Exploring startTransition in React: Boosting UI Responsiveness with Examples

With the introduction of React 18, a number of exciting new features were added to optimize performance and user experience. One of these is the startTransition API, which allows developers to handle state transitions more efficiently without blocking crucial UI updates. This helps create smoother, more responsive applications by prioritizing urgent updates over non-urgent ones.

In this article, we’ll dive into what startTransition is, how it works, and explore its usage with real-life examples.

What is startTransition?

The startTransition API allows you to mark certain state updates as “non-urgent.” By doing so, React can prioritize urgent updates (like user input or animations) over non-urgent updates (such as complex renders), ensuring a more fluid user experience.

Without startTransition, every state update is treated as urgent, meaning large or computationally expensive updates can cause noticeable delays in UI interactions. By marking non-urgent updates with startTransition, React will process the most critical updates first, improving responsiveness.

Basic Syntax

The startTransition API is used as a wrapper around a non-urgent state update. Here’s the basic syntax:

import { startTransition } from 'react';

// Non-urgent state update
startTransition(() => {
  // Perform state updates here
});

Example 1: Handling Complex State Updates

Let’s consider a scenario where a user types into a search input field that filters a large list of data. Without startTransition, every keystroke would immediately trigger a full re-render of the filtered list, which can make the UI feel slow and unresponsive.

import React, { useState, startTransition } from 'react';

const LargeList = ({ items }) => {
  return (
    <ul>
      {items.map((item) => (
        <li key={item.id}>{item.name}</li>
      ))}
    </ul>
  );
};

const App = () => {
  const [query, setQuery] = useState('');
  const [filteredItems, setFilteredItems] = useState([]);

  const handleSearch = (e) => {
    const value = e.target.value;
    setQuery(value);

    // Mark the filter operation as non-urgent
    startTransition(() => {
      const filtered = largeDataset.filter((item) =>
        item.name.toLowerCase().includes(value.toLowerCase())
      );
      setFilteredItems(filtered);
    });
  };

  return (
    <div>
      <input
        type="text"
        value={query}
        onChange={handleSearch}
        placeholder="Search..."
      />
      <LargeList items={filteredItems} />
    </div>
  );
};

export default App;

In this example:

• As the user types in the search input, the list is filtered. However, we wrap the filtering operation in startTransition, making it non-urgent.
• This ensures that the typing (an urgent update) is handled immediately without delays, while the list re-render (a non-urgent update) is deferred.

Example 2: Avoiding Janky UI During Expensive Re-Renders

Consider an application where selecting a filter triggers a heavy computation or complex UI update. Without startTransition, the UI could freeze or become unresponsive while React processes the update.

import React, { useState, startTransition } from 'react';

const ExpensiveComponent = ({ filter }) => {
  // Simulate an expensive operation
  for (let i = 0; i < 1000000000; i++) {}

  return <div>Filtered content based on: {filter}</div>;
};

const App = () => {
  const [filter, setFilter] = useState('All');
  const [currentFilter, setCurrentFilter] = useState('All');

  const handleFilterChange = (newFilter) => {
    setFilter(newFilter);

    // Mark the filter update as non-urgent
    startTransition(() => {
      setCurrentFilter(newFilter);
    });
  };

  return (
    <div>
      <button onClick={() => handleFilterChange('All')}>All</button>
      <button onClick={() => handleFilterChange('Category A')}>
        Category A
      </button>
      <button onClick={() => handleFilterChange('Category B')}>
        Category B
      </button>

      <ExpensiveComponent filter={currentFilter} />
    </div>
  );
};

export default App;

Here:

• The ExpensiveComponent performs a simulated heavy operation (like rendering or data processing). If this update were urgent, the UI might freeze when the user clicks a button to change the filter.
• By wrapping the state update with startTransition, the button click (an urgent event) is handled immediately, while the heavy render is processed in the background, leading to a smoother user experience.

Example 3: Prioritizing Animation with startTransition

Imagine you have an animation triggered by a user interaction. Without startTransition, if a non-urgent state update occurs during the animation, it can slow down the animation, making it feel janky.

import React, { useState, startTransition } from 'react';

const App = () => {
  const [data, setData] = useState([]);
  const [isAnimating, setIsAnimating] = useState(false);

  const fetchData = () => {
    setIsAnimating(true);

    // Simulate fetching large data
    startTransition(() => {
      const largeData = Array.from({ length: 10000 }, (_, index) => ({
        id: index,
        name: `Item ${index}`,
      }));

      setData(largeData);
      setIsAnimating(false);
    });
  };

  return (
    <div>
      <button onClick={fetchData}>Load Data</button>

      {isAnimating && <div className="loader">Loading animation...</div>}

      <ul>
        {data.map((item) => (
          <li key={item.id}>{item.name}</li>
        ))}
      </ul>
    </div>
  );
};

export default App;

n this example:

• When the user clicks the “Load Data” button, we simulate an animation while data is being fetched.
• Without startTransition, the UI might stutter or freeze during the large data load, affecting the smoothness of the animation. By using startTransition, the non-urgent data fetch is deprioritized, allowing the animation to run smoothly.

When to Use startTransition

While startTransition can improve responsiveness, it’s not needed for every state update. Here are some use cases where startTransition is especially beneficial:

• Complex UI Updates: When rendering large lists or performing heavy computations that don’t need to happen immediately.
• Search or Filtering Operations: Where the UI should prioritize responding to user input over updating the list or search results.
• Rendering with Animations: When an animation is running, and you want to ensure it’s not interrupted by background updates.

Limitations and Caveats

• Not for Urgent Updates: Avoid using startTransition for urgent updates like form inputs or actions that require immediate feedback.
• Concurrent Mode: startTransition works best with React’s concurrent features, introduced in React 18. It’s important to ensure your application is running in concurrent mode to fully benefit from startTransition.

Conclusion

The startTransition API in React is a game-changer for improving the responsiveness of complex applications. By allowing developers to mark certain updates as non-urgent, it prioritizes the most important interactions, like user input or animations, keeping the UI fast and responsive.

When used correctly, startTransition can prevent janky UI updates, especially when handling large datasets, running animations, or performing complex computations. It's one of many tools in React 18 aimed at enhancing performance and user experience.

By incorporating startTransition into your React projects, you can ensure smoother, more responsive interfaces that keep users engaged and satisfied.