describe('sum', () => {
  it('adds two positive numbers', () => {
    // Arrange
    const a = 2, b = 3;
    // Act
    const result = sum(a, b);
    // Assert
    expect(result).toBe(5);
  });

  it('handles negatives', () => {
    expect(sum(-1, 1)).toBe(0);
  });
});
```

### Parameterized
```typescript
import { describe, expect, it } from 'vitest';

describe.each([
  { a: 1, b: 2, want: 3 },
  { a: 0, b: 0, want: 0 },
  { a: -1, b: 1, want: 0 },
])('sum($a, $b)', ({ a, b, want }) => {
  it(`= ${want}`, () => {
    expect(sum(a, b)).toBe(want);
  });
});
```

### Mock collaborator
```typescript
import { vi, expect, it } from 'vitest';

interface Mailer {
  send(to: string, body: string): Promise<void>;
}

class SignupService {
  constructor(private mailer: Mailer) {}
  async signup(email: string) {
    await this.mailer.send(email, 'Welcome!');
  }
}

it('sends welcome email on signup', async () => {
  const mailer: Mailer = { send: vi.fn().mockResolvedValue(undefined) };
  const svc = new SignupService(mailer);

  await svc.signup('a@b.c');

  expect(mailer.send).toHaveBeenCalledWith('a@b.c', 'Welcome!');
});
```

### Module mock
```typescript
import { vi } from 'vitest';

vi.mock('./db', () => ({
  findUser: vi.fn().mockResolvedValue({ id: '1', name: 'A' }),
}));
```

### Time / clock
```typescript
import { afterEach, beforeEach, vi, expect, it } from 'vitest';

beforeEach(() => vi.useFakeTimers());
afterEach(() => vi.useRealTimers());

it('expires after 1h', () => {
  const session = createSession({ ttlMs: 3600_000 });
  expect(session.expired()).toBe(false);
  vi.advanceTimersByTime(3600_001);
  expect(session.expired()).toBe(true);
});
```

### Property-based (fast-check)
```typescript
import fc from 'fast-check';
import { it, expect } from 'vitest';

it('reverse(reverse(xs)) == xs', () => {
  fc.assert(
    fc.property(fc.array(fc.integer()), (xs) => {
      expect([...xs].reverse().reverse()).toEqual(xs);
    }),
  );
});
```

### Snapshot (use sparingly)
```typescript
it('renders config', () => {
  expect(buildConfig({ env: 'prod' })).toMatchInlineSnapshot(`
    {
      "apiUrl": "https://api.example.com",
      "retries": 3,
    }
  `);
});
```

### React component (Testing Library)
```tsx
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { Counter } from './Counter';

it('increments on click', async () => {
  const user = userEvent.setup();
  render(<Counter />);

  await user.click(screen.getByRole('button', { name: /\+/ }));

  expect(screen.getByText('Count: 1')).toBeInTheDocument();
});
```

### Coverage
```bash
vitest run --coverage
```
```typescript
// vitest.config.ts
export default {
  test: {
    coverage: {
      provider: 'v8',
      reporter: ['text', 'html'],
      thresholds: { lines: 80, functions: 80, branches: 70 },
    },
  },
};
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| 매 pure logic | Plain unit test |
| 매 collaborator-heavy | Sociable test (real collaborator) 또는 mock |
| 매 algorithmic invariants | Property-based (fast-check) |
| 매 React component | Testing Library — user-centric |
| 매 stable structured output | Snapshot (sparingly) |
| 매 DB / network | 매 integration test (not unit) |

**기본값**: 매 sociable + AAA + named cases. 매 mock 은 매 boundary 에서만.

## 🔗 Graph
- 부모: [[Testing]] · [[Test_Pyramid]]
- 변형: [[Integration_Test]] · [[E2E_Test]] · [[Property_Based_Test]]
- 응용: [[TDD]] · [[Mocking]] · [[Test_Doubles]]
- Adjacent: [[Vitest]] · [[Jest]] · [[Testing_Library]]

## 🤖 LLM 활용
**언제**: 매 test scaffolding, 매 edge case 생성, 매 missing branch coverage 의 fill.
**언제 X**: 매 fundamental design problem — 매 test 만으로는 fix 안됨.

## ❌ 안티패턴
- **Test implementation**: 매 internal 의 mock — 매 refactor breakage.
- **Slow units**: 매 DB / network → unit 아님.
- **Shared state**: 매 order-dependent flake.
- **Snapshot 남발**: 매 update 만 누름 — 매 검증 안됨.
- **Coverage as goal**: 매 100% line ≠ 매 quality.

## 🧪 검증 / 중복
- Verified (Vitest 2.x docs, Beck "Test-Driven Development", Meszaros "xUnit Test Patterns", 2026).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Vitest unit tests with mocks, fake timers, fast-check |