$ show facebook/stylex
StyleX is Meta's CSS-in-JS framework that compiles style definitions at build time into atomic CSS classes. The core is a Babel plugin that transforms `stylex.create()`, `stylex.defineVars()`, `stylex.keyframes()`, and other API calls into optimized JavaScript objects with deterministic class names and CSS rules. A small runtime injects the generated CSS into the DOM with priority-based ordering. The monorepo includes a comprehensive set of tools: an ESLint plugin for enforcing best practices, a CLI for pre-compilation, bundler plugins (Vite, Webpack, Rollup, esbuild, Rspack, PostCSS), a Chrome DevTools extension for debugging, and a documentation site built with StyleX itself. The architecture is modular, with separate packages for the runtime, compiler, and integrations, all sharing common utilities. The framework supports theming via CSS custom properties (`defineVars`/`createTheme`), compile-time constants (`defineConsts`), contextual selectors (`stylex.when`), and experimental nested token APIs. It is production-ready, used at Meta, and actively maintained with frequent releases.
The LLM identified 2 files worth highlighting for reuse:
A monadic parser combinator library for CSS values. Provides `TokenParser` class with `map`, `flatMap`, `or`, `sequence`, `oneOf`, `zeroOrMore`, `setOf` methods. Used across the style-value-parser package to parse CSS types like lengths, colors, easing functions, and media queries. The pattern is self-contained, well-tested, and could be extracted for any token-based parsing task beyond CSS.
A generic static expression evaluator for Babel AST paths. Extends Babel's own path evaluation to handle imports, object spreads, template literals, function calls, and custom identifier/member expression mappings. Used by the StyleX Babel plugin to resolve theme variable references and constant values at compile time. The `createVarGroupProxy` pattern for lazy resolution of CSS custom properties is particularly clever. This module is isolated and could be reused in any Babel plugin requiring compile-time constant folding.