io-ts

io-ts is a runtime type system for TypeScript that bridges the gap between compile-time types and runtime validation using functional programming concepts from the fp-ts library. io-ts codecs define both the runtime validator and the TypeScript type in a single declaration — t.type({ name: t.string, age: t.number }) creates a codec that validates unknown data at runtime and infers the TypeScript type { name: string; age: number } statically. The library uses the concept of codecs that can both decode (validate/parse incoming data) and encode (serialize outgoing data), making it suitable for API boundaries where data needs to be validated in both directions. io-ts supports primitive types, objects, arrays, intersections, unions, partial types, readonly types, branded types (nominal types with runtime validation), and recursive types. The decode method returns an Either type from fp-ts (Left for errors, Right for valid data), encouraging explicit error handling. io-ts is particularly popular in codebases that already use fp-ts for functional programming patterns. While Zod has become more popular for general use due to its simpler API, io-ts provides superior integration with the fp-ts ecosystem and functional programming patterns.