In this lesson, you'll learn everything about the useDeferredValue hook and how you can implement it in your React apps. By the end of this lesson, you'll have learned about the important role concurrency with this hook plays in building apps with a good user experience.

What is the useDeferredValue Hook?

useDeferredValue is a React Hook that help manage concurrency by allowing you to defer the value of a state update until React has enough time to render it without blocking urgent updates.

Unlike useTransition, which defers an entire state update process, useDeferredValue allows you to delay rendering a specific value while keeping the application responsive. This is particularly useful when dealing with slow-rendering components that depend on frequently changing values.

For example, when a user types into a search bar that filters a large dataset, useDeferredValue can ensure that the search results update smoothly without affecting the responsiveness of the input field.

How useDeferredValue Works in Concurrency

1. Prioritizing Urgent Updates Over Non-Urgent Ones:
useDeferredValue allows React to keep the UI responsive by:

• Instantly updating urgent values (e.g., user input).
• Delaying non-urgent updates (e.g., expensive calculations, filtering large lists).

2. Managing Render Performance:

• Helps avoid blocking the UI when rendering complex components.
• Allows React to render other high-priority components first.

3. Works Best with Slow Components:

• Ideal for cases where rendering takes longer, such as list filtering, expensive computations, or API responses.

Syntax and Basic Usage

The useDeferredValue Hook takes a state value and returns a deferred version of it that updates at a lower priority.

const deferredValue = useDeferredValue(value);

• value: The original state value.
• deferredValue: A delayed version of value that updates when React has time.

Example

Lets study an example that uses the useDeferredValue hook to improve search performance.

Imagine a user types into a search bar that filters a large dataset. Without useDeferredValue, the list updates synchronously, causing noticeable lag when dealing with thousands of items.

Without useDeferredValue (Causing UI Lag)

import { useState } from "react";

const ItemList = ({ items }) => {
  const [query, setQuery] = useState("");

  const filteredItems = items.filter(item => item.includes(query)); // Immediate update

  return (
    <div>
      <input 
        type="text" 
        value={query} 
        onChange={(e) => setQuery(e.target.value)} 
        placeholder="Search..." 
      />
      <ul>
        {filteredItems.map(item => <li key={item}>{item}</li>)}
      </ul>
    </div>
  );
};

The problem with this code example is that; the filtering happens immediately, which can cause UI lag when handling large lists.

With useDeferredValue (Smoother UI)

import { useState, useDeferredValue } from "react";

const ItemList = ({ items }) => {
  const [query, setQuery] = useState("");
  const deferredQuery = useDeferredValue(query); // Deferring query update

  const filteredItems = items.filter(item => item.includes(deferredQuery)); 

  return (
    <div>
      <input 
        type="text" 
        value={query} 
        onChange={(e) => setQuery(e.target.value)} 
        placeholder="Search..." 
      />
      {query !== deferredQuery && <p>Updating list...</p>} {/* Show loading indicator */}
      <ul>
        {filteredItems.map(item => <li key={item}>{item}</li>)}
      </ul>
    </div>
  );
};

After using the useDeferredValue hook in this code, here are some of the noticable improvements:

• The input field updates instantly (query state updates immediately).
• The list filtering is deferred, ensuring a smooth typing experience.
• The user sees a loading message while filtering occurs.

Benefits of useDeferredValue in Concurrency

1. Prevents UI Lag for Heavy Rendering Components

• Ensures that urgent updates (e.g., typing) remain instant while non-urgent updates (e.g., list filtering) wait for available rendering time.

2. Enhances User Experience with Smoother Transitions

• Users don’t experience sluggish interactions when interacting with slow components.

3. Works Seamlessly with Slow-Rendering Components

• When dealing with large datasets, expensive calculations, or animations, useDeferredValue helps maintain UI responsiveness.

Drawbacks and Challenges of useDeferredValue

1. Doesn't Prevent Re-Rendering

• Unlike useMemo, useDeferredValue does not memoize results. It only delays updates, meaning components still re-render when needed.

2. Not Always Noticeable in Fast Operations

• If the component updates quickly, using useDeferredValue may not provide any significant performance improvement.

3. Can Cause UI Inconsistencies if Used Incorrectly

• If developers rely on the deferred value for immediate UI updates, it might create temporary inconsistencies where the UI displays outdated data for a short period.

Best Practices for Using useDeferredValue

1. Use for Slow-Rendering Components

• Ideal for list filtering, complex animations, or charts that take time to update.

2. Combine with useMemo for Optimization

• Since useDeferredValue doesn’t memoize results, wrap it in useMemo if necessary to avoid unnecessary re-renders.

const filteredItems = useMemo(() => {
  return items.filter(item => item.includes(deferredQuery));
}, [deferredQuery, items]);

3. Avoid Using It for Urgent Updates

• For real-time interactions like form validation or button clicks, do not use useDeferredValue, as it may delay critical UI updates.

useDeferredValue vs. useTransition

In summary,

• Use useDeferredValue when a slow-rendering component depends on a frequently changing state.
• Use useTransition when you need to separate urgent and non-urgent state updates.

By leveraging useDeferredValue effectively, developers can enhance the perceived performance of their React applications, leading to a better user experience. 🚀