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

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

// 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

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

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

import { vi } from 'vitest';

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

Time / clock

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)

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)

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

React component (Testing Library)

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

vitest run --coverage

// 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: Test_Pyramid
• 응용: TDD · Mocking · Test_Doubles

🤖 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