在React中使用使用效應依賴性陣列時，什麼是常見的陷阱？

使用useEffect時依賴數組常見問題包括：1.未包含所有依賴導致閉包問題，應將effect中使用的變量均加入數組；2.添加不穩定依賴引發無限循環，需用useMemo或useCallback優化或移出組件；3.過度使用useEffect處理派生狀態，應改用useMemo；4.忽略異步操作的清理邏輯，需在return中取消未完成的任務。正確處理依賴項和副作用行為能避免多數問題。

When you're working with useEffect in React, the dependency array is one of those things that seems simple at first but can easily trip you up if not handled carefully. The main point is this: if your effect isn't behaving the way you expect, the problem is often in how you've set up (or missed setting up) the dependency array .

Here are some common pitfalls and what to watch out for.

? Not Including All Dependencies

This is probably the most frequent mistake. If your effect uses a variable from the component body — whether it's a prop, state, or any other value — and you don't include it in the dependency array, your effect will "see" an outdated version of that value.

For example:

 const [count, setCount] = useState(0);

useEffect(() => {
  const timer = setTimeout(() => {
    console.log(`Count is ${count}`);
  }, 1000);
}, []); // Oops! Missing count in dependencies

In this case, even if count changes, the effect still logs the initial value because it wasn't re-run. You might think it's a closure issue, but really, it's just missing from the deps.

✅ Fix: Add all values used inside the effect to the dependency array.

? Adding Unstable Dependencies That Cause Infinite Loops

Another trap is when you add something like an object or function as a dependency — especially if it's recreated on every render. This causes the effect to run again and again in a loop.

Example:

 const config = { delay: 1000 };

useEffect(() => {
  // do something with config
}, [config]); // This will trigger a re-run every time because config is new each render

Since config is created anew every time, its reference changes, so React thinks it's a new dependency, which triggers the effect again.

✅ Solutions:

• Move static values outside the component.
• Use useMemo for objects or useCallback for functions if they must be inside the component.

❌ Overusing useEffect for Derived State

Sometimes people reach for useEffect when they should be using useMemo or computing values directly. For instance, if you have a side-effect-free transformation of props or state, useEffect is not the right tool.

Bad example:

 useEffect(() => {
  setFormattedName(getName(user));
}, [user]);

If getName(user) has no side effects, this is unnecessary. It adds complexity and makes your code harder to follow.

✅ Better approach: Use useMemo instead.

 const formattedName = useMemo(() => getName(user), [user]);

? Ignoring How Functions Are Handled Inside Effects

Even if you're using useCallback , sometimes the logic inside your effect still needs to handle async behavior carefully. Especially when dealing with cleanup or race conditions.

Example:

 useEffect(() => {
  let isMounted = true;

  fetchData().then(result => {
    if (isMounted) {
      setData(result);
    }
  });

  return () => {
    isMounted = false;
  };
}, [someDependency]);

This pattern works, but if you forget to clean up or if someDependency changes rapidly (like in fast user input), you might end up updating state on an unmounted component or showing stale data.

✅ Tips:

• Always clean up async tasks in the return function.
• Be cautious with promises — consider switching to AbortController or libraries like useAsync for more control.

In Summary

The dependency array in useEffect is powerful but easy to misuse. Keep these in mind:

• Don't skip dependencies — React warns for a reason.
• Avoid unstable values by memoizing when necessary.
• Think twice before reaching for useEffect — maybe there's a simpler way.
• Handle async operations carefully, especially around cleanup.

It's not rocket science, but it does require attention to detail. Once you get into the habit of double-checking dependencies and understanding why effects re-run, things start to click.

基本上就這些。

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