We are thrilled to introduce StyleX. StyleX is an expressive, deterministic, reliable, and scalable styling system for ambitious applications. We've taken the best ideas from the styling libraries that have come before to create something that is simultaneously familiar and uniquely new.

What is StyleX?​

StyleX takes the developer experience of CSS-in-JS libraries and uses compile-time tooling to bridge it with the performance and scalability of static CSS. However, StyleX is not just another compiler-based CSS-in-JS library. StyleX has been carefully designed to meet the requirements of large application, reusable component libraries, and statically typed codebases.

1. StyleX supports an expressive subset of CSS. It avoids complex selectors and guarantees no specificity conflicts in the generated CSS.
2. StyleX transforms, organizes, and optimizes styles into "atomic" CSS class names. There's no need to learn or manage a separate library of utility class names.
3. StyleX allows styles to be merged across file and component boundaries, making it ideal for component libraries that allow user customization.
4. StyleX is fully typed and provides type utilities to allow fine-grained control over what properties and values can be accepted by a component.

What are the advantages of StyleX?​

Fast​

StyleX is designed to be fast at both compile-time and runtime. The babel transforms do not significantly slow down builds.

At runtime, StyleX entirely avoids the costs associated with using JavaScript to insert styles at runtime, and does little more than efficiently combine class name strings when necessary. And the generated CSS is optimized for size, ensuring that the styles for even the largest websites can be quickly parsed by browsers.

Scalable​

StyleX is designed to scale to extremely large codebases, like the ones we have at Meta. The babel plugin can handle processing styles in many thousands of components at compile-time by leveraging atomic builds and file-level caching. And because StyleX is designed to encapsulate styles, it allows new components to be developed in isolation with the expectation that they will render predictably once used within other components.

By generating atomic CSS class names, StyleX helps minimize the size of the CSS bundle. As the number of components in an application grows, the size of the CSS bundle starts to plateau. This frees developers from having to manually optimize or lazy-load CSS files.

Predictable​

StyleX automatically manages the specificity of CSS selectors to guarantee that there are no collisions between the generated rules. StyleX gives developers a system that reliably applies styles, and ensures that "the last style applied always wins"

Composable​

StyleX styles are easy to compose. Not only can multiple local styles be applied conditionally, styles can also be passed across files and components. Styles always merge with predictable results.

Type-Safe​

You can constrain the styles a component accepts by using Typescript or Flow types. Every style property and variable is fully typed.

Colocation​

StyleX allows and encourages authoring styles in the same file as the component that uses them. This co-location helps make styles more readable and maintainable in the long run. StyleX is able to use static analysis and build-time tools to de-duplicate styles across components and to remove unused styles.

Testable​

StyleX can be configured to output debug class names instead of functional atomic class names. This can be used to generate snapshots that don't change as often in response to minor design changes.

How does StyleX work?​

StyleX is a collection of tools that work together.

• A Babel plugin
• A small runtime library
• An ESlint plugin
• A growing collection of integrations with bundlers and frameworks.

The most important part of StyleX is the Babel plugin. It finds and extracts all the styles defined within your source code and converts them to atomic class names at compile time. A helper function deduplicates, sorts, and writes the collected styles to a CSS file. These tools are used to implement bundler plugins.

To make using StyleX feel as natural as possible, StyleX supports various static patterns to define your styles by using local constants and expressions. Additionally, in order to give you the best performance possible, the Babel plugin also pre-computes the final class names when possible to remove any runtime cost — even merging class names — from a given file. If a component is defining and using styles within the same file statically, the runtime cost will be ZERO.

When using more powerful patterns such as style composition, a tiny runtime merges objects of class names dynamically. This runtime has been optimized to be extremely fast and the results are then memoized.

The origins of StyleX​

The previous Facebook website used something akin to CSS modules and suffered from various problems that inspired the initial idea for CSS-in-JS. The average visitor to facebook.com would download tens of megabytes of CSS. Much of it unused. In order to optimize the initial load, we would lazy load our CSS which would, in turn, lead to slow update (or "Interaction to Next Paint") times. Usage of complex selectors would lead to conflicts or "specificity wars". Engineers would often resort to using !important or more complex selectors to solve their problems, making the entire system progressively worse.

A few years ago, when we were rebuilding facebook.com from the ground up using React, we knew we needed something better and built StyleX.

StyleX was designed to scale, and the design has proven itself in our years of experience using it. We've added new features to StyleX without regressing on performance or scalability while making StyleX more of a joy to use.

Using StyleX has been a massive improvement in both scalability and expressivity for us at Meta. On facebook.com we were able to bring down our CSS bundle from tens of megabytes of lazy-loaded CSS to a single bundle of a couple hundred kilobytes.

We created StyleX not only to meet the styling needs of React developers on the web, but to unify styling for React across web and native.

How does Meta use StyleX?​

StyleX has became the preferred way to style components for every web surface within Meta. StyleX is used to style React components for every major external and internal product at Meta including Facebook, WhatsApp, Instagram, Workplace, and Threads. It has changed the way we author components, and resolved the issues our teams previously had with not being able to encapsulate and scale their styled components.

We expanded the original capabilities of StyleX so that engineers at Meta can use StyleX to author both static and dynamic styles. Our teams are using StyleX theming APIs to develop "universal" components that are themed to take on the appearance of different design systems used within different Meta products. And we're gradually expanding support for cross-platform styling, thanks to StyleX being aligned with the principles of encapsulation introduced by React Native's styling system.

Open Source​

What we're open sourcing is what we use internally. We develop on Github first and sync it back to Meta. Although StyleX was originally created at Meta for Meta, it is not specific to Meta.

That said, this is still just the beginning. We look forward to working with the community to introduce further optimizations and more integrations.

We hope you love using StyleX as much as we do. ❤️