Why developers argue for keeping form validation logic out of UI libraries

A software developer argues that relying on libraries like Formik or React Hook Form for form validation embeds business logic into the UI layer, making it harder to reuse and test. The core problem emerges over time when the same validation rule, such as a VIN format check, ends up duplicated across multiple forms with subtle inconsistencies. The author proposes treating validation rules as domain logic — self-contained, testable units independent of any React component or third-party library. This approach allows rules to be shared across create, edit, and import workflows, tested without mounting a component, and potentially reused on the backend. The article outlines a lightweight contract-based system using plain classes and useState as an alternative to schema-resolver patterns.