DefinitelyTyped is the centralized repository for TypeScript type definitions, serving as critical infrastructure for the TypeScript ecosystem. This monorepo manages thousands of @types packages that provide static type checking and IntelliSense support for JavaScript libraries. The repository uses dtslint for validation, pnpm for package management, and follows a major.minor.9999 versioning scheme aligned with upstream library versions. Each type definition package contains index.d.ts declaration files, test files with $ExpectType assertions, and package.json metadata. The codebase is purely declarative - it contains no runtime logic, only type signatures, interfaces, and function signatures for external libraries. This makes it a foundational dependency for TypeScript projects rather than a consumable application.