**Question Translation and Refinement:** What are the similarities and differences between the behavior of useEffect with an empty dependency array ([]) and componentDidMount? **Answer and Explanation:** When the dependency array of useEffect is empty ([]), its callback function executes only once when the component is first mounted, and executes the cleanup function (if present) when the component is unmounted. This behavior is similar to componentDidMount in class components, but with key differences: 1. **Execution Timing Differences**: - componentDidMount executes synchronously after the component is mounted (before browser rendering). - useEffect(() => {}, []) is asynchronously scheduled, executing after the browser has completed rendering (falling into the "layout after, paint after" micro/macro task phase, depending on React version and whether createRoot is used). 2. **Blocking Rendering**: - componentDidMount does not block rendering. - useEffect also does not block rendering, but is more explicitly designed as a "side effect" hook, emphasizing non-blocking behavior. 3. **Use Case Differences**: - useEffect([], ...) is suitable for non-blocking side effects like data fetching or subscriptions. - useLayoutEffect should be used if immediate DOM measurements or synchronous DOM operations are required after component mount. 4. **Cleanup Mechanisms**: - useEffect supports automatic cleanup via returned functions, while componentDidMount requires manual cleanup with componentWillUnmount. **Key Takeaway**: Although useEffect([], ...) shares the "once" semantics with componentDidMount, its asynchronous nature and React's concurrency model make it distinct. For strictly synchronous behavior, useLayoutEffect is recommended.