@boostpack/joi-extract-type

TypeScript type extraction for Joi schemas via module augmentation.

Problem

Joi validates data at runtime, but doesn't provide compile-time types for validated values. You end up duplicating your schema as a TypeScript interface:

const schema = Joi.object({ name: Joi.string().required(), age: Joi.number() });

// manual duplication
interface Config {
  name: string;
  age?: number;
}

This package augments Joi's type definitions so TypeScript can infer the type directly from the schema — no duplication needed.

Installation

npm install @boostpack/joi-extract-type

joi >= 17 is required as a peer dependency.

Usage

Import the package once (side-effect import) to activate the type augmentation:

import '@boostpack/joi-extract-type';
import Joi from 'joi';

const schema = Joi.object({
  host: Joi.string().required(),
  port: Joi.number().default(3000),
  debug: Joi.boolean().optional(),
});

type Config = Joi.extractType<typeof schema>;
// { host: string; port: number; debug?: boolean | undefined }

Required vs optional

Fields are optional by default. Use .required() or .default() to make them required:

const schema = Joi.object({
  name: Joi.string().required(),  // required — always string
  age: Joi.number(),              // optional — number | undefined
  role: Joi.string().default('user'),  // has default — always string
});

Arrays

const schema = Joi.object({
  tags: Joi.array().items(Joi.string()).required(),
});

type Result = Joi.extractType<typeof schema>;
// { tags: string[] }

Alternatives

const schema = Joi.alternatives(Joi.string(), Joi.number());

type Result = Joi.extractType<typeof schema>;
// string | number

Acknowledgements

Based on the original joi-extract-type by Tiago de Carvalho Miranda, with fixes from panzelva and dp-franklin.

License

MIT