CSS-in-JS Performance Optimization

CSS-in-JS is a popular pattern where CSS is written using JavaScript, often within React components. Libraries like Styled Components, Emotion, and JSS help in achieving this by allowing dynamic styling and scoped styles at the component level. While this approach provides a great deal of flexibility, it can lead to performance issues if not used properly.

This guide will focus on optimizing the performance of CSS-in-JS in React applications by focusing on various techniques and best practices to ensure a smooth and fast experience.

Key Performance Concerns with CSS-in-JS

Style recalculation: Since styles are dynamically generated, repeated recalculations during re-renders can hurt performance.
Server-side rendering (SSR): With CSS-in-JS, rendering CSS on the server (SSR) needs to be handled properly to avoid flashing of unstyled content (FOUC).
Memory usage: Dynamically injecting styles can increase memory usage, especially in large apps with many components.
Critical rendering path: The CSS-in-JS solution might block rendering due to JavaScript execution, delaying the first paint.

1. Memoizing Styles

One of the most effective performance optimizations for CSS-in-JS is memoizing styles so that they are not recomputed on every render.

Example Using `styled-components`:

import styled from 'styled-components';

const Button = styled.button`
  background: ${({ primary }) => (primary ? 'blue' : 'gray')};
  color: white;
  padding: 10px 20px;
`;

const App = () => {
  return <Button primary>Click Me</Button>;
};

In this example, styled-components automatically optimizes by memoizing the styles so that if the props don’t change, the styles won’t be recalculated.

2. Using `React.memo` for Component Memoization

Wrapping components with React.memo helps avoid unnecessary re-renders of components that don’t have any changes in props. When you use CSS-in-JS inside components, this prevents recalculating styles for unchanged components.

import React from 'react';
import styled from 'styled-components';

const Button = styled.button`
  background: ${({ primary }) => (primary ? 'blue' : 'gray')};
  color: white;
  padding: 10px 20px;
`;

const MemoizedButton = React.memo(Button);

const App = () => {
  return <MemoizedButton primary>Click Me</MemoizedButton>;
};

Here, MemoizedButton only re-renders when the primary prop changes.

3. Minimizing Dynamic Styles

Avoid using dynamic styles that depend on every render or require calculations that are not necessary. For instance, don’t put heavy computations or functions directly inside the styled component.

Before (Inefficient):

const Button = styled.button`
  background: ${({ theme }) => theme.colors[theme.mode]}; 
  color: ${({ theme }) => theme.textColor};
`;

If the theme prop is recalculated on every render, the styles will be recalculated as well. This can result in performance issues, especially in large apps with lots of components.

After (Optimized):

Use a memoized value for the theme.

import React, { useMemo } from 'react';

const Button = styled.button`
  background: ${({ theme }) => theme.colors[theme.mode]}; 
  color: ${({ theme }) => theme.textColor};
`;

const App = ({ theme }) => {
  const themeMemo = useMemo(() => theme, [theme]);

  return <Button theme={themeMemo}>Click Me</Button>;
};

Here, we use useMemo to avoid recalculating the theme object unless necessary.

4. Critical CSS Optimization for SSR

For server-side rendering, CSS-in-JS solutions must be able to extract the critical CSS before sending the response to avoid FOUC (flash of unstyled content). Some libraries have built-in SSR support that extracts styles at the server level.

Styled Components SSR: When using styled-components with SSR, you should extract the styles during the server-side rendering process to prevent FOUC.

import { ServerStyleSheet } from 'styled-components';

const sheet = new ServerStyleSheet();
const html = ReactDOMServer.renderToString(sheet.collectStyles(<App />));
const styleTags = sheet.getStyleTags(); // styleTags will be inserted into the HTML head

Make sure to send the extracted CSS to the client to ensure the page is styled correctly before the JavaScript takes over.

5. Code-Splitting and Lazy-Loading CSS

One of the main benefits of CSS-in-JS is that it allows you to load only the CSS you need for each component. Combine React.lazy and Suspense for code-splitting and dynamically loading components only when needed.

Example with `React.lazy` and `Suspense`:

const Button = React.lazy(() => import('./Button'));

const App = () => {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <Button />
    </Suspense>
  );
};

When the Button component is loaded lazily, the associated CSS will be dynamically injected only when the component is rendered. This ensures that the CSS is not loaded prematurely, improving the initial load time.

6. Avoiding CSS Recalculation on Prop Changes

In many CSS-in-JS libraries, props changes (especially dynamic props) can lead to recalculation of styles. However, if the prop change doesn’t affect the styles, it would be a performance hit. A good practice is to only pass necessary props to styled components, avoiding unnecessary re-renders.

// Inefficient
const Component = styled.div`
  background: ${({ isActive, size }) => isActive ? 'green' : 'red'};
`;

const App = () => {
  return <Component isActive size="small" />;
};

This example recalculates the styles based on the isActive prop, but if size is not used in the styles, it should not be passed into the component, as that causes unnecessary recalculations.

7. Using the `css` Prop for Styling

With libraries like Emotion, you can use the css prop for inline styling. This method allows better optimization in some cases, as the styles are more directly tied to components rather than being wrapped in a function.

/** @jsxImportSource @emotion/react */
import { css } from '@emotion/react';

const buttonStyle = css`
  background: blue;
  color: white;
  padding: 10px;
`;

const App = () => {
  return <button css={buttonStyle}>Click Me</button>;
};

This eliminates the need to create new styled components on each render and gives you more control over when styles are recalculated.

8. Batched Styles Injection

CSS-in-JS libraries like Styled Components and Emotion batch styles injections to avoid adding excessive <style> tags into the DOM. Make sure that you use the batch injection feature to minimize the number of reflows and repaints in the browser.

You can control this with options in the library (e.g., Styled Components’ StyleSheetManager).

import { StyleSheetManager } from 'styled-components';

const App = () => {
  return (
    <StyleSheetManager disableVendorPrefixes>
      <div>Your components</div>
    </StyleSheetManager>
  );
};

This prevents excessive recalculations and ensures that styles are injected in an optimized manner.

9. Minimize Global Styles

Global styles can interfere with performance if overused or inefficiently applied. While global styles can be useful in some cases, they can lead to unnecessary recalculations in large applications, especially when using CSS-in-JS.

Instead, prefer using component-level styling as much as possible, ensuring that each component only manages its own styles and doesn’t rely on global styles that could change frequently.

10. Use CSS-in-JS Only for Dynamic Styles

Finally, try to limit the usage of CSS-in-JS to situations where dynamic styles are necessary. If your styles are static (e.g., for layout, typography, or branding), consider using traditional CSS or CSS modules instead of CSS-in-JS to optimize performance and reduce JavaScript bundle size.