JavaScript using 키워드 — try/finally 없이 리소스를 자동으로 정리하는 법

class FileHandle {
  #handle: number
 
  constructor(path: string) {
    this.#handle = openFile(path)
    console.log(`파일 열림: ${path}`)
  }
 
  read(): string {
    return readFromHandle(this.#handle)
  }
 
  [Symbol.dispose](): void {
    closeFile(this.#handle)
    console.log('파일 닫힘')
  }
}
 
function processFile() {
  using file = new FileHandle('/data/config.json')
  const content = file.read()
  // 블록이 끝나면 file[Symbol.dispose]() 자동 호출
  return JSON.parse(content)
}

class DatabaseConnection {
  #pool: ConnectionPool
 
  constructor(pool: ConnectionPool) {
    this.#pool = pool
  }
 
  async query(sql: string): Promise<Result> {
    return this.#pool.execute(sql)
  }
 
  async [Symbol.asyncDispose](): Promise<void> {
    await this.#pool.release()
    console.log('커넥션 반환 완료')
  }
}
 
async function fetchUsers() {
  await using conn = new DatabaseConnection(pool)
  const users = await conn.query('SELECT * FROM users')
  // 블록 끝에서 await conn[Symbol.asyncDispose]() 자동 호출
  return users
}

function processTransaction() {
  using stack = new DisposableStack()
 
  const conn = stack.use(getConnection())
  const lock = stack.use(acquireLock('users'))
  const tempFile = stack.use(createTempFile())
 
  conn.execute('UPDATE users SET active = true')
  tempFile.write(conn.execute('SELECT * FROM users'))
 
  // 블록 끝에서 역순 정리: tempFile → lock → conn
}

class TempDir {
  constructor(private path: string) {
    fs.mkdirSync(path, { recursive: true })
  }
 
  [Symbol.dispose]() {
    fs.rmSync(this.path, { recursive: true, force: true })
    console.log('temp dir removed')
  }
}
 
function buildPreview(enabled: boolean) {
  using tmp = new TempDir('.tmp/preview')
 
  if (!enabled) {
    return null // 여기서 바로 return해도 tmp는 정리됨
  }
 
  return renderPreview(tmp)
}

function runTask() {
  try {
    using lock = acquireUserLock('u:42')
    using file = openAuditFile('audit.log')
 
    file.write('start task')
    riskyOperation()
    file.write('done')
  } catch (e) {
    logger.error(e)
    // lock/file 정리는 여기서 수동으로 안 해도 됨
  }
}

async function processJobs(jobs: Job[]) {
  for (const job of jobs) {
    await using conn = await openJobConnection(job.id)
    await conn.run(job)
    // 각 루프 이터레이션이 끝날 때마다 conn이 자동 반환됨
  }
}

function createAbortable(): AbortController & Disposable {
  const controller = new AbortController()
  return Object.assign(controller, {
    [Symbol.dispose]() {
      controller.abort()
    }
  })
}
 
async function fetchWithTimeout(url: string) {
  using controller = createAbortable()
  setTimeout(() => controller.abort(), 5000)
  const response = await fetch(url, { signal: controller.signal })
  return response.json()
}

function onEvent(
  target: EventTarget,
  event: string,
  handler: EventListener
): Disposable {
  target.addEventListener(event, handler)
  return {
    [Symbol.dispose]() {
      target.removeEventListener(event, handler)
    }
  }
}
 
function setupUI() {
  using clickHandler = onEvent(button, 'click', handleClick)
  using scrollHandler = onEvent(window, 'scroll', handleScroll)
  // 스코프 끝에서 두 리스너 모두 자동 해제
}