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1.9.2 • Public • Published

Hyperjump - JSON Schema

A collection of modules for working with JSON Schemas.

  • Validate JSON-compatible values against a JSON Schemas
    • Dialects: draft-2020-12, draft-2019-09, draft-07, draft-06, draft-04
    • Schemas can reference other schemas using a different dialect
    • Work directly with schemas on the filesystem or HTTP
  • OpenAPI
    • Versions: 3.0, 3.1
    • Validate an OpenAPI document
    • Validate values against a schema from an OpenAPI document
  • 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.
  • Utilities for building non-validation JSON Schema tooling
  • Utilities for working with annotations


Includes support for node.js/bun.js (ES Modules, TypeScript) and browsers (works with CSP unsafe-eval).


npm install @hyperjump/json-schema


This package uses the package.json "exports" field. TypeScript understands "exports", but you need to change a couple settings in your tsconfig.json for it to work.

    "module": "Node16", // or "NodeNext"
    "moduleResolution": "Node16", // or "NodeNext"


The API for this library is divided into two categories: Stable and Experimental. The Stable API 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 { registerSchema, validate } from "@hyperjump/json-schema/draft-2020-12";

You can import support for additional versions as needed.

import { registerSchema, 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 near future.

Validate schema from JavaScript

  $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

Schemas that are available on the web can be loaded automatically without needing to load them manually.

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

When running on the server, you can also load schemas directly from the filesystem. When fetching from the file system, there are limitations for security reasons. You can only reference a schema identified by a file URI scheme (file:///path/to/my/schemas) from another schema identified by a file URI scheme. Also, a schema is not allowed to self-identify ($id) with a file: URI scheme.

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

If the schema URI is relative, the base URI in the browser is the browser location and the base URI on the server is the current working directory. This is the preferred way to work with file-based schemas on the server.

const output = await validate(`./string.schema.json`, "foo");

You can add/modify/remove support for any URI scheme using the plugin system provided by @hyperjump/browser.


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.

import { 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("./example.openapi.json#/components/schemas/foo", 42);

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 replace the JSON parts with YAML.

Media types

This library uses media types to determine how to parse a retrieved document. It will never assume the retrieved document is a schema. By default it's configured to accept documents with a application/schema+json Content-Type header (web) or a .schema.json file extension (filesystem).

You can add/modify/remove support for any media-type using the plugin system provided by @hyperjump/browser. The following example shows how to add support for JSON Schemas written in YAML.

import YAML from "yaml";
import contentTypeParser from "content-type";
import { addMediaTypePlugin } from "@hyperjump/browser";
import { buildSchemaDocument } from "@hyperjump/json-schema/experimental";

addMediaTypePlugin("application/schema+yaml", {
  parse: async (response) => {
    const contentType = contentTypeParser.parse(response.headers.get("content-type") ?? "");
    const contextDialectId = contentType.parameters.schema ?? contentType.parameters.profile;

    const foo = YAML.parse(await response.text());
    return buildSchemaDocument(foo, response.url, contextDialectId);
  fileMatcher: (path) => path.endsWith(".schema.yml")


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

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

    Add a schema the local schema registry. When this schema is needed, it will be loaded from the register rather than the filesystem or network. If a schema with the same identifier is already registered, an exception will be throw.

  • unregisterSchema: (uri: string) => void

    Remove a schema from the local schema registry.

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

    Load a schema manually rather than fetching it from the filesystem or over the network. Any schema already registered with the same identifier will be replaced with no warning.

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

    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.

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.



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

  "$schema": "https://json-schema.org/draft/2020-12/schema",

  "type": "object",
  "properties": {
    "foo": { "$ref": "/string" }
}, "https://example.com/main");

  "$schema": "https://json-schema.org/draft/2020-12/schema",

  "type": "string"
}, "https://example.com/string");

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


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

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

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


    • alwaysIncludeDialect: boolean (default: false) -- Include dialect even when it isn't strictly needed
    • definitionNamingStrategy: "uri" | "uuid" (default: "uri") -- By default the name used in definitions for embedded schemas will match the identifier of the embedded schema. Alternatively, you can use a UUID instead of the schema's URI.
    • externalSchemas: string[] (default: []) -- A list of schemas URIs that are available externally and should not be included in the bundle.


Output Formats

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. The official output format is still evolving, so these may change or be replaced in the future.

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";

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

Custom Keywords, Vocabularies, and Dialects

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.

Schemas are represented using the @hyperjump/browser package. You'll use that API to traverse schemas. @hyperjump/browser uses async generators to iterate over arrays and objects. If you like using higher order functions like map/filter/reduce, see @hyperjump/pact for utilities for working with generators and async generators.

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

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

  compile: async (schema, ast) => {
    const subSchemas = [];
    for await (const subSchema of Browser.iter(schema)) {
      subSchemas.push(Validation.compile(subSchema, ast));
    return subSchemas;

    // Alternative using @hyperjump/pact
    // return pipe(
    //   Browser.iter(schema),
    //   asyncMap((subSchema) => Validation.compile(subSchema, ast)),
    //   asyncCollectArray
    // );

  interpret: (implies, instance, ast, dynamicAnchors, quiet) => {
    return implies.reduce((valid, schema) => {
      return !valid || 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
  "$id": "https://example.com/meta/logic",
  "$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
  "$id": "https://example.com/dialect/logic",
  "$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
  "$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.

  "$id": "https://example.com/meta-schema1",
  "$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

  $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.

    • 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: Browser, ast: AST, parentSchema: Browser) => Promise<any>

        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: any, instance: JsonNode, ast: AST, dynamicAnchors: object, quiet: boolean, schemaLocation: string) => 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: any, instance: JsonNode, ast: AST, dynamicAnchors: object) => Set<string> | false

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

      • collectEvaluatedItems?: (compiledKeywordValue: A, instance: JsonNode, ast: AST, dynamicAnchors: object) => Set<number> | false

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

      • collectExternalIds?: (visited: Set<string>, parentSchema: Browser, schema: Browser) => Set<string> If the keyword is an applicator, it will need to implement this function to work properly with the bundle feature.

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

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

  • getKeywordId: (keywordName: string, dialectId: string) => string

    Get the identifier for a keyword by its name.

  • getKeyword: (keywordId: string) => Keyword

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

  • getKeywordByName: (keywordName: string, dialectId: string) => Keyword

    Get a keyword object by its name. 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] }, allowUnknownKeywords: boolean = false) => 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.

  • unloadDialect: (dialectId: string) => void

    Remove a dialect. You shouldn't need to use this function. It's called for you when you call unregisterSchema.

  • 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.

  • getSchema: (uri: string, browser?: Browser) => Promise<Browser>

    Get a schema by it's URI taking the local schema registry into account.

  • buildSchemaDocument: (schema: SchemaObject | boolean, retrievalUri?: string, contextDialectId?: string) => SchemaDocument

    Build a SchemaDocument from a JSON-compatible value. You might use this if you're creating a custom media type plugin, such as supporting JSON Schemas in YAML.

  • canonicalUri: (schema: Browser) => string

    Returns a URI for the schema.

  • toSchema: (schema: Browser, options: ToSchemaOptions) => object

    Get a raw schema from a Schema Document.

    • ToSchemaOptions: object

      • contextDialectId: string (default: "") -- If the dialect of the schema matches this value, the $schema keyword will be omitted.
      • includeDialect: "auto" | "always" | "never" (default: "auto") -- If "auto", $schema will only be included if it differs from contextDialectId.
      • selfIdentify: boolean (default: false) -- If true, $id will be included.
      • contextUri: string (default: "") -- $ids will be relative to this URI.
      • includeEmbedded: boolean (default: true) -- If false, embedded schemas will be unbundled from the schema.
  • compile: (schema: Browser) => Promise<CompiledSchema>

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

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

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

  • OutputFormat: FLAG | BASIC

    In addition to the FLAG output format in the Stable API, the Experimental API includes support for the BASIC format 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.

Instance API (experimental)

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

This library uses JsonNode objects to represent instances. You'll work with these objects if you create a custom keyword.

This API uses generators to iterate over arrays and objects. If you like using higher order functions like map/filter/reduce, see @hyperjump/pact for utilities for working with generators and async generators.

  • fromJs: (value: any, uri?: string) => JsonNode

    Construct a JsonNode from a JavaScript value.

  • cons: (baseUri: string, pointer: string, value: any, type: string, children: JsonNode[], parent?: JsonNode) => JsonNode

    Construct a JsonNode. This is used internally. You probably want fromJs instead.

  • get: (url: string, instance: JsonNode) => JsonNode

    Apply a same-resource reference to a JsonNode.

  • uri: (instance: JsonNode) => string

    Returns a URI for the value the JsonNode represents.

  • value: (instance: JsonNode) => any

    Returns the value the JsonNode represents.

  • has: (key: string, instance: JsonNode) => boolean

    Returns whether or not "key" is a property name in a JsonNode that represents an object.

  • typeOf: (instance: JsonNode) => string

    The JSON type of the JsonNode. In addition to the standard JSON types, there's also the property type that indicates a property name/value pair in an object.

  • step: (key: string, instance: JsonNode) => JsonType

    Similar to indexing into a object or array using the [] operator.

  • iter: (instance: JsonNode) => Generator<JsonNode>

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

  • entries: (instance: JsonNode) => Generator<[JsonNode, JsonNode]>

    Similar to Object.entries, but yields JsonNodes for keys and values.

  • values: (instance: JsonNode) => Generator<JsonNode>

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

  • keys: (instance: JsonNode) => Generator<JsonNode>

    Similar to Object.keys, but yields JsonNodes for keys.

  • length: (instance: JsonNode) => number

    Similar to Array.prototype.length.

Annotations (experimental)

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 a (JsonNode)[#/instance-api-experimental] AST. You can use this AST to traverse the data structure and get annotations for the values it represents.

import { registerSchema } from "@hyperjump/json-schema/draft/2020-12";
import { annotate } 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";

  "$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 [propertyNameNode, propertyInstance] of AnnotatedInstance.entries(instance)) {
  const propertyName = AnnotatedInstance.value(propertyName);
  console.log(propertyName, AnnotatedInstance.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(deprecated, "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 = BASIC) => Promise<JsonNode>

    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.

  • interpret: (compiledSchema: CompiledSchema, instance: JsonNode, outputFormat: OutputFormat = BASIC) => JsonNode

    Annotate a JsonNode object rather than a plain JavaScript value. This might be useful when building tools on top of the annotation functionality, but you probably don't need it.

  • ValidationError: Error & { output: OutputUnit } The output field contains an OutputUnit with information about the error.

AnnotatedInstance API (experimental)

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: JsonNode, keyword: string, dialect?: string): any[];

    Get the annotations for a keyword for the value represented by the JsonNode.

  • annotatedWith: (instance: JsonNode, keyword: string, dialect?: string): JsonNode[];

    Get all JsonNodes that are annotated with the given keyword.

  • setAnnotation: (instance: JsonNode, keywordId: string, value: any) => JsonNode

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



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npm test

Run the tests with a continuous test runner

npm test -- --watch

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