zod

Zod is a TypeScript-first schema validation library that provides a concise, chainable API for defining data shapes and validating them at runtime. Zod's key innovation is that schema definitions automatically infer TypeScript types — z.object({ name: z.string(), age: z.number() }) infers { name: string; age: number } — eliminating the duplication between type definitions and validation logic. Zod schemas are composable and support strings, numbers, booleans, dates, enums, arrays, objects, unions, intersections, tuples, records, maps, sets, promises, functions, and custom refinements. Each type includes built-in validators: z.string().email().min(1).max(100), z.number().int().positive().max(1000). The .transform() method converts validated data to a different shape, and .refine() adds custom validation logic with custom error messages. Zod supports schema composition through .merge(), .extend(), .pick(), .omit(), and .partial() methods that mirror TypeScript utility types. The library is used by tRPC for API input/output validation, React Hook Form for form validation, and many other tools that need runtime type checking. Zod's combination of type inference, composable schemas, and excellent error messages has made it the most popular validation library in the TypeScript ecosystem.