qcast-front/.agents/skills/vercel-react-best-practices/rules/rerender-use-deferred-value.md

title	impact	impactDescription	tags
Use useDeferredValue for Expensive Derived Renders	MEDIUM	keeps input responsive during heavy computation	rerender, useDeferredValue, optimization, concurrent

Use useDeferredValue for Expensive Derived Renders

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):

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):

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