# Re-render Optimization (rerender-*)

**Impact: MEDIUM.** Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.

Rules below are from Vercel's `react-best-practices` skill (MIT, vercel-labs/agent-skills). Rule IDs match upstream filenames for traceability.

---

## `rerender-defer-reads` — Defer State Reads to Usage Point

**Impact: MEDIUM (avoids unnecessary subscriptions)**

Don't subscribe to dynamic state (searchParams, localStorage) if you only read it inside callbacks.

**Incorrect (subscribes to all searchParams changes):**

```tsx
function ShareButton({ chatId }: { chatId: string }) {
  const searchParams = useSearchParams()

  const handleShare = () => {
    const ref = searchParams.get('ref')
    shareChat(chatId, { ref })
  }

  return <button onClick={handleShare}>Share</button>
}
```

**Correct (reads on demand, no subscription):**

```tsx
function ShareButton({ chatId }: { chatId: string }) {
  const handleShare = () => {
    const params = new URLSearchParams(window.location.search)
    const ref = params.get('ref')
    shareChat(chatId, { ref })
  }

  return <button onClick={handleShare}>Share</button>
}
```

---

## `rerender-dependencies` — Narrow Effect Dependencies

**Impact: LOW (minimizes effect re-runs)**

Specify primitive dependencies instead of objects to minimize effect re-runs.

**Incorrect (re-runs on any user field change):**

```tsx
useEffect(() => {
  console.log(user.id)
}, [user])
```

**Correct (re-runs only when id changes):**

```tsx
useEffect(() => {
  console.log(user.id)
}, [user.id])
```

**For derived state, compute outside effect:**

```tsx
// Incorrect: runs on width=767, 766, 765...
useEffect(() => {
  if (width < 768) {
    enableMobileMode()
  }
}, [width])

// Correct: runs only on boolean transition
const isMobile = width < 768
useEffect(() => {
  if (isMobile) {
    enableMobileMode()
  }
}, [isMobile])
```

---

## `rerender-derived-state-no-effect` — Calculate Derived State During Rendering

**Impact: MEDIUM (avoids redundant renders and state drift)**

If a value can be computed from current props/state, do not store it in state or update it in an effect. Derive it during render to avoid extra renders and state drift. Do not set state in effects solely in response to prop changes; prefer derived values or keyed resets instead.

**Incorrect (redundant state and effect):**

```tsx
function Form() {
  const [firstName, setFirstName] = useState('First')
  const [lastName, setLastName] = useState('Last')
  const [fullName, setFullName] = useState('')

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

  return <p>{fullName}</p>
}
```

**Correct (derive during render):**

```tsx
function Form() {
  const [firstName, setFirstName] = useState('First')
  const [lastName, setLastName] = useState('Last')
  const fullName = firstName + ' ' + lastName

  return <p>{fullName}</p>
}
```

References: [You Might Not Need an Effect](https://react.dev/learn/you-might-not-need-an-effect)

---

## `rerender-derived-state` — Subscribe to Derived State

**Impact: MEDIUM (reduces re-render frequency)**

Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.

**Incorrect (re-renders on every pixel change):**

```tsx
function Sidebar() {
  const width = useWindowWidth()  // updates continuously
  const isMobile = width < 768
  return <nav className={isMobile ? 'mobile' : 'desktop'} />
}
```

**Correct (re-renders only when boolean changes):**

```tsx
function Sidebar() {
  const isMobile = useMediaQuery('(max-width: 767px)')
  return <nav className={isMobile ? 'mobile' : 'desktop'} />
}
```

---

## `rerender-functional-setstate` — Use Functional setState Updates

**Impact: MEDIUM (prevents stale closures and unnecessary callback recreations)**

When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references.

**Incorrect (requires state as dependency):**

```tsx
function TodoList() {
  const [items, setItems] = useState(initialItems)
  
  // Callback must depend on items, recreated on every items change
  const addItems = useCallback((newItems: Item[]) => {
    setItems([...items, ...newItems])
  }, [items])  // ❌ items dependency causes recreations
  
  // Risk of stale closure if dependency is forgotten
  const removeItem = useCallback((id: string) => {
    setItems(items.filter(item => item.id !== id))
  }, [])  // ❌ Missing items dependency - will use stale items!
  
  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
}
```

The first callback is recreated every time `items` changes, which can cause child components to re-render unnecessarily. The second callback has a stale closure bug—it will always reference the initial `items` value.

**Correct (stable callbacks, no stale closures):**

```tsx
function TodoList() {
  const [items, setItems] = useState(initialItems)
  
  // Stable callback, never recreated
  const addItems = useCallback((newItems: Item[]) => {
    setItems(curr => [...curr, ...newItems])
  }, [])  // ✅ No dependencies needed
  
  // Always uses latest state, no stale closure risk
  const removeItem = useCallback((id: string) => {
    setItems(curr => curr.filter(item => item.id !== id))
  }, [])  // ✅ Safe and stable
  
  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
}
```

**Benefits:**

1. **Stable callback references** - Callbacks don't need to be recreated when state changes
2. **No stale closures** - Always operates on the latest state value
3. **Fewer dependencies** - Simplifies dependency arrays and reduces memory leaks
4. **Prevents bugs** - Eliminates the most common source of React closure bugs

**When to use functional updates:**

- Any setState that depends on the current state value
- Inside useCallback/useMemo when state is needed
- Event handlers that reference state
- Async operations that update state

**When direct updates are fine:**

- Setting state to a static value: `setCount(0)`
- Setting state from props/arguments only: `setName(newName)`
- State doesn't depend on previous value

**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs.

---

## `rerender-lazy-state-init` — Use Lazy State Initialization

**Impact: MEDIUM (wasted computation on every render)**

Pass a function to `useState` for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.

**Incorrect (runs on every render):**

```tsx
function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs on EVERY render, even after initialization
  const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  // When query changes, buildSearchIndex runs again unnecessarily
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs on every render
  const [settings, setSettings] = useState(
    JSON.parse(localStorage.getItem('settings') || '{}')
  )
  
  return <SettingsForm settings={settings} onChange={setSettings} />
}
```

**Correct (runs only once):**

```tsx
function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs ONLY on initial render
  const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs only on initial render
  const [settings, setSettings] = useState(() => {
    const stored = localStorage.getItem('settings')
    return stored ? JSON.parse(stored) : {}
  })
  
  return <SettingsForm settings={settings} onChange={setSettings} />
}
```

Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.

For simple primitives (`useState(0)`), direct references (`useState(props.value)`), or cheap literals (`useState({})`), the function form is unnecessary.

---

## `rerender-memo-with-default-value` — Extract Default Non-primitive Parameter Value from Memoized Component to Constant

**Impact: MEDIUM (restores memoization by using a constant for default value)**

When memoized component has a default value for some non-primitive optional parameter, such as an array, function, or object, calling the component without that parameter results in broken memoization. This is because new value instances are created on every rerender, and they do not pass strict equality comparison in `memo()`.

To address this issue, extract the default value into a constant.

**Incorrect (`onClick` has different values on every rerender):**

```tsx
const UserAvatar = memo(function UserAvatar({ onClick = () => {} }: { onClick?: () => void }) {
  // ...
})

// Used without optional onClick
<UserAvatar />
```

**Correct (stable default value):**

```tsx
const NOOP = () => {};

const UserAvatar = memo(function UserAvatar({ onClick = NOOP }: { onClick?: () => void }) {
  // ...
})

// Used without optional onClick
<UserAvatar />
```

---

## `rerender-memo` — Extract to Memoized Components

**Impact: MEDIUM (enables early returns)**

Extract expensive work into memoized components to enable early returns before computation.

**Incorrect (computes avatar even when loading):**

```tsx
function Profile({ user, loading }: Props) {
  const avatar = useMemo(() => {
    const id = computeAvatarId(user)
    return <Avatar id={id} />
  }, [user])

  if (loading) return <Skeleton />
  return <div>{avatar}</div>
}
```

**Correct (skips computation when loading):**

```tsx
const UserAvatar = memo(function UserAvatar({ user }: { user: User }) {
  const id = useMemo(() => computeAvatarId(user), [user])
  return <Avatar id={id} />
})

function Profile({ user, loading }: Props) {
  if (loading) return <Skeleton />
  return (
    <div>
      <UserAvatar user={user} />
    </div>
  )
}
```

**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, manual memoization with `memo()` and `useMemo()` is not necessary. The compiler automatically optimizes re-renders.

---

## `rerender-move-effect-to-event` — Put Interaction Logic in Event Handlers

**Impact: MEDIUM (avoids effect re-runs and duplicate side effects)**

If a side effect is triggered by a specific user action (submit, click, drag), run it in that event handler. Do not model the action as state + effect; it makes effects re-run on unrelated changes and can duplicate the action.

**Incorrect (event modeled as state + effect):**

```tsx
function Form() {
  const [submitted, setSubmitted] = useState(false)
  const theme = useContext(ThemeContext)

  useEffect(() => {
    if (submitted) {
      post('/api/register')
      showToast('Registered', theme)
    }
  }, [submitted, theme])

  return <button onClick={() => setSubmitted(true)}>Submit</button>
}
```

**Correct (do it in the handler):**

```tsx
function Form() {
  const theme = useContext(ThemeContext)

  function handleSubmit() {
    post('/api/register')
    showToast('Registered', theme)
  }

  return <button onClick={handleSubmit}>Submit</button>
}
```

Reference: [Should this code move to an event handler?](https://react.dev/learn/removing-effect-dependencies#should-this-code-move-to-an-event-handler)

---

## `rerender-no-inline-components` — Don't Define Components Inside Components

**Impact: HIGH (prevents remount on every render)**

**Impact: HIGH (prevents remount on every render)**

Defining a component inside another component creates a new component type on every render. React sees a different component each time and fully remounts it, destroying all state and DOM.

A common reason developers do this is to access parent variables without passing props. Always pass props instead.

**Incorrect (remounts on every render):**

```tsx
function UserProfile({ user, theme }) {
  // Defined inside to access `theme` - BAD
  const Avatar = () => (
    <img
      src={user.avatarUrl}
      className={theme === 'dark' ? 'avatar-dark' : 'avatar-light'}
    />
  )

  // Defined inside to access `user` - BAD
  const Stats = () => (
    <div>
      <span>{user.followers} followers</span>
      <span>{user.posts} posts</span>
    </div>
  )

  return (
    <div>
      <Avatar />
      <Stats />
    </div>
  )
}
```

Every time `UserProfile` renders, `Avatar` and `Stats` are new component types. React unmounts the old instances and mounts new ones, losing any internal state, running effects again, and recreating DOM nodes.

**Correct (pass props instead):**

```tsx
function Avatar({ src, theme }: { src: string; theme: string }) {
  return (
    <img
      src={src}
      className={theme === 'dark' ? 'avatar-dark' : 'avatar-light'}
    />
  )
}

function Stats({ followers, posts }: { followers: number; posts: number }) {
  return (
    <div>
      <span>{followers} followers</span>
      <span>{posts} posts</span>
    </div>
  )
}

function UserProfile({ user, theme }) {
  return (
    <div>
      <Avatar src={user.avatarUrl} theme={theme} />
      <Stats followers={user.followers} posts={user.posts} />
    </div>
  )
}
```

**Symptoms of this bug:**
- Input fields lose focus on every keystroke
- Animations restart unexpectedly
- `useEffect` cleanup/setup runs on every parent render
- Scroll position resets inside the component

---

## `rerender-simple-expression-in-memo` — Do not wrap a simple expression with a primitive result type in useMemo

**Impact: LOW-MEDIUM (wasted computation on every render)**

When an expression is simple (few logical or arithmetical operators) and has a primitive result type (boolean, number, string), do not wrap it in `useMemo`.
Calling `useMemo` and comparing hook dependencies may consume more resources than the expression itself.

**Incorrect:**

```tsx
function Header({ user, notifications }: Props) {
  const isLoading = useMemo(() => {
    return user.isLoading || notifications.isLoading
  }, [user.isLoading, notifications.isLoading])

  if (isLoading) return <Skeleton />
  // return some markup
}
```

**Correct:**

```tsx
function Header({ user, notifications }: Props) {
  const isLoading = user.isLoading || notifications.isLoading

  if (isLoading) return <Skeleton />
  // return some markup
}
```

---

## `rerender-split-combined-hooks` — Split Combined Hook Computations

**Impact: MEDIUM (avoids recomputing independent steps)**

When a hook contains multiple independent tasks with different dependencies, split them into separate hooks. A combined hook reruns all tasks when any dependency changes, even if some tasks don't use the changed value.

**Incorrect (changing `sortOrder` recomputes filtering):**

```tsx
const sortedProducts = useMemo(() => {
  const filtered = products.filter((p) => p.category === category)
  const sorted = filtered.toSorted((a, b) =>
    sortOrder === "asc" ? a.price - b.price : b.price - a.price
  )
  return sorted
}, [products, category, sortOrder])
```

**Correct (filtering only recomputes when products or category change):**

```tsx
const filteredProducts = useMemo(
  () => products.filter((p) => p.category === category),
  [products, category]
)

const sortedProducts = useMemo(
  () =>
    filteredProducts.toSorted((a, b) =>
      sortOrder === "asc" ? a.price - b.price : b.price - a.price
    ),
  [filteredProducts, sortOrder]
)
```

This pattern also applies to `useEffect` when combining unrelated side effects:

**Incorrect (both effects run when either dependency changes):**

```tsx
useEffect(() => {
  analytics.trackPageView(pathname)
  document.title = `${pageTitle} | My App`
}, [pathname, pageTitle])
```

**Correct (effects run independently):**

```tsx
useEffect(() => {
  analytics.trackPageView(pathname)
}, [pathname])

useEffect(() => {
  document.title = `${pageTitle} | My App`
}, [pageTitle])
```

**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, it automatically optimizes dependency tracking and may handle some of these cases for you.

---

## `rerender-transitions` — Use Transitions for Non-Urgent Updates

**Impact: MEDIUM (maintains UI responsiveness)**

Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.

**Incorrect (blocks UI on every scroll):**

```tsx
function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0)
  useEffect(() => {
    const handler = () => setScrollY(window.scrollY)
    window.addEventListener('scroll', handler, { passive: true })
    return () => window.removeEventListener('scroll', handler)
  }, [])
}
```

**Correct (non-blocking updates):**

```tsx
import { startTransition } from 'react'

function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0)
  useEffect(() => {
    const handler = () => {
      startTransition(() => setScrollY(window.scrollY))
    }
    window.addEventListener('scroll', handler, { passive: true })
    return () => window.removeEventListener('scroll', handler)
  }, [])
}
```

---

## `rerender-use-deferred-value` — Use useDeferredValue for Expensive Derived Renders

**Impact: MEDIUM (keeps input responsive during heavy computation)**

When user input triggers expensive computations or renders, use `useDeferredValue` to keep the input responsive. The deferred value lags behind, allowing React to prioritize the input update and render the expensive result when idle.

**Incorrect (input feels laggy while filtering):**

```tsx
function Search({ items }: { items: Item[] }) {
  const [query, setQuery] = useState('')
  const filtered = items.filter(item => fuzzyMatch(item, query))

  return (
    <>
      <input value={query} onChange={e => setQuery(e.target.value)} />
      <ResultsList results={filtered} />
    </>
  )
}
```

**Correct (input stays snappy, results render when ready):**

```tsx
function Search({ items }: { items: Item[] }) {
  const [query, setQuery] = useState('')
  const deferredQuery = useDeferredValue(query)
  const filtered = useMemo(
    () => items.filter(item => fuzzyMatch(item, deferredQuery)),
    [items, deferredQuery]
  )
  const isStale = query !== deferredQuery

  return (
    <>
      <input value={query} onChange={e => setQuery(e.target.value)} />
      <div style={{ opacity: isStale ? 0.7 : 1 }}>
        <ResultsList results={filtered} />
      </div>
    </>
  )
}
```

**When to use:**

- Filtering/searching large lists
- Expensive visualizations (charts, graphs) reacting to input
- Any derived state that causes noticeable render delays

**Note:** Wrap the expensive computation in `useMemo` with the deferred value as a dependency, otherwise it still runs on every render.

Reference: [React useDeferredValue](https://react.dev/reference/react/useDeferredValue)

---

## `rerender-use-ref-transient-values` — Use useRef for Transient Values

**Impact: MEDIUM (avoids unnecessary re-renders on frequent updates)**

When a value changes frequently and you don't want a re-render on every update (e.g., mouse trackers, intervals, transient flags), store it in `useRef` instead of `useState`. Keep component state for UI; use refs for temporary DOM-adjacent values. Updating a ref does not trigger a re-render.

**Incorrect (renders every update):**

```tsx
function Tracker() {
  const [lastX, setLastX] = useState(0)

  useEffect(() => {
    const onMove = (e: MouseEvent) => setLastX(e.clientX)
    window.addEventListener('mousemove', onMove)
    return () => window.removeEventListener('mousemove', onMove)
  }, [])

  return (
    <div
      style={{
        position: 'fixed',
        top: 0,
        left: lastX,
        width: 8,
        height: 8,
        background: 'black',
      }}
    />
  )
}
```

**Correct (no re-render for tracking):**

```tsx
function Tracker() {
  const lastXRef = useRef(0)
  const dotRef = useRef<HTMLDivElement>(null)

  useEffect(() => {
    const onMove = (e: MouseEvent) => {
      lastXRef.current = e.clientX
      const node = dotRef.current
      if (node) {
        node.style.transform = `translateX(${e.clientX}px)`
      }
    }
    window.addEventListener('mousemove', onMove)
    return () => window.removeEventListener('mousemove', onMove)
  }, [])

  return (
    <div
      ref={dotRef}
      style={{
        position: 'fixed',
        top: 0,
        left: 0,
        width: 8,
        height: 8,
        background: 'black',
        transform: 'translateX(0px)',
      }}
    />
  )
}
```