# useEffect Anti-Patterns

## 1. Redundant State for Derived Values

```tsx
// BAD: Extra state + Effect for derived value
function Form() {
  const [firstName, setFirstName] = useState('Taylor');
  const [lastName, setLastName] = useState('Swift');
  const [fullName, setFullName] = useState('');

  useEffect(() => {
    setFullName(firstName + ' ' + lastName);
  }, [firstName, lastName]);
}

// GOOD: Calculate during rendering
function Form() {
  const [firstName, setFirstName] = useState('Taylor');
  const [lastName, setLastName] = useState('Swift');
  const fullName = firstName + ' ' + lastName; // Just compute it
}
```

**Why it's bad**: Causes extra render pass with stale value, then re-renders with updated value.

---

## 2. Filtering/Transforming Data in Effect

```tsx
// BAD: Effect to filter list
function TodoList({ todos, filter }) {
  const [visibleTodos, setVisibleTodos] = useState([]);

  useEffect(() => {
    setVisibleTodos(getFilteredTodos(todos, filter));
  }, [todos, filter]);
}

// GOOD: Filter during render (memoize if expensive)
function TodoList({ todos, filter }) {
  const visibleTodos = useMemo(
    () => getFilteredTodos(todos, filter),
    [todos, filter]
  );
}
```

---

## 3. Resetting State on Prop Change

```tsx
// BAD: Effect to reset state
function ProfilePage({ userId }) {
  const [comment, setComment] = useState('');

  useEffect(() => {
    setComment('');
  }, [userId]);
}

// GOOD: Use key prop
function ProfilePage({ userId }) {
  return <Profile userId={userId} key={userId} />;
}

function Profile({ userId }) {
  const [comment, setComment] = useState(''); // Resets automatically
}
```

**Why key works**: React treats components with different keys as different components, recreating state.

---

## 4. Event-Specific Logic in Effect

```tsx
// BAD: Effect for button click result
function ProductPage({ product, addToCart }) {
  useEffect(() => {
    if (product.isInCart) {
      showNotification(`Added ${product.name}!`);
    }
  }, [product]);

  function handleBuyClick() {
    addToCart(product);
  }
}

// GOOD: Handle in event handler
function ProductPage({ product, addToCart }) {
  function handleBuyClick() {
    addToCart(product);
    showNotification(`Added ${product.name}!`);
  }
}
```

**Why it's bad**: Effect fires on page refresh (isInCart is true), showing notification unexpectedly.

---

## 5. Chains of Effects

```tsx
// BAD: Effects triggering each other
function Game() {
  const [card, setCard] = useState(null);
  const [goldCardCount, setGoldCardCount] = useState(0);
  const [round, setRound] = useState(1);
  const [isGameOver, setIsGameOver] = useState(false);

  useEffect(() => {
    if (card?.gold) setGoldCardCount(c => c + 1);
  }, [card]);

  useEffect(() => {
    if (goldCardCount > 3) {
      setRound(r => r + 1);
      setGoldCardCount(0);
    }
  }, [goldCardCount]);

  useEffect(() => {
    if (round > 5) setIsGameOver(true);
  }, [round]);
}

// GOOD: Calculate in event handler
function Game() {
  const [card, setCard] = useState(null);
  const [goldCardCount, setGoldCardCount] = useState(0);
  const [round, setRound] = useState(1);
  const isGameOver = round > 5; // Derived!

  function handlePlaceCard(nextCard) {
    if (isGameOver) throw Error('Game ended');

    setCard(nextCard);
    if (nextCard.gold) {
      if (goldCardCount < 3) {
        setGoldCardCount(goldCardCount + 1);
      } else {
        setGoldCardCount(0);
        setRound(round + 1);
        if (round === 5) alert('Good game!');
      }
    }
  }
}
```

**Why it's bad**: Multiple re-renders (setCard -> setGoldCardCount -> setRound -> setIsGameOver). Also fragile for features like history replay.

---

## 6. Notifying Parent via Effect

```tsx
// BAD: Effect to notify parent
function Toggle({ onChange }) {
  const [isOn, setIsOn] = useState(false);

  useEffect(() => {
    onChange(isOn);
  }, [isOn, onChange]);

  function handleClick() {
    setIsOn(!isOn);
  }
}

// GOOD: Notify in same event
function Toggle({ onChange }) {
  const [isOn, setIsOn] = useState(false);

  function updateToggle(nextIsOn) {
    setIsOn(nextIsOn);
    onChange(nextIsOn); // Same event, batched render
  }

  function handleClick() {
    updateToggle(!isOn);
  }
}

// BEST: Fully controlled component
function Toggle({ isOn, onChange }) {
  function handleClick() {
    onChange(!isOn);
  }
}
```

---

## 7. Passing Data Up to Parent

```tsx
// BAD: Child fetches, passes up via Effect
function Parent() {
  const [data, setData] = useState(null);
  return <Child onFetched={setData} />;
}

function Child({ onFetched }) {
  const data = useSomeAPI();

  useEffect(() => {
    if (data) onFetched(data);
  }, [onFetched, data]);
}

// GOOD: Parent fetches, passes down
function Parent() {
  const data = useSomeAPI();
  return <Child data={data} />;
}
```

**Why**: Data should flow down. Upward flow via Effects makes debugging hard.

---

## 8. Fetching Without Cleanup (Race Condition)

```tsx
// BAD: No cleanup - race condition
function SearchResults({ query }) {
  const [results, setResults] = useState([]);

  useEffect(() => {
    fetchResults(query).then(json => {
      setResults(json); // "hello" response may arrive after "hell"
    });
  }, [query]);
}

// GOOD: Cleanup ignores stale responses
function SearchResults({ query }) {
  const [results, setResults] = useState([]);

  useEffect(() => {
    let ignore = false;

    fetchResults(query).then(json => {
      if (!ignore) setResults(json);
    });

    return () => { ignore = true; };
  }, [query]);
}
```

---

## 9. App Initialization in Effect

```tsx
// BAD: Runs twice in dev, may break auth
function App() {
  useEffect(() => {
    loadDataFromLocalStorage();
    checkAuthToken(); // May invalidate token on second call!
  }, []);
}

// GOOD: Module-level guard
let didInit = false;

function App() {
  useEffect(() => {
    if (!didInit) {
      didInit = true;
      loadDataFromLocalStorage();
      checkAuthToken();
    }
  }, []);
}

// ALSO GOOD: Module-level execution
if (typeof window !== 'undefined') {
  checkAuthToken();
  loadDataFromLocalStorage();
}
```