CSS-in-JS has become a popular solution for styling modern web applications, especially within JavaScript frameworks like React and Vue. By writing your styles directly in your JavaScript code, CSS-in-JS offers dynamic theming, component-based styling, and even improved maintainability in certain cases. However, like any approach, it comes with its own set of challenges and potential pitfalls.
In this article, we’ll explore the pitfalls of CSS-in-JS that can impact performance, scalability, and maintainability. Understanding these challenges will help you make informed decisions about when—and when not—to use CSS-in-JS in your projects.
What Is CSS-in-JS?
CSS-in-JS refers to a styling approach where CSS is written directly within JavaScript files. This method allows developers to define styles inside components or even dynamically generate styles based on component state or props.
Here’s a simple example of CSS-in-JS using the popular Styled Components library in React:
import styled from 'styled-components';
const Button = styled.button`
background-color: ${(props) => (props.primary ? 'blue' : 'gray')};
color: white;
padding: 10px 20px;
border: none;
cursor: pointer;
`;
export default Button;
In this example, the Button
component can dynamically change its background color based on the primary
prop. This flexibility and tight integration with JavaScript logic are some of the main reasons CSS-in-JS is attractive to developers.
However, as appealing as it is, CSS-in-JS introduces certain trade-offs that you should be aware of before fully committing to it.
Pitfall #1: Performance Overhead
One of the primary concerns with CSS-in-JS is the performance overhead it introduces. Since styles are generated dynamically within JavaScript, they need to be processed and applied at runtime. This can cause performance bottlenecks, particularly in large applications or when rendering many components.
The Problem:
In traditional CSS, styles are parsed and applied by the browser before the page is rendered, ensuring fast performance. CSS-in-JS, on the other hand, requires additional computation because the styles are generated as part of the JavaScript execution. This process can lead to slower rendering times, especially when you’re using a large number of components that rely on dynamic styles.
For example, each time a component is rendered or updated, CSS-in-JS libraries like Styled Components or Emotion need to generate and inject the necessary styles into the document. While this may not be an issue in small applications, the performance hit can be noticeable in larger projects with thousands of styled components.
The Fix: Optimize Where Possible
To mitigate performance issues, consider using static styles where possible. For components that don’t need dynamic styling, defining styles outside the render cycle can help improve performance.
Here’s an example of using static styles instead of dynamically generating them:
const Button = styled.button`
background-color: blue;
color: white;
padding: 10px 20px;
border: none;
cursor: pointer;
`;
By avoiding dynamic props where they’re not needed, you reduce the overhead of generating styles at runtime. Additionally, some CSS-in-JS libraries offer optimizations like server-side rendering (SSR) to reduce the performance impact of generating styles on the client side.
Advanced Fix: Use Critical CSS
If performance is a major concern, another solution is to use critical CSS techniques. Critical CSS involves extracting only the styles required for above-the-fold content during the initial load, reducing the amount of CSS the browser needs to process. Libraries like emotion and Styled Components provide tools for server-side rendering and critical CSS extraction to mitigate performance bottlenecks.

Pitfall #2: Lack of Global Styles Management
Another challenge with CSS-in-JS is the difficulty in managing global styles. CSS-in-JS is component-based, meaning each component is styled in isolation. While this works well for smaller, self-contained components, it can be problematic when you need to apply global styles across your entire application, such as typography, layout grids, or form elements.
The Problem:
Traditional CSS has the benefit of cascading and global selectors, allowing you to define styles that affect multiple elements across the site with ease. CSS-in-JS lacks this global styling capability, and while it’s possible to define global styles in CSS-in-JS, doing so often requires workarounds that go against the component-based paradigm.
For example, using Styled Components for global styles looks like this:
import { createGlobalStyle } from 'styled-components';
const GlobalStyle = createGlobalStyle`
body {
margin: 0;
font-family: Arial, sans-serif;
}
h1 {
font-size: 2em;
color: blue;
}
`;
export default GlobalStyle;
While this works, it feels cumbersome and doesn’t take advantage of CSS’s natural cascading behavior.
The Fix: Use a Hybrid Approach
For managing global styles, consider combining CSS-in-JS with traditional CSS files or CSS modules. While CSS-in-JS excels at local, component-specific styling, using traditional CSS for global styles can simplify the management of typography, layout, and other site-wide styles.
For example, you can include a global CSS file for site-wide styles, while still using CSS-in-JS for component-level styling:
/* global.css */
body {
margin: 0;
font-family: Arial, sans-serif;
}
h1 {
font-size: 2em;
color: blue;
}
Then, import this global CSS file in your entry point:
import './global.css';
This hybrid approach allows you to take advantage of both methods, ensuring that you manage global styles efficiently while still keeping your component styles scoped and maintainable.
Pitfall #3: Bloated Bundles and Code Duplication
A common issue with CSS-in-JS is bundle size bloat. Since CSS is generated dynamically, the final bundle can include duplicated styles, especially if the same styles are used across multiple components. This leads to code duplication and increases the size of your JavaScript bundles, which can affect loading times and overall performance.
The Problem:
Each time a styled component is rendered, CSS-in-JS libraries create and inject styles into the document. If you’re using the same styles in multiple places, these styles may get duplicated, resulting in a bloated CSS payload. In large applications, this can lead to significant performance degradation, as the browser has to parse and apply increasingly large amounts of dynamically injected CSS.
The Fix: Use Style Deduplication Tools
To address this issue, many CSS-in-JS libraries have built-in style deduplication mechanisms that prevent duplicate styles from being injected. Make sure you’re using these features, especially in production environments, where minimizing bundle size is critical.
For example, libraries like Styled Components and Emotion handle deduplication automatically by creating unique class names based on the styles. However, it’s important to ensure that your build process is optimized for production, where these deduplication techniques are most effective.
Another way to avoid style duplication is by reusing shared styles or creating utility components for commonly used styles. Instead of duplicating styles across components, define them in a central location and import them as needed.
Example of shared utility styles:
const sharedStyles = css`
padding: 10px;
color: white;
`;
const Button = styled.button`
${sharedStyles};
background-color: blue;
`;
const LinkButton = styled.a`
${sharedStyles};
background-color: red;
`;
In this case, the sharedStyles
are reused across different components, reducing code duplication and keeping the CSS payload lean.

Pitfall #4: Limited Debugging and Tooling Support
One often overlooked downside of CSS-in-JS is the limited support for debugging and developer tools compared to traditional CSS. While CSS-in-JS libraries do provide some tools to inspect and debug styles, they don’t always offer the same level of visibility and ease of use as traditional CSS workflows.
The Problem:
With traditional CSS, developers can easily inspect styles in the browser’s DevTools, view the cascade, and identify which styles are being applied to specific elements. However, with CSS-in-JS, styles are often scoped to components and generated dynamically, making it harder to trace which styles are being applied at runtime.
Additionally, since styles are embedded in JavaScript, the browser’s Source Maps for CSS can become more difficult to navigate, complicating the debugging process.
The Fix: Use DevTools Extensions for CSS-in-JS Libraries
Most popular CSS-in-JS libraries, like Styled Components and Emotion, offer DevTools extensions that enhance the debugging experience. These extensions make it easier to inspect dynamically generated styles and trace them back to their component source.
For example, the Styled Components DevTools extension allows you to see the component name, associated styles, and where those styles are coming from in your JavaScript code. This helps bridge the gap between dynamic CSS generation and traditional browser debugging.
Example of installing the Styled Components DevTools extension:
npm install --save-dev styled-components-devtools
Using these tools can make debugging CSS-in-JS styles much easier, especially in large applications where managing multiple components and styles can become overwhelming.
Pitfall #5: Difficulty in Learning Curve and Developer Adoption
CSS-in-JS introduces a steeper learning curve for developers, especially those who are accustomed to traditional CSS. This can slow down development and make it harder to onboard new team members who aren’t familiar with JavaScript-based styling approaches.
The Problem:
CSS-in-JS blurs the lines between presentation (CSS) and logic (JavaScript), which can be confusing for developers who are used to separating concerns. Learning to work with JavaScript objects for styling, dealing with dynamic props, and understanding the intricacies of runtime style generation can be overwhelming for new developers or those who have only worked with traditional CSS.
Additionally, CSS-in-JS often introduces syntax quirks and requires developers to write styles in a more programmatic way, which can slow down productivity during the initial learning phase.
The Fix: Provide Training and Documentation for Developers
To overcome the learning curve, teams adopting CSS-in-JS should invest in training and documentation for their developers. Make sure your team is familiar with the benefits and limitations of CSS-in-JS, and provide clear documentation on how to use the library within your project’s architecture.
Example of team documentation:
- How to create styled components
- When to use dynamic styling
- Best practices for sharing styles across components
- Debugging CSS-in-JS issues
By providing adequate support and guidelines, you can smooth the learning curve and ensure that developers feel confident using CSS-in-JS without slowing down the development process.
The Future of CSS-in-JS: Where Are We Heading?
As the web development landscape evolves, CSS-in-JS is becoming more refined. Newer tools and libraries are emerging that address some of the pitfalls we’ve discussed, such as better performance optimizations, enhanced tooling, and improved debugging support.
1. Zero-runtime CSS-in-JS
One of the exciting developments in this space is the rise of zero-runtime CSS-in-JS solutions. Libraries like Linaria and Vanilla Extract aim to provide the benefits of CSS-in-JS while eliminating runtime overhead by compiling styles at build time.
Linaria, for example, lets you write styles in JavaScript, but instead of generating styles dynamically, it compiles them into static CSS files during the build process. This approach avoids the performance costs associated with runtime CSS generation while still giving developers the convenience of writing styles in JavaScript.
Example with Linaria:
import { css } from 'linaria';
const buttonClass = css`
background-color: blue;
color: white;
padding: 10px 20px;
`;
function Button() {
return <button className={buttonClass}>Click me</button>;
}
In this case, the styles are compiled into a separate CSS file during build time, avoiding the performance issues related to dynamic runtime CSS generation.
2. CSS-in-JS with Static Extraction
Libraries like Styled Components and Emotion are introducing static extraction capabilities to allow for server-side rendering and critical CSS extraction. These features help mitigate the performance overhead by allowing you to extract the critical styles needed for the initial page load, reducing the load time and improving performance metrics like First Contentful Paint (FCP).
Static extraction helps to generate a minimal set of critical CSS for above-the-fold content, ensuring that only the necessary styles are loaded during the initial render, while the rest of the styles are loaded asynchronously.
Example with static extraction (Emotion):
npm install @emotion/babel-plugin
By using Babel plugins, you can enable static extraction for your CSS-in-JS setup, optimizing the performance without giving up the benefits of writing styles in JavaScript.
Conclusion: Balancing the Pros and Cons of CSS-in-JS
CSS-in-JS offers a modern, flexible approach to styling web applications, especially in the context of component-based frameworks like React. However, it’s important to understand the potential pitfalls that come with it—performance overhead, difficulty managing global styles, bundle size bloat, limited debugging support, and a steep learning curve.
Here’s a recap of how to navigate these challenges:
- Optimize performance by using static styles where possible, leveraging server-side rendering, and using critical CSS techniques.
- Manage global styles effectively with a hybrid approach that combines CSS-in-JS with traditional CSS or CSS modules.
- Reduce bundle size by avoiding duplicate styles and reusing shared utility components.
- Improve debugging with DevTools extensions specifically built for CSS-in-JS libraries like Styled Components and Emotion.
- Support developer onboarding with clear documentation and training.
By understanding and managing these trade-offs, you can make informed decisions about when to use CSS-in-JS in your projects, ensuring that your codebase remains efficient, maintainable, and scalable. At PixelFree Studio, we believe in leveraging the right tools for the job, and CSS-in-JS is no exception—it’s a powerful approach, but only when used wisely and in the right context.
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