TypeScript architecture for systems that cannot drift

Large TypeScript systems fail when compile-time confidence is mistaken for runtime safety. Start by separating domain rules from transport, persistence, and framework code. A domain module should be testable without a server, database, or browser.

Validate every external boundary. API payloads, environment variables, queue messages, and database results are runtime data, regardless of how confidently they are typed. Convert unknown input into trusted domain objects once, close to the boundary.

Finally, model failure explicitly. Expected business outcomes should be values your application can handle; unexpected infrastructure failures should preserve context, correlation IDs, and safe retry information.