Translate the following 500-word English technical documentation into Simplified Chinese, preserving code blocks and variable names verbatim. Source: 'The useEffect hook in React 19 accepts a callback and a dependency array. When dependencies change, the effect re-runs. Cleanup functions run before re-run and on unmount. Avoid side effects that do not need synchronization. Prefer useMemo for pure derivations. Strict Mode double-invokes effects in dev to surface bugs. With Server Components, effects only run on the client.'

Fluent zh-CN, technical accuracy, consistent terminology, code preserved.