Validate, sanitize and document JSON schemas

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Validate, sanitize and document JSON schemas


Explicit JSON schemas describing objects passed around in your system are good!

  • they are a living testable documentation instead of manual Wiki editing
  • provide examples for tests and integrations
  • validate inputs and outputs of the API calls



Each individual schema object should have 3 parts: a version, an example and a JSON schema describing its properties. See test/example-schemas.ts. Start with a single ObjectSchema that describes a particular version of an object

import { ObjectSchema } from '@cypress/schema-tools'
const person100: ObjectSchema = {
  // has semantic version numbers
  version: {
    major: 1,
    minor: 0,
    patch: 0,
  // JSON schema
  schema: {
    type: 'object',
    title: 'Person',
    description: 'An example schema describing a person',
    properties: {
      name: {
        type: 'string',
        format: 'name',
        description: 'this person needs a name',
      age: {
        type: 'integer',
        minimum: 0,
        description: 'Age in years',
    required: ['name', 'age'],
    // note: you can just use required: true to require all properties
  // has typical example
  example: {
    name: 'Joe',
    age: 10,

You can have multiple separate versions of the "Person" schema, and then combine them into single object.

import {ObjectSchema, VersionedSchema, versionSchemas} from '@cypress/schema-tools'
const person100: ObjectSchema = { ... }
// maybe added another property
const person110: ObjectSchema = { ... }
// some big changes
const person200: ObjectSchema = { ... }
const personVersions: VersionedSchema = versionSchemas(person100, person110, person200)

Finally, you probably have "Person" versioned schema, and maybe "Organization" and maybe some other schemas. So put them into a single collection

import { SchemaCollection, combineSchemas } from '@cypress/schema-tools'
export const schemas: SchemaCollection = combineSchemas(

Now you can use the schemas object to validate and sanitize any object.


In addition to the formats included with JSON-schemas you can define custom formats that will be used to validate values. Start with a single custom format to describe an UUID for example

// single custom format
import { CustomFormat, CustomFormats } from '@cypress/schema-tools'
const uuid: CustomFormat = {
  name: 'uuid', // the name
  description: 'GUID used through the system',
  // regular expression to use to validate value
  detect: /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/,
  // (optional) replace actual value with this default value
  // when using to sanitize an object
  defaultValue: 'ffffffff-ffff-ffff-ffff-ffffffffffff',
// export all custom formats, in our case just 1
export const formats: CustomFormats = { uuid }

Now every time you use your schemas, pass the formats too so that the validator knows how to check values from custom formats.

// example JSON schema using uuid custom format
const employee100: ObjectSchema = {
  // has semantic version numbers
  version: {
    major: 1,
    minor: 0,
    patch: 0,
  // JSON schema
  schema: {
    type: 'object',
    title: 'Employee',
    properties: {
      id: {
        type: 'string',
        format: 'uuid',
  example: {
    id: 'a368dbfd-08e4-4698-b9a3-b2b660a11835',
// employee100 goes into "schemas", then
assertSchema(schemas, formats)('Employee', '1.0.0')(someObject)



Returns true if the given schema exists in the collection. Curried function.

import { hasSchema } from '@cypress/schema-tools'
import { schemas } from './schemas'
hasSchema(schemas, 'Name', '1.0.0') // true
hasSchema(schemas)('FooBarBaz')('1.0.0') // false


You can document your schemas using provided method. Example code file

import { documentSchemas } from '@cypress/schema-tools'
import { schemas } from './schemas'
import { formats } from './formats'
console.log(documentSchemas(schemas, formats))

Call it from your NPM scripts

  "scripts": {
    "document": "ts-node ./document.ts > schemas.md"
  "devDependencies": {
    "ts-node": "5.0.1",
    "typescript": "2.8.1"

If you want to tell where a schema is coming from, you can set package name, which will add a note to the output Markdown

import { setPackageName, documentSchemas } from '@cypress/schema-tools'
import { schemas } from './schemas'
setPackageName(schemas, 'my-schemas')
console.log(documentSchemas(schemas, formats))
// each schema will have a note that it was defined in "my-schemas"


Checks a given object against a schema and returns list of errors if the object does not pass the schema. Returns true if the object passes schema, and a list of strings if there are errors (I know, we should use Either or Validation).

import { validate } from '@cypress/schema-tools'
// see example in ./test/example-schemas.ts
import { schemas } from './my-schemas'
import { formats } from './my-formats'
const validatePerson100 = validate(schemas, formats)('person', '1.0.0')
const result = validatePerson100(someObject)
if (result === true) {
  // all good
} else {
  const errorMessage = result.join('\n')

Typical validation messages are

data.createdAt is required
data.createdAt must be date-time format

To stop after finding initial set of errors, pass greedy = false flag

const validatePerson100 = validate(schemas, formats, false)('person', '1.0.0')


Checks a given object against schemas (and formats) and throws a SchemaError if the object violates the given schema.

try {
  assertSchema(schemas, formats)('Person', '1.0.0')(object)
} catch (e) {
  // can also inspect individual fields, see SchemaError

You can substitute some fields from example object to help with dynamic data. For example, to avoid breaking on invalid id value, we can tell assertSchema to use id value from the example object.

const o = {
  name: 'Mary',
  age: -1,
assertSchema(schemas, formats)('Person', '1.0.0', {
  substitutions: ['age'],
// everything is good, because the actual object asserted was
// {name: 'Mary', age: 10}

You can also limit the error message and omit some properties. Typically the error message with include list of errors, current and example objects, which might create a wall of text. To omit object and example but leave other fields when forming error message use

const o = {
  name: 'Mary',
  age: -1,
assertSchema(schemas, formats)('Person', '1.0.0', {
  omit: {
    object: true,
    example: true,
// Error message is much much shorter, only "errors" and label will be there

By default the json schema check is greedy but you can limit it via an option

assertSchema(schemas, formats)('Person', '1.0.0', { greedy: false })


Often you have an object that has more properties than the schema allows. For example if you have new result that should go to "older" clients, you might want to trim the result object and then assert schema.

import { trim } from '@cypress/schema-tools'
const trimPerson = trim(schemas, 'Person', '1.0.0')
const person = ... // some result with lots of properties
const trimmed = trimPerson(person)
// trimmed should be valid Person 1.0.0 object
// if the values are actually matching Person@1.0.0
// all extra properties should have been removed


The opposite of trim. Tries to fill missing object properties with explicit default values from the schema. See test/fill-test.ts for example.


If you schema has dynamic data, like timestamps or uuids, it is impossible to compare objects without first deleting some fields, breaking the schema. To solve this you can mark some properties with format and if that format has a default value, you can replace all dynamic values with default ones.

In the example below the name property has format called name like this

name: {
  type: 'string',
  format: 'name'

Now we can sanitize any object which will replace name value with default value, but will keep other properties unchanged.

import { sanitize, getDefaults } from '@cypress/schema-tools'
const name: CustomFormat = {
  name: 'name',
  description: 'Custom name format',
  detect: /^[A-Z][a-z]+$/,
  defaultValue: 'Buddy',
const exampleFormats: CustomFormats = {
const formatDefaults = getDefaults(exampleFormats)
const object = {
  name: 'joe',
  age: 21,
const sanitizePerson = sanitize(schemas, formatDefaults)('person', '1.0.0')
// now pass any object with dynamic "name" property
const result = sanitizePerson(object)
// result is {name: 'Buddy', age: 21}

For another example see test/sanitize-test.ts


There are multiple methods to validate, assert or sanitize an object against a schema. All take schemas and (optional) formats. But a project using schema tools probably has a single collection of schemas that it wants to use again and again. The bind method makes it easy to bind the first argument in each function to a schema collection and just call methods with an object later.

import { schemas } from './my-schemas'
import { formats } from './my-formats'
import { bind } from '@cypress/schema-tools'
const api = bind({ schemas, formats })
api.assertSchema('name', '1.0.0')(someObject)

See test/bind-test.ts for examples


When asserting an object against a schema a custom error is thrown. It is an instance of Error, with a very detailed message. It also has additional properties.

  • errors is the list of strings with individual validation errors
  • object the object being validated
  • example example object for the schema
  • schemaName is the title of the schema, like Person
  • schemaVersion the version like 1.0.0 of the schema violated, if known.


You can easily extend existing schema using included addProperty function. See src/actions.ts and test/add-property-test.ts for examples.


Rather than add a single property at a time, you can simply use extend(existingSchema, newSchemaObj).

The existingSchema will be deep cloned and have the newSchemaObj properties merged in.

If newSchemaObj.version is not provided, then the previous schema's semver minor property will be bumped by one.

Fields like required are automatically unioned.

See src/actions.ts and test/extend-schema-test.ts for examples.


A little utility function to create a regular expression to match only the given strings.

import { oneOfRegex } from '@cypress/schema-tools'
const r = oneOfRegex('foo', 'bar')
r.test('foo') // true
r.test('bar') // true
r.toString() // "/^(foo|bar)$/"


To see log messages from this module, run with DEBUG=schema-tools


Uses ava-ts to run Ava test runner directly against TypeScript test files. Use npm t to build and test everything in the test folder.

To run a single test file, use command

npx ava-ts test/<file-name.ts>

To update snapshots and use verbose reporter (prints test names)

npx ava-ts test/<file-name.ts> --verbose -u


This project is licensed under the terms of the MIT license.