A collection of modules for working with JSON Schemas.

• Validate JSON-compatible values against a JSON Schema
  • Dialects: draft-2020-12, draft-2019-09, draft-07, draft-06, draft-04
  • OpenAPI
    • Versions/Dialects: 3.0, 3.1
    • Validate an OpenAPI document
    • Validate values against a schema from an OpenAPI document
  • Schemas can reference other schemas using a different dialect
  • Work directly with schemas on the filesystem or HTTP
• Create custom keywords, vocabularies, and dialects
• Bundle multiple schemas into one document
  • Uses the process defined in the 2020-12 specification but works with any dialect.
• Provides utilities for building non-validation JSON Schema tooling
• Provides utilities for working with annotations

Includes support for node.js (ES Modules, TypeScript) and browsers.

npm install @hyperjump/json-schema

When in a browser context, this library is designed to use the browser's fetch implementation instead of a node.js fetch clone. The Webpack bundler does this properly without any extra configuration, but if you are using the Rollup bundler you will need to include the browser: true option in your Rollup configuration.

  plugins: [
    resolve({
      browser: true
    })
  ]

The API for this library is divided into two categories: Stable and Experimental. The Stable API strictly follows semantic versioning, but the Experimental API may have backward-incompatible changes between minor versions.

All experimental features are segregated into exports that include the word "experimental" so you never accidentally depend on something that could change or be removed in future releases.

This library supports many versions of JSON Schema. Use the pattern @hyperjump/json-schema/* to import the version you need.

import { addSchema, validate } from "@hyperjump/json-schema/draft-2020-12";

You can import support for additional versions as needed.

import { addSchema, validate } from "@hyperjump/json-schema/draft-2020-12";
import "@hyperjump/json-schema/draft-07";

Note: The default export (@hyperjump/json-schema) is reserved for the stable version of JSON Schema that will hopefully be released in 2023.

Validate schema from JavaScript

addSchema({
  $schema: "https://json-schema.org/draft/2020-12/schema",
  type: "string"
}, "http://example.com/schemas/string");

const output = await validate("http://example.com/schemas/string", "foo");
if (output.valid) {
  console.log("Instance is valid :-)");
} else {
  console.log("Instance is invalid :-(");
}

Compile schema

If you need to validate multiple instances against the same schema, you can compile the schema into a reusable validation function.

const isString = await validate("http://example.com/schemas/string");
const output1 = isString("foo");
const output2 = isString(42);

Fetching schemas

You can fetch schemas from the web or from the file system, but when fetching from the file system, there are limitations for security reasons. If your schema has an identifier with an http(s) scheme (https://example.com), it's not allowed to reference schemas with a file scheme (file:///path/to/my/schemas).

const output = await validate("http://example.com/schemas/string", "foo");

const output = await validate(`file://${__dirname}/string.schema.json`, "foo");

Media type plugins

There is a plugin system for adding support for different media types. By default it's configured to accept schemas that have the application/schema+json Content-Type (web) or a .schema.json file extension (filesystem). If, for example, you want to fetch schemas that are written in YAML, you can add a MediaTypePlugin to support that.

import { addMediaTypePlugin, validate } from "@hyperjump/json-schema/draft-2020-12";
import YAML from "yaml";


// Add support for JSON Schemas written in YAML
addMediaTypePlugin("application/schema+yaml", {
  parse: async (response) => [YAML.parse(await response.text()), undefined],
  matcher: (path) => path.endsWith(".schema.yaml")
});

// Example: Fetch schema with Content-Type: application/schema+yaml from the web
const isString = await validate("http://example.com/schemas/string");

// Example: Fetch from file with JSON Schema YAML file extension
const isString = await validate(`file://${__dirname}/string.schema.yaml`);

// Then validate against your schema like normal
const output = isString("foo");

OpenAPI

The OpenAPI 3.0 and 3.1 meta-schemas are pre-loaded and the OpenAPI JSON Schema dialects for each of those versions is supported. A document with a Content-Type of application/openapi+json (web) or a file extension of openapi.json (filesystem) is understood as an OpenAPI document.

Use the pattern @hyperjump/json-schema/* to import the version you need. The available versions are openapi-3-0 for 3.0 and openapi-3-1 for 3.1.

YAML support isn't built in, but you can add it by writing a MediaTypePlugin. You can use the one at lib/openapi.js as an example and replacing the JSON parts with YAML.

import { addSchema, validate } from "@hyperjump/json-schema/openapi-3-1";


// Validate an OpenAPI document
const output = await validate("https://spec.openapis.org/oas/3.1/schema-base", openapi);

// Validate an instance against a schema in an OpenAPI document
const output = await validate(`file://${__dirname}/example.openapi.json#/components/schemas/foo`, 42);

These are available from any of the exports that refer to a version of JSON Schema, such as @hyperjump/json-schema/draft-2020-12.

• addSchema: (schema: object, retrievalUri?: string, defaultDialectId?: string) => void

  Load a schema manually rather than fetching it from the filesystem or over the network.

• validate: (schemaURI: string, instance: any, outputFormat: OutputFormat = FLAG) => Promise

  Validate an instance against a schema. This function is curried to allow compiling the schema once and applying it to multiple instances.

• validate: (schemaURI: string) => Promise<(instance: any, outputFormat: OutputFormat = FLAG) => OutputUnit>

  Compiling a schema to a validation function.

• FLAG: "FLAG"

  An identifier for the FLAG output format as defined by the 2019-09 and 2020-12 specifications.

• InvalidSchemaError: Error & { output: OutputUnit }

  This error is thrown if the schema being compiled is found to be invalid. The output field contains an OutputUnit with information about the error. You can use the setMetaSchemaOutputFormat configuration to set the output format that is returned in output.

• setMetaSchemaOutputFormat: (outputFormat: OutputFormat) => void

  Set the output format used for validating schemas.

• getMetaSchemaOutputFormat: () => OutputFormat

  Get the output format used for validating schemas.

• setShouldMetaValidate: (isEnabled: boolean) => void

  Enable or disable validating schemas.

• getShouldMetaValidate: (isEnabled: boolean) => void

  Determine if validating schemas is enabled.

• addMediaTypePlugin: (contentType: string, plugin: MediaTypePlugin) => void

  Add a custom media type handler to support things like YAML or to change the way JSON is supported.

Type Definitions

The following types are used in the above definitions

• OutputFormat: FLAG

  Only the FLAG output format is part of the Stable API. Additional output formats are included as part of the Experimental API.

• OutputUnit: { valid: boolean }

  Output is an experimental feature of the JSON Schema specification. There may be additional fields present in the OutputUnit, but only the valid property should be considered part of the Stable API.

• MediaTypePlugin: object

  • parse: (response: Response, mediaTypeParameters: object) => [object | boolean, string?]

    Given a fetch Response object, parse the body of the request. Return the parsed schema and an optional default dialectId.

  • matcher: (path) => boolean

    Given a filesystem path, return whether or not the file should be considered a member of this media type.

  • quality (optional): string

    The registered media type plugins are used to create the Accept header for HTTP requests. This property allows you to specify a quality value for your media type. A quality value is a string representation of a number between 0 and 1 with up to three digits.

You can bundle schemas with external references into a single deliverable using the official JSON Schema bundling process introduced in the 2020-12 specification. Given a schema with external references, any external schemas will be embedded in the schema resulting in a Compound Schema Document with all the schemas necessary to evaluate the given schema in a single JSON document.

The bundling process allows schemas to be embedded without needing to modify any references which means you get the same output details whether you validate the bundle or the original unbundled schemas.

import { addSchema } from "@hyperjump/json-schema/draft-2020-12";
import { bundle } from "@hyperjump/json-schema/bundle";

addSchema({
  "$id": "https://example.com/main",
  "$schema": "https://json-schema.org/draft/2020-12/schema",

  "type": "object",
  "properties": {
    "foo": { "$ref": "/string" }
  }
});

addSchema({
  "$id": "https://example.com/string",
  "$schema": "https://json-schema.org/draft/2020-12/schema",

  "type": "string"
});

const bundledSchema = await bundle("https://example.com/main"); // {
//   "$id": "https://example.com/main",
//   "$schema": "https://json-schema.org/draft/2020-12/schema",
//
//   "type": "object",
//   "properties": {
//     "foo": { "$ref": "/string" }
//   },
//
//   "$defs": {
//     "https://example.com/string": {
//       "$id": "https://example.com/string",
//       "type": "string"
//     }
//   }
// }

These are available from the @hyperjump/json-schema/bundle export.

• bundle: (uri: string, options: Options) => Promise

  Create a bundled schema starting with the given schema. External schemas will be fetched from the filesystem, the network, or internally as needed.

  Options:

  • alwaysIncludeDialect: boolean (default: false) -- Include dialect even when it isn't strictly needed
  • bundleMode: "flat" | "full" (default: "flat") -- When bundling schemas that already contain bundled schemas, "flat" mode with remove nested embedded schemas and put them all in the top level $defs. When using "full" mode, it will keep the already embedded schemas around, which will result in some embedded schema duplication.
  • definitionNamingStrategy: "uri" | "uuid" (default: "uri") -- By default the name used in definitions for embedded schemas will match the identifier of the embedded schema. This naming is unlikely to collide with actual definitions, but if you want to be sure, you can use the "uuid" strategy instead to be sure you get a unique name.
  • externalSchemas: string[] (default: []) -- A list of schemas URIs that are available externally and should not be included in the bundle.

Change the validation output format

The FLAG output format isn't very informative. You can change the output format used for validation to get more information about failures.

import { BASIC } from "@hyperjump/json-schema/experimental";

const output = await validate("https://example.com/schema1", 42, BASIC);

Change the schema validation output format

The output format used for validating schemas can be changed as well.

import { validate, setMetaSchemaOutputFormat } from "@hyperjump/json-schema/draft-2020-12";
import { BASIC } from "@hyperjump/json-schema/experimental";

setMetaSchemaOutputFormat(BASIC);
try {
  const output = await validate("https://example.com/invalid-schema");
} catch (error) {
  console.log(error.output);
}

Type Definitions

• OutputFormat: FLAG | BASIC | DETAILED | VERBOSE

  In addition to the FLAG output format in the Stable API, the Experimental API includes support for the BASIC, DETAILED, and VERBOSE formats as specified in the 2019-09 specification (with some minor customizations). This implementation doesn't include annotations or human readable error messages. The output can be processed to create human readable error messages as needed.

In order to create and use a custom keyword, you need to define your keyword's behavior, create a vocabulary that includes that keyword, and then create a dialect that includes your vocabulary.

import { addSchema, validate } from "@hyperjump/json-schema/draft-2020-12";
import { addKeyword, defineVocabulary, Validation } from "@hyperjump/json-schema/experimental";
import * as Schema from "@hyperjump/json-schema/schema/experimental";


// Define a keyword that's an array of schemas that are applied sequentially
// using implication: A -> B -> C -> D
addKeyword({
  id: "https://example.com/keyword/implication",

  compile: (schema, ast) => {
    return Schema.map(async (itemSchema) => Validation.compile(await itemSchema, ast), schema);
  },

  interpret: (implies, instance, ast, dynamicAnchors, quiet) => {
    return implies.reduce((acc, schema) => {
      return !acc || Validation.interpret(schema, instance, ast, dynamicAnchors, quiet);
    }, true);
  }
});

// Create a vocabulary with this keyword and call it "implies"
defineVocabulary("https://example.com/vocab/logic", {
  "implies": "https://example.com/keyword/implication"
});

// Create a vocabulary schema for this vocabulary
addSchema({
  "$id": "https://example.com/meta/logic",
  "$schema": "https://json-schema.org/draft/2020-12/schema",

  "$dynamicAnchor": "meta",
  "properties": {
    "implies": {
      "type": "array",
      "items": { "$dynamicRef": "meta" },
      "minItems": 2
    }
  }
});

// Create a dialect schema adding this vocabulary to the standard JSON Schema
// vocabularies
addSchema({
  "$id": "https://example.com/dialect/logic",
  "$schema": "https://json-schema.org/draft/2020-12/schema",

  "$vocabulary": {
    "https://json-schema.org/draft/2020-12/vocab/core": true,
    "https://json-schema.org/draft/2020-12/vocab/applicator": true,
    "https://json-schema.org/draft/2020-12/vocab/unevaluated": true,
    "https://json-schema.org/draft/2020-12/vocab/validation": true,
    "https://json-schema.org/draft/2020-12/vocab/meta-data": true,
    "https://json-schema.org/draft/2020-12/vocab/format-annotation": true,
    "https://json-schema.org/draft/2020-12/vocab/content": true
    "https://example.com/vocab/logic": true
  },

  "$dynamicAnchor": "meta",

  "allOf": [
    { "$ref": "https://json-schema.org/draft/2020-12/schema" },
    { "$ref": "/meta/logic" }
  ]
});

// Use your dialect to validate a JSON instance
addSchema({
  "$schema": "https://example.com/dialect/logic",

  "type": "number",
  "implies": [
    { "minimum": 10 },
    { "multipleOf": 2 }
  ]
}, "https://example.com/schema1");
const output = await validate("https://example.com/schema1", 42);

Custom Meta Schema

You can use a custom meta-schema to restrict users to a subset of JSON Schema functionality. This example requires that no unknown keywords are used in the schema.

addSchema({
  "$id": "https://example.com/meta-schema1",
  "$schema": "https://json-schema.org/draft/2020-12/schema",

  "$vocabulary": {
    "https://json-schema.org/draft/2020-12/vocab/core": true,
    "https://json-schema.org/draft/2020-12/vocab/applicator": true,
    "https://json-schema.org/draft/2020-12/vocab/unevaluated": true,
    "https://json-schema.org/draft/2020-12/vocab/validation": true,
    "https://json-schema.org/draft/2020-12/vocab/meta-data": true,
    "https://json-schema.org/draft/2020-12/vocab/format-annotation": true,
    "https://json-schema.org/draft/2020-12/vocab/content": true
  },

  "$dynamicAnchor": "meta",

  "$ref": "https://json-schema.org/draft/2020-12/schema",
  "unevaluatedProperties": false
});

addSchema({
  $schema: "https://example.com/meta-schema1",
  type: "number",
  foo: 42
}, "https://example.com/schema1");

const output = await validate("https://example.com/schema1", 42); // Expect InvalidSchemaError

These are available from the @hyperjump/json-schema/experimental export.

• addKeyword: (keywordHandler: Keyword) => void

  Define a keyword for use in a vocabulary.

• defineVocabulary: (id: string, keywords: { [keyword: string]: string }) => void

  Define a vocabulary that maps keyword name to keyword URIs defined using addKeyword.

• getKeyword: (keywordId: string) => Keyword

  Get a keyword object by its URI. This is useful for building non-validation tooling.

• getKeywordName: (dialectId: string, keywordId: string) => string

  Determine a keyword's name given its URI a dialect URI. This is useful when defining a keyword that depends on the value of another keyword (such as how contains depends on minContains and maxContains).

• loadDialect: (dialectId: string, dialect: { [vocabularyId: string] }) => void

  Define a dialect. In most cases, dialects are loaded automatically from the $vocabulary keyword in the meta-schema. The only time you would need to load a dialect manually is if you're creating a distinct version of JSON Schema rather than creating a dialect of an existing version of JSON Schema.

• Validation: Keyword

  A Keyword object that represents a "validate" operation. You would use this for compiling and evaluating sub-schemas when defining a custom keyword.

• Keyword: object

  • id: string

    A URI that uniquely identifies the keyword. It should use a domain you own to avoid conflict with keywords defined by others.

  • compile: (schema: SchemaDocument, ast: AST, parentSchema: SchemaDocument) => Promise

    This function takes the keyword value, does whatever preprocessing it can on it without an instance, and returns the result. The returned value will be passed to the interpret function. The ast parameter is needed for compiling sub-schemas. The parentSchema parameter is primarily useful for looking up the value of an adjacent keyword that might effect this one.

  • interpret: (compiledKeywordValue: A, instance: JsonDocument, ast: AST, dynamicAnchors: Anchors, quiet: boolean) => boolean

    This function takes the value returned by the compile function and the instance value that is being validated and returns whether the value is valid or not. The other parameters are only needed for validating sub-schemas.

  • collectEvaluatedProperties?: (compiledKeywordValue: A, instance: JsonDocument, ast: AST, dynamicAnchors: Anchors) => string[] | false

    If the keyword is an applicator, it will need to implements this function for unevaluatedProperties to work as expected.

  • collectEvaluatedItems?: (compiledKeywordValue: A, instance: JsonDocument, ast: AST, dynamicAnchors: Anchors) => Set | false

    If the keyword is an applicator, it will need to implements this function for unevaluatedItems to work as expected.

These functions are available from the @hyperjump/json-schema/schema/experimental export.

This library uses SchemaDocument objects to represent a value in a schema. You'll work with these objects if you create a custom keyword. This module is a set of functions for working with SchemaDocuments.

• Schema.add: (schema: object, retrievalUri?: string, dialectId?: string) => string

  Load a schema. Returns the identifier for the schema.

• Schema.get: (url: string, contextDoc?: SchemaDocument) => Promise

  Fetch a schema. Schemas can come from an HTTP request, a file, or a schema that was added with Schema.add.

• Schema.uri: (doc: SchemaDocument) => string

  Returns a URI for the value the SchemaDocument represents.

• Schema.value: (doc: SchemaDocument) => any

  Returns the value the SchemaDocument represents.

• Schema.typeOf: (doc: SchemaDocument, type: string) => boolean

  Determines if the JSON type of the given doc matches the given type.

• Schema.has: (key: string, doc: SchemaDocument) => Promise

  Similar to key in schema.

• Schema.step: (key: string, doc: SchemaDocument) => Promise

  Similar to schema[key], but returns an SchemaDocument.

• Schema.iter: (doc: SchemaDocument) => AsyncGenerator

  Iterate over the items in the array that the SchemaDocument represents

• Schema.entries: (doc: SchemaDocument) => AsyncGenerator<[string, SchemaDocument]>

  Similar to Object.entries, but yields SchemaDocuments for values.

• Schema.values: (doc: SchemaDocument) => AsyncGenerator

  Similar to Object.values, but yields SchemaDocuments for values.

• Schema.keys: (doc: SchemaDocument) => Generator

  Similar to Object.keys.

• Schema.length: (doc: SchemaDocument) => number

  Similar to Array.prototype.length.

• Schema.toSchema: (doc: SchemaDocument, options: ToSchemaOptions) => object

  Get a raw schema from a Schema Document.

Type Definitions

The following types are used in the above definitions

• ToSchemaOptions: object

  • parentId: string (default: "") -- file:// URIs will be generated relative to this path.
  • parentDialect: string (default: "") -- If the dialect of the schema
  • matches this value, the $schema keyword will be omitted.
  • includeEmbedded: boolean (default: true) -- If false, embedded schemas will be unbundled from the schema.

These functions are available from the @hyperjump/json-schema/instance/experimental export.

This library uses InstanceDocument objects to represent a value in an instance. You'll work with these objects if you create a custom keyword. This module is a set of functions for working with InstanceDocuments.

• Instance.cons: (instance: any, uri?: string) => InstanceDocument

  Construct an InstanceDocument from a value.

• Instance.get: (url: string, contextDoc: InstanceDocument) => InstanceDocument

  Apply a same-resource reference to a InstanceDocument.

• Instance.uri: (doc: InstanceDocument) => string

  Returns a URI for the value the InstanceDocument represents.

• Instance.value: (doc: InstanceDocument) => any

  Returns the value the InstanceDocument represents.

• Instance.has: (key: string, doc: InstanceDocument) => any

  Similar to key in instance.

• Instance.typeOf: (doc: InstanceDocument, type: string) => boolean

  Determines if the JSON type of the given doc matches the given type.

• Instance.step: (key: string, doc: InstanceDocument) => InstanceDocument

  Similar to schema[key], but returns a InstanceDocument.

• Instance.iter: (doc: InstanceDocument) => Generator

  Iterate over the items in the array that the SchemaDocument represents.

• Instance.entries: (doc: InstanceDocument) => Generator<[string, InstanceDocument]>

  Similar to Object.entries, but yields InstanceDocuments for values.

• Instance.values: (doc: InstanceDocument) => Generator

  Similar to Object.values, but yields InstanceDocuments for values.

• Instance.keys: (doc: InstanceDocument) => Generator

  Similar to Object.keys.

• Instance.length: (doc: InstanceDocument) => number

  Similar to Array.prototype.length.

JSON Schema is for annotating JSON instances as well as validating them. This module provides utilities for working with JSON documents annotated with JSON Schema.

An annotated JSON document is represented as an AnnotatedInstance object. This object is a wrapper around your JSON document with functions that allow you to traverse the data structure and get annotations for the values within.

import { annotate, annotatedWith, addSchema } from "@hyperjump/json-schema/annotations/experimental";
import * as AnnotatedInstance from "@hyperjump/json-schema/annotated-instance/experimental";


const schemaId = "https://example.com/foo";
const dialectId = "https://json-schema.org/draft/2020-12/schema";

addSchema({
  "$schema": dialectId,

  "title": "Person",
  "unknown": "foo",

  "type": "object",
  "properties": {
    "name": {
      "$ref": "#/$defs/name",
      "deprecated": true
    },
    "givenName": {
      "$ref": "#/$defs/name",
      "title": "Given Name"
    },
    "familyName": {
      "$ref": "#/$defs/name",
      "title": "Family Name"
    }
  },

  "$defs": {
    "name": {
      "type": "string",
      "title": "Name"
    }
  }
}, schemaId);

const instance = await annotate(schemaId, {
  name: "Jason Desrosiers",
  givenName: "Jason",
  familyName: "Desrosiers"
});

// Get the title of the instance
const titles = AnnotatedInstance.annotation(instance, "title", dialectId); // => ["Person"]

// Unknown keywords are collected as annotations
const unknowns = AnnotatedInstance.annotation(instance, "unknown", dialectId); // => ["foo"]

// The type keyword doesn't produce annotations
const types = AnnotatedInstance.annotation(instance, "type", dialectId); // => []

// Get the title of each of the properties in the object
for (const [propertyName, propertyInstance] of AnnotatedInstance.entries(instance)) {
  console.log(propertyName, Instance.annotation(propertyInstance, "title", dialectId));
}

// List all locations in the instance that are deprecated
for (const deprecated of AnnotatedInstance.annotatedWith(instance, "deprecated", dialectId)) {
  if (AnnotatedInstance.annotation(instance, "deprecated", dialectId)[0]) {
    logger.warn(`The value at '${deprecated.pointer}' has been deprecated.`); // => (Example) "WARN: The value at '/name' has been deprecated."
  }
}

These are available from the @hyperjump/json-schema/annotations/experimental export.

• annotate: (schemaUri: string, instance: any, outputFormat: OutputFormat = FLAG) => Promise

  Annotate an instance using the given schema. The function is curried to allow compiling the schema once and applying it to multiple instances. This may throw an InvalidSchemaError if there is a problem with the schema or a ValidationError if the instance doesn't validate against the schema.

• ValidationError: output: OutputUnit -- The errors that were found while validating the instance.

These are available from the @hyperjump/json-schema/annotated-instance/experimental export. The following functions are available in addition to the functions available in the Instance API.

• annotation: (instance: AnnotatedInstance, keyword: string, dialectId?: string) => [any]

  Get the annotations for a given keyword at the location represented by the instance object.

• annotatedWith: (instance: AnnotatedInstance, keyword: string, dialectId?: string) => [AnnotatedInstance]

  Get an array of instances for all the locations that are annotated with the given keyword.

• annotate: (instance: AnnotatedInstance, keywordId: string, value: any) => AnnotatedInstance

  Add an annotation to an instance. This is used internally, you probably don't need it.

These are available from the @hyperjump/json-schema/experimental export.

• compile: (schema: SchemaDocument) => Promise

  Return a compiled schema. This is useful if you're creating tooling for something other than validation.

• interpret: (schema: CompiledSchema, instance: Instance, outputFormat: OutputFormat = BASIC) => OutputUnit

  A curried function for validating an instance against a compiled schema. This can be useful for creating custom output formats.

Run the tests

npm test

Run the tests with a continuous test runner

npm test -- --watch