Build DDD aggregates and repositories in TypeScript with TDD. Enforce invariants and test application flows with an in-memory repository.

Learn how Value Objects and Entities make invalid states impossible in TypeScript. Build self-validating domain code with TDD — the blocks that can't break.

Learn why test-driven development is really about permission to refactor, not catching bugs. With TypeScript examples, Result<T, E> patterns, and behavior-based testing from 3 years in production.