Build a Yup schema from a JSON Schema. Also supports custom/alternative schema models such as GraphQL type defs

Downloads in past


278101.8.85 years ago6 years agoMinified + gzip package size for json-schema-to-yup in KB


Schema to Yup schema
Build a Yup schema from a JSON Schema, GraphQL schema (type definition) or any other similar type/class and field/properties model or schema :)

The builder currently supports the most commonly used JSON Schema layout and GraphQL type definition exports using graphSchemaToJson (see GraphQL schema).
It also supports some extra convenience schema properties that make it more "smooth" to define validation requirements declaratively (see below).
According to the JSON schema specs, you are free to add extra metadata to the field schema definitions beyond those supported "natively".

Quick start

npm install json-schema-to-yup -S or yarn add json-schema-to-yup
const schema = {
  $schema: "http://json-schema.org/draft-07/schema#",
  $id: "http://example.com/person.schema.json",
  title: "Person",
  description: "A person",
  type: "object",
  properties: {
    name: {
      description: "Name of the person",
      type: "string"
    email: {
      type: "string",
      format: "email"
    fooorbar: {
      type: "string",
      matches: "(foo|bar)"
    age: {
      description: "Age of person",
      type: "number",
      exclusiveMinimum: 0,
      required: true
    characterType: {
      enum: ["good", "bad"],
      enum_titles: ["Good", "Bad"],
      type: "string",
      title: "Type of people",
      propertyOrder: 3
  required: ["name", "email"]

const config = {
  // for error messages...
  errMessages: {
    age: {
      required: "A person must have an age"
    email: {
      required: "You must enter an email address",
      format: "Not a valid email address"

const { buildYup } = require("json-schema-to-yup");
const yupSchema = buildYup(json, config);
// console.dir(schema)
const valid = await yupSchema.isValid({
  name: "jimmy",
  age: 24

// => {valid: true}

This would generate the following Yup validation schema:
const schema = yup.object().shape({
  name: yup.string().required(),
  age: yup

Note the "required": true for the age property (not natively supported by JSON schema).

Types and keys

Mixed (any type)

  • strict
  • default
  • nullable
  • required
  • notRequired
  • oneOf (enum)
  • notOneOf


  • ensure
  • compact
  • items (of)
  • maxItems (max)
  • minItems (min)


No keys


  • maxDate (max)
  • minDate (min)


  • integer
  • moreThan (exclusiveMinimum)
  • lessThan (exclusiveMaximum)
  • positive
  • negative
  • min (minimum)
  • max (maximum)
  • truncate
  • round


  • camelCase
  • constantCase
  • noUnknown (propertyNames)


  • minLength (min)
  • maxLength (max)
  • pattern (matches or regex)
  • email (format: 'email')
  • url (format: 'url')
  • lowercase
  • uppercase
  • trim

Complex example

Here a more complete example of the variations currently possible
  "title": "Person",
  "description": "A person",
  "type": "object",
  "properties": {
    "name": {
      "description": "Name of the person",
      "type": "string",
      "required": true,
      "matches": "[a-zA-Z- ]+",
      "mix": 3,
      "maxLength": 40,
    "age": {
      "description": "Age of person",
      "type": "integer",
      "moreThan": 0,
      "max": 130,
      "default": 32,
      "required": false,
      "nullable": true
    "birthday": {
      "type": "date",
      "min": "1-1-1900",
      "maxDate": "1-1-2015"
    "married": {
      "type": "boolean"
    "boss": {
      "type": "object",
      "noUnknown": [
      "properties": {
        "name": {
          "type": "string",
          "notOneOf": ["Dr. evil", "bad ass"]
    "colleagues": {
      "type": "array",
      "items": {
        "type": "object",
        "propertyNames": [
        "properties": {
          "name": {
            "type": "string"
    "programming": {
        "type": "object",
        "properties": {
          "languages": {
            "type": "array",
            "of": {
              "type": "string",
              "enum": ["javascript", "java", "C#"]
            "min": 1,
            "max": 3

Custom models

This library now also supports non JSON schema models. See the types/defaults mappings.
module.exports {
  getProps: obj => obj.properties,
  getType: obj => obj.type,
  getName: obj => obj.name || obj.title,
  getConstraints: obj => obj,
  isString: obj => obj.type === "string",
  isArray: obj => obj.type === "array",
  isInteger: obj => obj.type === "integer",
  isBoolean: obj => obj.type === "boolean",
  hasDateFormat: obj => ["date", "date-time"].find(t => t === obj.format),
  isDate: obj => obj.type === "string" && defaults.hasDateFormat(obj.format),
  isNumber: obj => obj.type === "number" || defaults.isInteger(obj.type),
  isObject: obj => obj.type === "object",
  isRequired: obj => obj.required

GraphQL schema

To support another model, such as GraphQL schema (type definitions) via graphSchemaToJson
  Person: {
    name: 'Person',
    fields: {
      name: {
        type: 'String',
        directives: {
          constraints: {
            minLength: 2
        isNullable: false,
        isList: false
    directives: {},
    type: 'Object',
    implements: []

Create a map of methods to match your model layout:
const typeDefConf = {
  getProps: obj => obj.fields,
  getType: obj => obj.type,
  getName: obj => obj.name,
  getConstraints: obj => (obj.directives || {}).constraints || {},
  isString: obj => obj.type === "String",
  isArray: obj => obj.isList,
  isInteger: obj => obj.type === "Int",
  isBoolean: obj => obj.type === "Boolean",
  isDate: obj => obj.type === "Date" || obj.directives.date,
  isNumber: obj => obj.type === "Int" || obj.type === "Float",
  isObject: obj => obj.type === "Object",
  isRequired: obj => !obj.isNullable

Please note that getConstraints can be used to control where the constraints of the field will be taken from (depending on the type of model/schema or your preference).
Pass overrides to match your model in config as follows:
const schema = buildYup(nameJsonSchema, { ...typeDefConf, log: true });

The type definition mappings above are already built-in and available. To switch the schema type, pass schemaType in config as follows.
const schema = buildYup(nameJsonSchema, { schemaType: "type-def", log: true });

Feel free to make PRs to make more common schema models conveniently available!

Custom logs and error handling

You can enable logging py passing a log option in the config argument. If set to true, it will by default assign the internal log function to console.log
const schema = buildYup(nameJsonSchema, { log: true });

You can also pass a log function in the log option to handle log messages and an err option with a custom error handler function.
See Custom errors in Node for how to design custom errors
class ValidationError extends Error {}

const schema = buildYup(nameJsonSchema, {
  log: (name, msg) => console.log(`[${name}] ${msg}`)
  err: (msg) => {
    console.error(`[${name}] ERROR: ${msg}`
    throw new ValidationError(msg)


You can supply a createYupSchemaEntry function as an entry in the config object. This function will then be used to build each Yup Schema entry in the Yup Schema being built.
Use the Yup Type classes such as types.YupArray to act as building blocks or create your own custom logic as you see fit.

Customization example

const { YupSchemaEntry, buildYup, types } = require("json-schema-to-yup");

class CustomYupArray extends types.YupArray {
  // ...

class CustomYupSchemaEntry extends YupSchemaEntry {
  // ...

function createYupSchemaEntry(key, value, config) {
  const builder = new CustomYupSchemaEntryBuilder(key, value, config);
  builder.types.array = config => createYupArray(config);
  return builder.toEntry();

// use some localized error messages
const messages = i18n.locale(LOCALE);

const yupSchema = buildYup(json, {

Extend Yup API to bridge other validators

You can use extendYupApi to extend the Yup API with extra validation methods:
const validator = require("validator");
const { extendYupApi } = require("json-schema-to-yup/validator-bridge");

// by default extends with string format validation methods of validator
// See https://www.npmjs.com/package/validator
extendYupApi({ validator });

You can optionally pass in a custom validator and a constraints map of your choice. You can either extend the default constraints or override them with your own map.
PS: Check out src/validator-bridge for more many options for fine control
const myValidator = new MyValidator();
const constraints = ["creditCard", "currency", { name: "hash", opts: "algo" }];
extendYupApi({ validator: myValidator, override: true, constraints });

const { buildYup } = require("json-schema-to-yup");
// type def sample schema, using credit-card format validator
const schema = {
  name: "BankAccount",
  fields: {
    accountNumber: {
      type: "String",
      format: "credit-card"

// opt in to use generic string format validation, via format: true config option
const yupSchema = buildYup(schema, { format: true, schemaType: "type-def" });
// ...do your validation
const valid = await yupSchema.isValid({
  accountNumber: "123-4567-1828-2929"

Now the bridge includes tests. Seems to work ;)


You can sublass YupBuilder or any of the internal classes to create your own custom infrastructure to suit your particular needs, expand with support for extra features etc.
const { YupBuilder } = require("json-schema-to-yup");

class MyYupBuilder extends YupBuilder {
  // ... custom overrides etc

const builder = new MyYupBuilder(schema, config);
const { yupSchema } = builder;
// ...

Error messages

You can pass an errMessages object in the optional config object argument with key mappings for your custom validation error messages.
Internally the validator error messages are resolved with the instance method valErrMessage (from Mixed class)
notOneOf() {
  const {not, notOneOf} = this.value
  const $oneOf = notOneOf || (not && (not.enum || not.oneOf))
  $oneOf && this
    .notOneOf($oneOf, this.valErrMessage('notOneOf'))
  return this

The key entries can be either a function, taking a value argument or a static string. Here are some of the defaults that you can override as needed.
export const errValKeys = [

export const defaults = {
  errMessages: (keys = errValKeys) =>
    keys.reduce((acc, key) => {
      const fn = ({ key, value }) =>
        `${key}: invalid for ${value.name || value.title}`;
      acc[key] = fn;
      return acc;
    }, {})

Custom validation messages using select defaults

const { buildYup, types } = require("json-schema-to-yup");
const { defaults } = types;

const myErrMessages = require("./err-messages");
const valKeys = ["lowercase", "integer"];

// by default Yup built-in validation error messages will be used if not overridden here
const errMessages = {

const yupSchema = buildYup(json, {

Adding custom constraints

See the number type for the current best practice to add type constraints.
For simple cases: use addConstraint from the superclass YupMixed
required() {
  return this.addConstraint("required");

For types with several of these, we should map through a list or map to add them all.
strict() {
  return this.addValueConstraint("strict");

required() {
  return this.addConstraint("required");

notRequired() {
  return this.addConstraint("notRequired");

Can be rewritten to use conventions, iterating a map:
addMappedConstraints() {
  Object.keys(this.constraintsMap).map(key => {
    const list = constraintsMap[key];
    const fnName = key === 'value' ? 'addValueConstraint' : 'addConstraint'

get constraintsMap() {
  return {
    simple: ["required", "notRequired", "nullable"],
    value: ["default", "strict"]

For more complex contraints that:
  • have multiple valid constraint names
  • require validation
  • optional transformation

You can create a separate Constraint subclass, to offload and handle it all separately. Here is a sample RangeConstraint used by number.
class RangeConstraint extends NumericConstraint {
  constructor(typer) {

  get $map() {
    return {
      moreThan: ["exclusiveMinimum", "moreThan"],
      lessThan: ["exclusiveMaximum", "lessThan"],
      max: ["maximum", "max"],
      min: ["minimum", "min"]

Instead of wrapping a Constraint you can call it directly with a map
// this would be an instance such as YupNumber
// map equivalent to $map in the RangeConstraint
range() {
  return createNumericConstraint(this, map);

For the core type constraint class (such as YupNumber) you should now be able to simplify it to:
get enabled() {
  return ["range", "posNeg", "integer"];

convert() {
  this.enabled.map(name => this.processConstraint(name));
  return this;

The following constraint classes are available for use:
  • NumericConstraint
  • StringConstraint
  • RegExpConstraint
  • DateConstraint

Currently only YupNumber has been (partly) refactored to take advantage of this new infrastructure. Please help refactor the rest!
YupNumber also has the most unit test coverage, used to test the current infrastructure!

Similar projects

The library JSON Schema model builder is a powerful toolset to build a framework to create any kind of output model from a JSON schema.
If you enjoy this declarative/generator approach, try it out!


Uses jest for unit testing.
  • Have unit tests that cover most of the constraints supported.
  • Could use some refactoring using the latest infrastructure (see NumericConstraint)
  • Please help add more test coverage and help refactor to make this lib even more awesome :)


Current development is taking place on refactoring branch.
On his branch:
  • all the code has been converted to TypeScript
  • constraint classes for String, Numeric, RegExp etc.
  • Validator building has been extracted so you can add support for any Validator, such as Forg
  • more...

If you would like to further improved this library or add support for more validators than Yup, please help on this branch. Cheers!

Ideas and suggestions

Please feel free to come with ideas and suggestions on how to further improve this library.


2018 Kristian Mandrup (CTO@Tecla5)


``` ```