Object schema description language and validator for JavaScript objects.

Imagine you run facebook and you want visitors to sign up on the website with real names and not something like l337_p@nda in the first name field. How would you define the limitations of what can be inputted and validate it against the set rules?

This is joi, joi allows you to create blueprints or schemas for JavaScript objects (an object that stores information) to ensure validation of key information.

See the detailed API Reference.

const Joi = require('@hapi/joi');

const schema = Joi.object().keys({
    username: Joi.string().alphanum().min(3).max(30).required(),
    password: Joi.string().regex(/^[a-zA-Z0-9]{3,30}$/),
    access_token: [Joi.string(), Joi.number()],
    birthyear: Joi.number().integer().min(1900).max(2013),
    email: Joi.string().email({ minDomainSegments: 2 })
}).with('username', 'birthyear').without('password', 'access_token');

// Return result.
const result = schema.validate({ username: 'abc', birthyear: 1994 });
// result.error === null -> valid

// You can also pass a callback which will be called synchronously with the validation result.
schema.validate({ username: 'abc', birthyear: 1994 }, function (err, value) { });  // err === null -> valid

The above schema defines the following constraints:

• username
  • a required string
  • must contain only alphanumeric characters
  • at least 3 characters long but no more than 30
  • must be accompanied by birthyear
• password
  • an optional string
  • must satisfy the custom regex
  • cannot appear together with access_token
• access_token
  • an optional, unconstrained string or number
• birthyear
  • an integer between 1900 and 2013
• email
  • a valid email address string
  • must have two domain parts e.g. example.com

Usage is a two steps process:

First, a schema is constructed using the provided types and constraints:

const schema = Joi.object({
    a: Joi.string()
});

Note that joi schema objects are immutable which means every additional rule added (e.g. .min(5)) will return a new schema object.

Second, the value is validated against the defined schema:

const { error, value } = schema.validate({ a: 'a string' });

// or

schema.validate({ a: 'a string' }, function (error, value) { });

If the input is valid, then the error will be null. If the input is invalid, error is assigned a ValidationError object providing more information.

The schema can be a plain JavaScript object where every key is assigned a joi type, or it can be a joi type directly:

const schema = Joi.string().min(10);

If the schema is a joi type, the schema.validate(value, callback) can be called directly on the type. When passing a non-type schema object, the module converts it internally to an object() type equivalent to:

const schema = Joi.object().keys({
    a: Joi.string()
});

When validating a schema:

• Values (or keys in case of objects) are optional by default.

  Joi.string().validate(undefined); // validates fine

  To disallow this behavior, you can either set the schema as required(), or set presence to "required" when passing options:

  Joi.string().required().validate(undefined);
  // or
  Joi.string().validate(undefined, /* options */ { presence: "required" });

• Strings are utf-8 encoded by default.

• Rules are defined in an additive fashion and evaluated in order, first the inclusive rules, then the exclusive rules.

Joi doesn't directly support browsers, but you could use joi-browser for an ES5 build of Joi that works in browsers, or as a source of inspiration for your own builds.