30. What is the useDeferredValue hook in React and how does it improve performance?
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The useDeferredValue hook is a performance optimization tool introduced in React 18. It allows you to defer the update of a value, ensuring that less critical updates (e.g., rendering search results) do not block more critical updates (e.g., user input). This helps improve the responsiveness of your application, especially when dealing with expensive computations or frequent updates.

Deferred Updates:

  • Priority Scheduling: React schedules updates based on their priority. High-priority updates (e.g., user interactions) are processed immediately, while low-priority updates (e.g., rendering large lists) are deferred.
  • Deferring Updates: The useDeferredValue hook defers the update of a value, allowing React to prioritize more important tasks.

How It Works:

  • The hook takes a value as input and returns a deferred version of that value.
  • React updates the deferred value only when the main thread is idle or when the deferred value is needed.

Use Cases:

  • Deferring the rendering of search results while the user is typing.
  • Optimizing performance for large lists or complex UI updates.

Step-by-Step Breakdown

  1. Input Value: You provide a value (e.g., a state variable) to useDeferredValue.
  2. Deferred Value: React returns a deferred version of the value, which updates at a lower priority.
  3. Rendering Optimization: Use the deferred value in expensive computations or rendering logic to avoid unnecessary re-renders.
  4. Priority Handling: React ensures that high-priority updates (e.g., user input) are processed first, while the deferred value is updated later.

Example: Using useDeferredValue in Search Results

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

function SearchResults({ query }) {
  // Defer the query value
  const deferredQuery = useDeferredValue(query);

  // Simulate an expensive computation
  const results = useMemo(() => {
    return expensiveSearch(deferredQuery);
  }, [deferredQuery]);

  return (
    <div>
      {results.map((result) => (
        <div key={result.id}>{result.text}</div>
      ))}
    </div>
  );
}

function App() {
  const [query, setQuery] = useState('');

  return (
    <div>
      <input
        type='text'
        value={query}
        onChange={(e) => setQuery(e.target.value)}
        placeholder='Search...'
      />
      <SearchResults query={query} />
    </div>
  );
}

function expensiveSearch(query) {
  const results = [];
  for (let i = 0; i < 10000; i++) {
    results.push({ id: i, text: `Result ${i} for "${query}"` });
  }
  return results;
}

Explanation of the Example

  • Input Handling: The query state is updated on every keystroke in the input field.
  • Deferred Value: useDeferredValue creates a deferred version of query (deferredQuery), which updates at a lower priority.
  • Expensive Computation: The expensiveSearch function is only recomputed when deferredQuery changes, reducing the frequency of expensive operations.
  • Responsive UI: By deferring the update of deferredQuery, React ensures that the input field remains responsive to user interactions, even while the search results are being computed.

Conclusion

The useDeferredValue hook is a powerful tool for optimizing rendering performance in React applications. It defers less critical updates, ensuring that high-priority tasks (e.g., user interactions) are processed first. By using useDeferredValue, you can improve the responsiveness of your application, especially when dealing with frequently updating values and expensive computations.

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