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JSON Safe Parser & Schema Validator





A code-generating JSON Schema validator that attempts to be reasonably secure.

Supports draft-04/06/07/2019-09 and the discriminator OpenAPI keyword.

Node CI Status npm codecov Total alerts


  • Converts schemas to self-contained JavaScript files, can be used in the build process.
    Integrates nicely with bundlers, so one won't need to generate code in runtime, and that works with CSP.
  • Optional requireValidation: true mode enforces full validation of the input object.
    Using mode: "strong" is recommended, — it combines that option with additional schema safety checks.
  • Does not fail open on unknown or unprocessed keywords — instead throws at build time if schema was not fully understood. That is implemented by tracking processed keywords and ensuring that none remain uncovered.
  • Does not fail open on schema problems — instead throws at build time.
    E.g. it will detect mistakes like {type: "array", "maxLength": 2}.
  • Under 1700 lines of code, non-minified.
  • Uses secure code generation approach to prevent data from schema from leaking into the generated code without being JSON-wrapped.
  • 0 dependencies
  • Very fast
  • Supports JSON Schema draft-04/06/07/2019-09 and a strict subset of the discriminator OpenAPI keyword.
  • Can assign defaults and/or remove additional properties when schema allows to do that safely. Throws at build time if those options are used with schemas that don't allow to do that safely.


npm install --save @exodus/schemasafe


Simply pass a schema to compile it:

const { validator } = require('@exodus/schemasafe')

const validate = validator({
  type: 'object',
  required: ['hello'],
  properties: {
    hello: {
      type: 'string'

console.log('should be valid', validate({ hello: 'world' }))
console.log('should not be valid', validate({}))

Or use the parser mode (running in strong mode by default):

const { parser } = require('.')

const parse = parser({
  $schema: '',
  type: 'object',
  required: ['hello'],
  properties: {
    hello: {
      pattern: '^[a-z]+$',
      type: 'string'
  additionalProperties: false

console.log('returns { valid: true, value }:', parse('{"hello": "world" }'))
console.log('returns { valid: false }:', parse('{}'))


See options documentation for the full list of supported options.

Custom formats

@exodus/schemasafe supports the formats specified in JSON schema v4 (such as date-time). If you want to add your own custom formats pass them as the formats options to the validator

const validate = validator({
  type: 'string',
  format: 'only-a'
}, {
  formats: {
    'only-a': /^a+$/

console.log(validate('aa')) // true
console.log(validate('ab')) // false

External schemas

You can pass in external schemas that you reference using the $ref attribute as the schemas option

const ext = {
  type: 'string'

const schema = {
  $ref: 'ext#' // references another schema called ext

// pass the external schemas as an option
const validate = validator(schema, { schemas: { ext: ext }})

console.log(validate('hello')) // true
console.log(validate(42)) // false

schemas can be either an object as shown above, a Map, or plain array of schemas (given that those have corresponding $id set at top level inside schemas themselves).

Enabling errors shows information about the source of the error

When the includeErrors option is set to true, @exodus/schemasafe also outputs:

  • keywordLocation: a JSON pointer string as an URI fragment indicating which sub-schema failed, e.g. #/properties/item/type
  • instanceLocation: a JSON pointer string as an URI fragment indicating which property of the object failed validation, e.g. #/item
const schema = {
  type: 'object',
  required: ['hello'],
  properties: {
    hello: {
      type: 'string'
const validate = validator(schema, { includeErrors: true })

validate({ hello: 100 });
// [ { keywordLocation: '#/properties/hello/type', instanceLocation: '#/hello' } ]

Only the first error is reported by default unless allErrors option is also set to true in addition to includeErrors.

See Error handling for more information.

Generate Modules

See the doc/samples directory to see how @exodus/schemasafe compiles the draft/2019-09 test suite.

To compile a validator function to an IIFE, call validate.toModule():

const { validator } = require('@exodus/schemasafe')

const schema = {
  type: 'string',
  format: 'hex'

// This works with custom formats as well.
const formats = {
  hex: (value) => /^0x[0-9A-Fa-f]*$/.test(value),

const validate = validator(schema, { formats })

/** Prints:
 * (function() {
 * 'use strict'
 * const format0 = (value) => /^0x[0-9A-Fa-f]*$/.test(value);
 * return (function validate(data) {
 *   if (data === undefined) data = null
 *   if (!(typeof data === "string")) return false
 *   if (!format0(data)) return false
 *   return true
 * })})();


@exodus/schemasafe uses code generation to turn a JSON schema into javascript code that is easily optimizeable by v8 and extremely fast.

See Performance for information on options that might affect performance both ways.

Previous work

This is based on a heavily rewritten version of the amazing (but outdated) is-my-json-valid by @mafintosh.

Compared to is-my-json-valid, @exodus/schemasafe adds security-first design, many new features, newer spec versions support, slimmer and more maintainable code, 0 dependencies, self-contained JS module generation, fixes bugs and adds better test coverage, and drops support for outdated Node.js versions.