What is Domain-Driven Design (DDD) and how is it applied in Node.js?

Domain-Driven Design (DDD) is a software design approach that focuses on modeling software to match the business domain, with collaboration between technical and domain experts. Key concepts applied to Node.js: (1) Domain models: rich objects with business logic (not anemic data containers) — a User class with methods like user.changeEmail() that enforce business rules; (2) Value Objects: immutable objects defined by their attributes, no identity — Money { amount, currency }, Email { value }; (3) Aggregates: cluster of entities treated as a single unit — Order (aggregate root) contains OrderLine entities; (4) Repositories: abstract data access behind an interface — UserRepository.findById() — separating domain from infrastructure; (5) Domain Events: record things that happened — UserRegistered, OrderPlaced; (6) Application Services: orchestrate domain objects for use cases; (7) Bounded Contexts: clear boundaries between subdomains (auth context, billing context). In Node.js, DDD works well with TypeScript (for type-safe domain models) and NestJS (for clear architectural boundaries). It is most valuable for complex business domains, not simple CRUD apps.