JSON to Joi Schema — Node.js Validation Made Easy
Joi is one of the most battle-tested validation libraries in the Node.js ecosystem. Originally built for Hapi framework, it's now used across Express, Fastify, and standalone validation scenarios. But writing Joi schemas from scratch is verbose and error-prone. This tool generates a complete schema from your actual JSON data.
Paste a sample API request body or configuration object, and you get a Joi schema that validates data matching that structure. Strings become Joi.string(), numbers becomeJoi.number(), nested objects get their own named schema variables, and arrays are validated with Joi.array().items().
Express Middleware Validation
The most common use case is validating request bodies in Express routes. Create a middleware that runs schema.validate(req.body) before your handler. If validation fails, return a 400 with the error details. The generated schema gives you the structure — you then add .required(), .min(), .max(), and other constraints based on your business rules.
Joi vs Zod vs Yup
Joi is JavaScript-first and works great in vanilla Node.js projects. If you're using TypeScript, Zod might be a better fit since it infers TypeScript types. Yup is popular in React ecosystems with Formik. We offer converters for all three — use whichever matches your stack.
Custom Validation Rules
The generated schema handles type validation, but real-world validation needs more. Add.email() for email fields, .regex() for patterns, .valid()for enum values, and .custom() for complex business logic. The generated base gives you the structure so you're not starting from a blank file.
API Contract Testing
Joi schemas aren't just for input validation. You can also use them to validate API responses in your integration tests. Generate a schema from a known-good response, then assert that future responses match the same structure. It catches breaking API changes early.
Related JSON Tools
For TypeScript projects, consider Zod schemas which provide type inference alongside validation. React/Formik users may prefer Yup schemas. Generate the TypeScript interfaces too.