FBJS is a monorepo containing Facebook's internal JavaScript utility library and associated tooling. It provides a Babel preset for transforming JavaScript with Flow types, ESLint configurations enforcing Facebook's code style, a core utility library with functional programming helpers, data structures, DOM utilities, and cross-browser compatibility helpers. The repository also includes build scripts for dependency checking, module mapping, and test environment setup. Designed primarily for Facebook's internal projects (like React and Relay), it explicitly warns that external consumers may experience instability due to non-semver adherence and API changes. The architecture consists of multiple loosely coupled packages under a monorepo structure. Key entry points include the Babel preset (`babel-preset-fbjs/index.js`), ESLint config (`eslint-config-fbjs/index.js`), core utility index (`fbjs/index.js` which directs users to sub-packages), and build scripts (`fbjs-scripts/index.js`). Data flows involve source code transformation via Babel, linting via ESLint, runtime utility consumption via the fbjs package, and build automation via gulp-based scripts. External dependencies include Babel, ESLint, Jest, Gulp, and Flow, with versions locked in package.json files. Notable reusable components include the Deferred.js promise-like utility, PromiseMap for request deduplication, Heap and CircularBuffer data structures, areEqual for deep equality with circular reference handling, normalizeWheel for cross-browser wheel event handling, and UserAgent/VersionRange for browser detection and version checking. These components are self-contained, demonstrate non-obvious engineering (e.g., circular reference safety, cross-browser normalization), and could be extracted with minimal modification. The codebase shows strong craftsmanship in utility design but is coupled to Facebook-specific tooling and internal conventions, limiting external reuse without adaptation.