JSON-Schema + fake data generators
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Use JSON Schema along with fake generators to provide consistent and meaningful fake data for your system.

We are looking for contributors! If you wanna help us make jsf more awesome, simply write us so!

Join us!

We've recently setup a gitter room for this project, if you want contribute, talk about specific issues from the library, or you need help on json-schema topics just reach us!

Have some ❤ for JSON-Schema-Faker? You can support the project via:

What's new?

A release candidate for v0.5.x series was released in order to support local/remote reference downloading thanks to json-schema-ref-parser, this change forced jsf to be completely async.

var jsf = require('json-schema-faker');

var REMOTE_REF = 'https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json';

var schema = {
  $ref: REMOTE_REF

jsf.resolve(schema).then(function(result) {
  console.log(JSON.stringify(result, null, 2));


  • All code examples from previous versions (v0.4.x and below) will not work, however the fix is easy
  • All dependent tools are of course outdated and they may not work as expected, please take care of this: If you're a maintainer please upgrade your API.

Until a polished v0.5.0 version is released we encourage you to use and test around this RC, the main API will remain intact but probably option names, or subtle behaviors can be introduced.

Examples, new ideas, tips and any kind of kindly feedback is greatly appreciated.


  • Since 0.5.0-rc3 we introduced a jsf.resolve() method for full-async results.
  • Since 0.5.0-rc3 the methods jsf.sync() is REMOVED and the API for jsf() will remain sync.

Thanks for all your feedback in advance to everyone!

Table of contents

Online demo

See online demo. You can save your schemas online and share the link with your collaborators.


jsf is installable through 3 different channels:


Install json-schema-faker with npm:

npm install json-schema-faker --save


Install json-schema-faker with bower:

bower install json-schema-faker --save


JSON-Schema-faker is also available at cdnjs.com. This means you can just include the script file into your HTML:

# remember to update the version number!
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/json-schema-faker/0.5.0-rc1/json-schema-faker.min.js"></script>

It will be fetched from the Content Delivery Network without installing any node.js package.

You can see an example JS fiddle based on jsf loaded from cdnjs.


JSON-Schema-faker (or jsf for short) combines two things:

  • The JSON-schema specification, that defines what is the allowed content of a JSON document
  • Fake data generators, that are used to generate basic or complex data, conforming to the schema.

Since v0.5.x external generators are not longer bundled with jsf, however built-in defaults are shipped for all basic types and formats.

Example usage

var jsf = require('json-schema-faker');

var schema = {
  type: 'object',
  properties: {
    user: {
      type: 'object',
      properties: {
        id: {
          $ref: '#/definitions/positiveInt'
        name: {
          type: 'string',
          faker: 'name.findName'
        email: {
          type: 'string',
          format: 'email',
          faker: 'internet.email'
      required: ['id', 'name', 'email']
  required: ['user'],
  definitions: {
    positiveInt: {
      type: 'integer',
      minimum: 0,
      exclusiveMinimum: true

jsf.resolve(schema).then(function(sample) {
  // "[object Object]"

  // "John Doe"

(demo »)

jsf.version attribute is available to check which version you're using:

var jsf = require('json-schema-faker');
// "0.5.0-rc1"

More examples

Gist demos

Clone these gists and execute them locally (each gist has its own readme with instructions):

  • jsf console - minimal example of jsf working directly under command line
  • jsf grunt - example of jsf working under grunt.js



Use angular-jsf module (installable via npm and bower) to get jsf working in your angular app out of the box! And check out angular-jsf demo.

Grunt plugin

Use grunt-jsonschema-faker to automate running json-schema-faker against your JSON schemas.


Use json-schema-faker-cli to run jsf from your command line.

Webpack loader

Use json-schema-faker-loader to execute jsf as a webpack loader.

JSON Schema specification support

Currently jsf supports the JSON-Schema specification draft-04 only.

If you want to use draft-03, you may find useful information here.

Supported keywords

Below is the list of supported keywords:

  • $ref — Resolve internal references only, and/or external if provided.
  • required — All required properties are guaranteed, if not can be omitted.
  • pattern — Generate samples based on RegExp values.
  • format — Core formats v4-draft only: date-time, email, hostname, ipv4, ipv6 and uri -- demo »
  • enum — Returns any of these enumerated values.
  • minLength, maxLength — Applies length constraints to string values.
  • minimum, maximum — Applies constraints to numeric values.
  • exclusiveMinimum, exclusiveMaximum — Adds exclusivity for numeric values.
  • multipleOf — Multiply constraints for numeric values.
  • items — Support for subschema and fixed item values.
  • minItems, maxItems — Adds length constraints for array items.
  • uniqueItems — Applies uniqueness constraints for array items.
  • additionalItems — Partially supported (?)
  • allOf, oneOf, anyOf — Subschema combinators.
  • properties — Object properties to be generated.
  • minProperties, maxProperties — Adds length constraints for object properties.
  • patternProperties — RegExp-based object properties.
  • additionalProperties — Partially supported (?)
  • dependencies — Not supported yet (?)
  • not — Not supported yet (?)

Using references

Inline references are fully supported (json-pointers) but external can't be resolved by jsf.

Remote en local references are automatically resolved thanks to json-schema-ref-parser.

var schema = {
  type: 'object',
  properties: {
    someValue: {
      $ref: 'otherSchema'

var refs = [
    id: 'otherSchema',
    type: 'string'

jsf.resolve(schema, refs).then(function(sample) {
  // "voluptatem"

Local references are always resolved from the process.cwd(), of course you can specify a custom folder to look-up: jsf(schema, refs, cwd)

Faking values

jsf has built-in generators for core-formats, Faker.js and Chance.js (and others) are also supported but they require setup:

jsf.extend('faker', function() {
  return require('faker');
  "type": "string",
  "faker": "internet.email"

(demo »)

The above schema will invoke faker.internet.email().

Note that both generators has higher precedence than format.

You can also use standard JSON Schema keywords, e.g. pattern:

  "type": "string",
  "pattern": "yes|no|maybe|i don't know"

(demo »)

Advanced usage of faker.js and Chance.js

In following inline code examples the faker and chance variables are assumed to be created with, respectively:

var faker = require('faker');

var Chance = require('chance'),
  chance = new Chance();

Another example of faking values is passing arguments to the generator:

  "type": "string",
  "chance": {
    "email": {
      "domain": "fake.com"

(demo »)

which will invoke chance.email({ "domain": "fake.com" }). This example works for single-parameter generator function.

However, if you pass multiple arguments to the generator function, just pass them wrapped in an array. In the example below we use the faker.finance.amount(min, max, dec, symbol) generator which has 4 parameters. We just wrap them with an array and it's equivalent to faker.finance.amount(100, 10000, 2, "$"):

  "type": "object",
  "properties": {
    "cash": {
      "type": "string",
      "faker": {
        "finance.amount": [100, 10000, 2, "$"]
  "required": [

(demo »)

However, if you want to pass a single parameter that is an array itself, e.g. chance.pickone(["banana", "apple", "orange"]), just like described here, then you need to wrap it with an array once more (twice in total). The outer brackets determine that the content is gonna be a list of params injected into the generator. The inner brackets are just the value itself - the array we pass:

  "type": "object",
  "properties": {
    "food": {
      "type": "string",
      "chance": {
        "pickone": [
  "required": [

(demo »)

Custom formats

Additionally, you can add custom generators for those:

jsf.format('semver', function() {
  return jsf.random.randexp('\\d\\.\\d\\.[1-9]\\d?');

Now that format can be generated:

  "type": "string",
  "format": "semver"


  • format() — Return all registered formats (custom only)
  • format(obj) — Register formats by key/value → name/callback
  • format(name) — Returns that format generator (undefined if not exists)
  • format(name, callback) — Register a custom format by name/callback


  • schema (object) — The schema for input

Note that custom generators has lower precedence than core ones.

Custom Options

You may define following options for jsf that alter its behavior:

  • failOnInvalidTypes: boolean - don't throw exception when invalid type passed
  • defaultInvalidTypeProduct: - default value generated for a schema with invalid type (works only if failOnInvalidTypes is set to false)
  • failOnInvalidFormat: boolean - don't throw exception when invalid format passed
  • maxItems: number - Configure a maximum amount of items to generate in an array. This will override the maximum items found inside a JSON Schema.
  • maxLength: number - Configure a maximum length to allow generating strings for. This will override the maximum length found inside a JSON Schema.
  • random: Function - a replacement for Math.random to support pseudorandom number generation.
  • alwaysFakeOptionals: boolean - When true, all object-properties will be generated regardless they're required or not.
  • optionalsProbability: number - A decimal number from 0 to 1 that indicates the probability to fake a non-required object property (default: 0). When 0.0, only required properties will be generated; when 1.0, all properties are generated. This option is overwritten to 1 when alwaysFakeOptionals = true.

Set options just as below:

  failOnInvalidTypes: false

Extending dependencies

You may extend Faker.js:

var jsf = require('json-schema-faker');

jsf.extend('faker', function(){
  var faker = require('faker');

  faker.locale = "de"; // or any other language
  faker.custom = {
    statement: function(length) {
      return faker.name.firstName() + " has " + faker.finance.amount() + " on " + faker.finance.account(length) + ".";
  return faker;

var schema = {
  "type": "string",
  "faker": {
    "custom.statement": [19]


or if you want to use faker's individual localization packages, simply do the following:

jsf.extend('faker', function() {
  // just ignore the passed faker instance
  var faker = require('faker/locale/de');
  // do other stuff
  return faker;

You can also extend Chance.js, using built-in chance.mixin function:

var jsf = require('json-schema-faker');

jsf.extend('chance', function(){
  var Chance = require('chance');
  var chance = new Chance();

    'user': function() {
      return {
        first: chance.first(),
        last: chance.last(),
        email: chance.email()

  return chance;

var schema = {
  "type": "string",
  "chance": "user"


The first parameter of extend function is the generator name (faker, chance, etc.). The second one is the function that must return the dependency library.

Inferred Types

JSON Schema does not require you to provide the type property for your JSON Schema documents and document fragments.

But since jsf uses the type property to create the proper fake data, we attempt to infer the type whenever it is not provided. We do this based on the JSON Schema validation properties you use.

Now this means that if you do not use any of the JSON Schema validation properties, jsf will not be able to infer the type for you and you will need to explicitly set your type manually.)

Below is the list of JSON Schema validation properties and the inferred type based on the property:


  • additionalItems
  • items
  • maxItems
  • minItems
  • uniqueItems

integer (Number uses the same properties so if you need number, set your type explicitly)

  • exclusiveMaximum
  • exclusiveMinimum
  • maximum
  • minimum
  • multipleOf


  • additionalProperties
  • dependencies
  • maxProperties
  • minProperties
  • patternProperties
  • properties
  • required


  • maxLength
  • minLength
  • pattern

Swagger extensions

jsf supports OpenAPI Specification vendor extensions, i.e.

  • x-faker property that stands for faker property (demo »)
  • x-chance property that stands for chance property (demo »)

Thanks to it, you can use valid swagger definitions for jsf data generation.


JSON-Schema-faker might be used in Node.js as well as in the browser. In order to execute jsf in a browser, you should include the distribution file from dist directory. Each new version of jsf is bundled using Rollup.js and stored by the library maintainers. The bundle includes full versions of all dependencies.

From v0.5.x and beyond we'll not longer bundle external generators, locales and such due the unnecessary waste of time and space.


We are more than happy to welcome new contributors, our project is heavily developed, but we need more power :) Please see contribution guide, you can always contact us to ask how you can help.

Technical Documentation

If you want to contribute, take a look at the technical documentation page. You may find some important information there making it easier to start.

Moreover, if you find something unclear (e.g. how does something work) or would like to suggest improving the docs, please submit an issue, we'll gladly provide more info for future contributors.



There were some existing projects or services trying to achieve similar goals as jsf:

but they were either incomplete, outdated, broken or non-standard. That's why jsf was created.