joi

name	joi
description	Use when building joi schemas, validating input data, defining custom types, conditional validation with .when(), cross-field references, custom error messages, or writing joi extensions. Standalone package that integrates with the hapijs ecosystem.

Joi

Quick Start

const Joi = require('joi');

const schema = Joi.object({
    name: Joi.string().min(1).max(100).required(),
    age: Joi.number().integer().min(0),
    email: Joi.string().email()
});

const { error, value } = schema.validate(input);

Critical Rules

1. Schemas are immutable - Every method returns a new schema instance; never mutate
2. Validate at boundaries - Use validate() or attempt() at input boundaries; see validation
3. Types extend base - All types inherit from any(); see types overview
4. Refs for cross-field - Use Joi.ref() for dynamic values across fields; see references
5. Extend for custom types - Use Joi.extend() to create custom types; see extensions

Workflow

1. Choose a type - types overview for all built-in types
2. Add constraints - Chain rules like .min(), .max(), .pattern(), .valid()
3. Compose schemas - Nest Joi.object(), Joi.array(), Joi.alternatives()
4. Add conditionals - Use .when() for dynamic schemas; see conditionals
5. Customize errors - Override messages via .messages() or .error(); see errors

Common Patterns

Conditional validation with .when()

const schema = Joi.object({
    type: Joi.string().valid('email', 'sms').required(),
    address: Joi.when('type', {
        is: 'email',
        then: Joi.string().email().required(),
        otherwise: Joi.string().pattern(/^\+?[1-9]\d{1,14}$/).required()
    })
});

Cross-field references with Joi.ref()

const schema = Joi.object({
    password: Joi.string().min(8).required(),
    confirmPassword: Joi.string().valid(Joi.ref('password')).required()
        .messages({ 'any.only': 'passwords must match' }),
    startDate: Joi.date().required(),
    endDate: Joi.date().greater(Joi.ref('startDate')).required()
});

Key Patterns

Topic	Reference
All built-in types	types
Validation & options	validation
References & templates	references
Conditional schemas	conditionals
Error handling	errors
Custom extensions	extensions
Metadata & introspection	metadata
Common methods (any)	any
Testing patterns	testing