React useInsertionEffect: Optimizing CSS-in-JS Libraries for Performance

React's useInsertionEffect is a relatively new hook designed to address a specific performance bottleneck in certain situations, particularly when working with CSS-in-JS libraries. This article provides a comprehensive guide to understanding useInsertionEffect, its purpose, how it works, and how it can be used to optimize CSS-in-JS libraries for improved performance and reduced layout thrashing. The information contained here is important for any React developer working on performance sensitive applications, or looking to improve the perceived performance of their web applications.

Understanding the Problem: CSS-in-JS and Layout Thrashing

CSS-in-JS libraries offer a powerful way to manage CSS styles within your JavaScript code. Popular examples include:

• styled-components
• Emotion
• stitches

These libraries typically work by generating CSS rules dynamically based on your component's props and state. While this approach provides excellent flexibility and composability, it can introduce performance challenges if not handled carefully. The main concern is layout thrashing.

What is Layout Thrashing?

Layout thrashing occurs when the browser is forced to recalculate the layout (the positions and sizes of elements on the page) multiple times during a single frame. This happens when JavaScript code:

1. Modifies the DOM.
2. Immediately requests layout information (e.g., offsetWidth, offsetHeight, getBoundingClientRect).
3. The browser then recalculates the layout.

If this sequence occurs repeatedly within the same frame, the browser spends a significant amount of time recalculating the layout, leading to performance issues such as:

• Slow rendering
• Janky animations
• Poor user experience

CSS-in-JS libraries can contribute to layout thrashing because they often inject CSS rules into the DOM after React has updated the component's DOM structure. This can trigger a layout recalculation, especially if the styles affect the size or position of elements. In the past, libraries would often use useEffect to add the styles, which occurs after the browser has already painted. Now, we have better tools.

Introducing useInsertionEffect

useInsertionEffect is a React hook designed to address this specific performance issue. It allows you to run code before the browser paints, but after the DOM has been updated. This is crucial for CSS-in-JS libraries because it allows them to inject CSS rules before the browser performs its initial layout calculation, thus minimizing layout thrashing. Consider it a more specialized version of useLayoutEffect.

Key Characteristics of useInsertionEffect:

• Runs Before Painting: The effect runs before the browser paints the screen.
• Limited Scope: Primarily intended for injecting styles, mutations to the DOM outside of the specified scope will likely cause unexpected results or issues.
• Runs After DOM Mutations: The effect runs after the DOM has been mutated by React.
• Server-Side Rendering (SSR): It won't execute on the server during server-side rendering. This is because server-side rendering doesn't involve painting or layout calculations.

How useInsertionEffect Works

To understand how useInsertionEffect helps with performance, it's essential to understand the React rendering lifecycle. Here's a simplified overview:

1. Render Phase: React determines what changes need to be made to the DOM based on the component's state and props.
2. Commit Phase: React applies the changes to the DOM.
3. Browser Paint: The browser calculates the layout and paints the screen.

Traditionally, CSS-in-JS libraries would inject styles using useEffect or useLayoutEffect. useEffect runs after the browser has painted, which can lead to a flash of unstyled content (FOUC) and potential layout thrashing. useLayoutEffect runs before the browser paints, but after the DOM mutations. While useLayoutEffect is generally better than useEffect for injecting styles, it can still contribute to layout thrashing because it forces the browser to recalculate the layout after the DOM has been updated, but before the initial paint.

useInsertionEffect solves this problem by running before the browser paints, but after the DOM mutations and before useLayoutEffect. This allows CSS-in-JS libraries to inject styles before the browser performs its initial layout calculation, minimizing the need for subsequent recalculations.

Practical Example: Optimizing a CSS-in-JS Component

Let's consider a simple example using a hypothetical CSS-in-JS library called my-css-in-js. This library provides a function called injectStyles that injects CSS rules into the DOM.

Naive Implementation (Using useEffect):

            
import React, { useEffect } from 'react';
import { injectStyles } from 'my-css-in-js';

const MyComponent = ({ color }) => {
  useEffect(() => {
    const styles = `
      .my-component {
        color: ${color};
        font-size: 16px;
      }
    `;
    injectStyles(styles);
  }, [color]);

  return <div className="my-component">Hello, world!</div>;
};

export default MyComponent;

            

              
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This implementation uses useEffect to inject the styles. While it works, it can lead to a FOUC and potential layout thrashing.

Optimized Implementation (Using useInsertionEffect):

            
import React, { useInsertionEffect } from 'react';
import { injectStyles } from 'my-css-in-js';

const MyComponent = ({ color }) => {
  useInsertionEffect(() => {
    const styles = `
      .my-component {
        color: ${color};
        font-size: 16px;
      }
    `;
    injectStyles(styles);
  }, [color]);

  return <div className="my-component">Hello, world!</div>;
};

export default MyComponent;

            

              
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By switching to useInsertionEffect, we ensure that the styles are injected before the browser paints, reducing the likelihood of layout thrashing.

Best Practices and Considerations

When using useInsertionEffect, keep the following best practices and considerations in mind:

• Use it Specifically for Style Injection: useInsertionEffect is primarily designed for injecting styles. Avoid using it for other types of side effects, as it may lead to unexpected behavior.
• Minimize Side Effects: Keep the code within useInsertionEffect as minimal and efficient as possible. Avoid complex calculations or DOM manipulations that could slow down the rendering process.
• Understand the Execution Order: Be aware that useInsertionEffect runs before useLayoutEffect. This can be important if you have dependencies between these effects.
• Test Thoroughly: Test your components thoroughly to ensure that useInsertionEffect is correctly injecting styles and not introducing any performance regressions.
• Measure Performance: Use browser developer tools to measure the performance impact of useInsertionEffect. Compare the performance of your component with and without useInsertionEffect to verify that it is providing a benefit.
• Be Mindful of Third-Party Libraries: When using third-party CSS-in-JS libraries, check if they already utilize useInsertionEffect internally. If they do, you may not need to use it directly in your components.

Real-World Examples and Use Cases

While the previous example demonstrated a basic use case, useInsertionEffect can be particularly beneficial in more complex scenarios. Here are a few real-world examples and use cases:

• Dynamic Theming: When implementing dynamic theming in your application, you can use useInsertionEffect to inject theme-specific styles before the browser paints. This ensures that the theme is applied smoothly without causing layout shifts.
• Component Libraries: If you're building a component library, using useInsertionEffect can help improve the performance of your components when used in different applications. By injecting styles efficiently, you can minimize the impact on the overall application performance.
• Complex Layouts: In applications with complex layouts, such as dashboards or data visualizations, useInsertionEffect can help reduce layout thrashing caused by frequent style updates.

Example: Dynamic Theming with useInsertionEffect

Consider an application that allows users to switch between light and dark themes. The theme styles are defined in a separate CSS file and injected into the DOM using useInsertionEffect.

            
import React, { useInsertionEffect, useState } from 'react';
import { injectStyles } from 'my-css-in-js';

const themes = {
  light: `
    body {
      background-color: #fff;
      color: #000;
    }
  `,
  dark: `
    body {
      background-color: #000;
      color: #fff;
    }
  `,
};

const ThemeSwitcher = () => {
  const [theme, setTheme] = useState('light');

  useInsertionEffect(() => {
    injectStyles(themes[theme]);
  }, [theme]);

  const toggleTheme = () => {
    setTheme(theme === 'light' ? 'dark' : 'light');
  };

  return (
    <div>
      <button onClick={toggleTheme}>Toggle Theme</button>
      <p>Current Theme: {theme}</p>
    </div>
  );
};

export default ThemeSwitcher;

            

              
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In this example, useInsertionEffect ensures that the theme styles are injected before the browser paints, resulting in a smooth theme transition without any noticeable layout shifts.

When Not to Use useInsertionEffect

While useInsertionEffect can be a valuable tool for optimizing CSS-in-JS libraries, it's important to recognize when it's not necessary or appropriate:

• Simple Applications: In simple applications with minimal styling or infrequent style updates, the performance benefits of useInsertionEffect may be negligible.
• When Library Already Handles Optimization: Many modern CSS-in-JS libraries already use useInsertionEffect internally or have other optimization techniques in place. In these cases, you may not need to use it directly in your components.
• Non-Style-Related Side Effects: useInsertionEffect is specifically designed for injecting styles. Avoid using it for other types of side effects, as it may lead to unexpected behavior.
• Server-Side Rendering: This effect won't execute during server-side rendering, since there is no painting.

Alternatives to useInsertionEffect

While useInsertionEffect is a powerful tool, there are other approaches you can consider for optimizing CSS-in-JS libraries:

• CSS Modules: CSS Modules offer a way to scope CSS rules locally to components, avoiding global namespace collisions. While they don't provide the same level of dynamic styling as CSS-in-JS libraries, they can be a good alternative for simpler styling needs.
• Atomic CSS: Atomic CSS (also known as utility-first CSS) involves creating small, single-purpose CSS classes that can be composed together to style elements. This approach can lead to more efficient CSS and reduced code duplication.
• Optimized CSS-in-JS Libraries: Some CSS-in-JS libraries are designed with performance in mind and offer built-in optimization techniques such as CSS extraction and code splitting. Research and choose a library that aligns with your performance requirements.

Conclusion

useInsertionEffect is a valuable tool for optimizing CSS-in-JS libraries and minimizing layout thrashing in React applications. By understanding how it works and when to use it, you can improve the performance and user experience of your web applications. Remember to use it specifically for style injection, minimize side effects, and test your components thoroughly. With careful planning and implementation, useInsertionEffect can help you build high-performance React applications that deliver a smooth and responsive user experience.

By carefully considering the techniques discussed in this article, you can effectively address the challenges associated with CSS-in-JS libraries and ensure that your React applications deliver a smooth, responsive, and performant experience for users around the world.