How to build advanced forms in Next.js using a rule engine

This article explores the use of a rule engine to handle complex conditional logic in a multi-step business loan application form built with Next.js, as opposed to relying solely on React Hook Form or Zod for simple field-level validation. It introduces a five-step form where business rules, such as field visibility and step progression based on previous inputs, are managed through a declarative rule engine. This engine evaluates rules both on the client side for responsiveness and on the server side during form submission for security and consistency. The architecture divides the form into distinct layers for rules, form state, and UI components, ensuring that components are decoupled from business logic and instead consume evaluation results. The article emphasizes the benefits of using a rule engine for forms governed by complex, versioned business rules, highlighting its suitability for scenarios involving eligibility, compliance, or pricing logic, while acknowledging the added complexity and necessity for maintaining synchronization between client and server evaluations.