Joi is a schema description language and data validation library for JavaScript.
The library addresses the need to validate data structures with a declarative, chainable API that lets developers define schemas and apply them to objects, arrays, and other values. Rather than writing imperative validation logic scattered throughout an application, Joi centralizes validation rules in reusable schema definitions. The approach combines a fluent interface with built-in support for complex validation scenarios including nested objects, conditional validation, and custom error messages.
Joi suits projects where data integrity matters—APIs receiving untrusted input, configuration file parsers, form validation in web applications, and any system processing structured data. The library works well for teams that prefer declarative validation over hand-written checks, and for codebases where validation logic needs to be maintainable and testable. It integrates naturally with the Hapi framework but functions as a standalone tool in any JavaScript environment.
The project maintains active development with regular updates to its documentation portal, versioning guidance, and changelog. The maintainers publish clear project policies and provide a dedicated developer portal with tutorials and API reference material, indicating sustained investment in developer experience and long-term support.
