useDeferredValue for Derived State

Introduction

While useTransition wraps the state update, useDeferredValue wraps the value itself. It's a simpler API when you want to defer a value without needing the isPending state.

Key Concepts

Signature:

tsx
const deferredValue = useDeferredValue(value);

• Takes any value and returns a "deferred" version
• During urgent updates, returns the old value first
• Then updates to the new value in a background render

Real World Context

Perfect for:

• Search-as-you-type interfaces
• Filtering derived data from props
• Any scenario where you receive a value and need to derive expensive UI from it

Deep Dive

How It Works

1. You update some state (e.g., search query)
2. useDeferredValue initially returns the old value
3. React renders with the new state but old deferred value
4. In the background, React prepares a render with the new deferred value
5. When ready, React switches to the new render

Comparing to useTransition

Detecting Stale Content

Check if the deferred value differs from the current value:

tsx
const query = props.searchQuery;
const deferredQuery = useDeferredValue(query);
const isStale = query !== deferredQuery;

return (
  <div style={{ opacity: isStale ? 0.5 : 1 }}>
    <Results query={deferredQuery} />
  </div>
);

Important: Memoization

For useDeferredValue to actually skip work, the child component must be memoized:

tsx
const deferredQuery = useDeferredValue(query);

// Without memo, SlowList still re-renders!
// With memo, it only re-renders when deferredQuery changes
<SlowList query={deferredQuery} />

Common Pitfalls

1. Forgetting to memoize children: Without memo, the optimization is lost because the child re-renders anyway.
2. Using for urgent updates: Don't use for UI that must update immediately (input values, hover states).
3. Creating objects inline: Passing a new object to useDeferredValue each render defeats the purpose.

Best Practices

• Always wrap the component consuming the deferred value with memo()
• Use for props you receive, useTransition for state you own
• Show a visual indicator when content is stale (isStale = current !== deferred)
• Don't defer the input value itself - defer the derived computation
• Combine with Suspense for data fetching scenarios

Summary

useDeferredValue is a simpler alternative to useTransition for deferring values you receive. It automatically returns old values during urgent updates, then updates in the background. Always pair with memo() for the optimization to work.

import { useState, useDeferredValue, memo } from 'react';

// Must be memoized for useDeferredValue to skip re-renders
const SearchResults = memo(function SearchResults({ query }: { query: string }) {
  // Expensive search/filter operation
  const results = expensiveSearch(query);
  
  return (
    <ul>
      {results.map(result => (
        <li key={result.id}>{result.title}</li>
      ))}
    </ul>
  );
});

function SearchPage() {
  const [query, setQuery] = useState('');
  
  // Deferred version of query
  const deferredQuery = useDeferredValue(query);
  
  // Detect if showing stale content
  const isStale = query !== deferredQuery;

  return (
    <div>
      <input
        value={query}
        onChange={(e) => setQuery(e.target.value)}
        placeholder="Search..."
      />
      
      {/* Visual feedback for stale content */}
      <div style={{
        opacity: isStale ? 0.5 : 1,
        transition: 'opacity 200ms'
      }}>
        {isStale && <span className="updating">Updating...</span>}
        
        {/* Uses deferred value - won't block typing */}
        <SearchResults query={deferredQuery} />
      </div>
    </div>
  );
}