# D12. TypeScript workspace packages, run directly by Node (revised 6 Oct 2026)

*Replaces "Plain JavaScript (ES modules), no build step for the sources".* Plain JavaScript was right for a prototype that agents read and run, but not for a core that a desktop app, a server and browser plugins will consume: the format, the operations and the course data shipped to the player are contracts that types should hold. The core is TypeScript in `strict` mode, split into workspace packages (`format`, `player`, `core`, `node`, `engine`, `cli`; docs/architecture.md), and Node 24 runs the `.ts` sources directly (type stripping), so agents and contributors still read and run the sources without a build step. Only erasable syntax is used. Types of the format are generated from the JSON Schemas (D1 stays: the schemas are the source). The migration was checked by an oracle: the three examples build to the same bytes before and after.
*Revisit if:* Node's type stripping stops being enough (then a build step with `tsc` emitting JavaScript; publishing to npm already needs one, because Node does not strip types under `node_modules`).