joi

Joi is a JavaScript validation library used to describe and validate data through schemas. It provides a descriptive, chainable API for defining the shape and constraints of objects, arrays, and primitive values. Joi schemas are commonly used to enforce the structure of API inputs, configuration objects, and other data exchanged between a client and server.

Origin and scope: Joi was developed as part of the Hapijs project and is distributed as a standalone npm package. It is framework-agnostic and can be used with Express, Fastify, Koa, or in any Node.js code that requires runtime validation and type coercion.

Core features: Joi supports schemas for strings, numbers, booleans, dates, arrays, and objects, with a wide range of validation rules (min, max, length, pattern, email, etc.). It offers required and optional fields, default values, and custom validation via .custom or .external. Conditional validation can be expressed with when/alternatives. Validation results include a value after coercion and a detailed error object. The API is chainable, readable, and designed for use in request validation pipelines.

Usage example: const Joi = require('joi'); const schema = Joi.object({ name: Joi.string().min(3).max(30).required(), age: Joi.number().integer().min(0).optional() }); const { value, error } = schema.validate({ name: 'Ada', age: 29 }); if (error) { /* handle errors */ }

Notes: Joi is one of several options for runtime validation in JavaScript; it emphasizes developer-friendly error reporting and flexible schema definitions, at the cost of some additional boilerplate compared to lighter-weight validators.