Zod

The TypeScript validation library that made runtime type checking feel inevitable. Zod lets you define schemas once and get both TypeScript types and runtime validation from the same source of truth. If you've ever had a type that says string but the API sends null, Zod catches that at the boundary.

Yup was the previous standard but lacks TypeScript-first design. Joi is battle-tested for Node but heavy and not TS-native. Valibot is the newer, smaller alternative (tree-shakeable). TypeBox generates JSON Schema from TS types. ArkType is the performance-focused newcomer.

Zod's ecosystem is enormous: tRPC, React Hook Form, Astro, and dozens of frameworks integrate directly. The .parse() and .safeParse() API is clean and predictable. Schema composition with .merge(), .extend(), and .transform() handles real-world complexity.

The catch: Zod is not small — bundle size is 13KB+ gzipped, which matters for edge functions and client-side code. Valibot does the same job at a fraction of the size. Performance is adequate but not stellar for high-throughput validation. And Zod schemas can get verbose for deeply nested objects — what starts as a clean one-liner becomes a wall of chained methods.