Zod

Zod is a TypeScript-first schema library: you describe shapes with a small API, get static types via inference, and parse unknown input (JSON, env vars, form data) at runtime. It bridges the gap between compile-time types and untrusted boundaries—APIs, configs, and user input—without a separate code generator. Often used with Next.js route handlers, NestJS DTOs, and anywhere TypeScript alone is not enough at runtime.