The fastest JSON Schema Validator

.zip .tar.gz

Ajv: Another JSON Schema Validator

The fastest JSON Schema validator for Node.js and browser. Supports draft-04/06/07.

Build Status npm npm (beta) npm downloads Coverage Status Gitter GitHub Sponsors

Ajv v7 beta is released

Ajv version 7.0.0-beta.0 is released with these changes:

Please note:

See release notes for the details.

To install the new version:

npm install ajv@beta

See Getting started with v7 for code example.

Mozilla MOSS grant and OpenJS Foundation


Ajv has been awarded a grant from Mozilla’s Open Source Support (MOSS) program in the “Foundational Technology” track! It will sponsor the development of Ajv support of JSON Schema version 2019-09 and of JSON Type Definition.

Ajv also joined OpenJS Foundation – having this support will help ensure the longevity and stability of Ajv for all its users.

This blog post has more details.

I am looking for the long term maintainers of Ajv – working with ReadySet, also sponsored by Mozilla, to establish clear guidelines for the role of a “maintainer” and the contribution standards, and to encourage a wider, more inclusive, contribution from the community.

Please sponsor Ajv development

Since I asked to support Ajv development 40 people and 6 organizations contributed via GitHub and OpenCollective - this support helped receiving the MOSS grant!

Your continuing support is very important - the funds will be used to develop and maintain Ajv once the next major version is released.

Please sponsor Ajv via:

Thank you.

Open Collective sponsors

Using version 6

JSON Schema draft-07 is published.

Ajv version 6.0.0 that supports draft-07 is released. It may require either migrating your schemas or updating your code (to continue using draft-04 and v5 schemas, draft-06 schemas will be supported without changes).

Please note: To use Ajv with draft-06 schemas you need to explicitly add the meta-schema to the validator instance:


To use Ajv with draft-04 schemas in addition to explicitly adding meta-schema you also need to use option schemaId:

var ajv = new Ajv({schemaId: 'id'});
// If you want to use both draft-04 and draft-06/07 schemas:
// var ajv = new Ajv({schemaId: 'auto'});



Ajv generates code using doT templates to turn JSON Schemas into super-fast validation functions that are efficient for v8 optimization.

Currently Ajv is the fastest and the most standard compliant validator according to these benchmarks:

Performance of different validators by json-schema-benchmark:

[![performance](,y&cht=bhs&chco=76A4FB&chls=2.0&chbh=32,4,1&chs=600x416&chxl=-1: djv ajv json-schema-validator-generator jsen is-my-json-valid themis z-schema jsck skeemas json-schema-library tv4&chd=t:100,98,72.1,66.8,50.1,15.1,6.1,3.8,1.2,0.7,0.2)](



npm install ajv

Getting started

Try it in the Node.js REPL:

The fastest validation call:

// Node.js require:
var Ajv = require('ajv');
// or ESM/TypeScript import
import Ajv from 'ajv';

var ajv = new Ajv(); // options can be passed, e.g. {allErrors: true}
var validate = ajv.compile(schema);
var valid = validate(data);
if (!valid) console.log(validate.errors);

or with less code

// ...
var valid = ajv.validate(schema, data);
if (!valid) console.log(ajv.errors);
// ...


// ...
var valid = ajv.addSchema(schema, 'mySchema')
               .validate('mySchema', data);
if (!valid) console.log(ajv.errorsText());
// ...

See API and Options for more details.

Ajv compiles schemas to functions and caches them in all cases (using schema serialized with fast-json-stable-stringify or a custom function as a key), so that the next time the same schema is used (not necessarily the same object instance) it won’t be compiled again.

The best performance is achieved when using compiled functions returned by compile or getSchema methods (there is no additional function call).

Please note: every time a validation function or ajv.validate are called errors property is overwritten. You need to copy errors array reference to another variable if you want to use it later (e.g., in the callback). See Validation errors

Note for TypeScript users: ajv provides its own TypeScript declarations out of the box, so you don’t need to install the deprecated @types/ajv module.

Using in browser

You can require Ajv directly from the code you browserify - in this case Ajv will be a part of your bundle.

If you need to use Ajv in several bundles you can create a separate UMD bundle using npm run bundle script (thanks to siddo420).

Then you need to load Ajv in the browser:

<script src="ajv.min.js"></script>

This bundle can be used with different module systems; it creates global Ajv if no module system is found.

The browser bundle is available on cdnjs.

Ajv is tested with these browsers:

Sauce Test Status

Please note: some frameworks, e.g. Dojo, may redefine global require in such way that is not compatible with CommonJS module format. In such case Ajv bundle has to be loaded before the framework and then you can use global Ajv (see issue #234).

Ajv and Content Security Policies (CSP)

If you’re using Ajv to compile a schema (the typical use) in a browser document that is loaded with a Content Security Policy (CSP), that policy will require a script-src directive that includes the value 'unsafe-eval'. :warning: NOTE, however, that unsafe-eval is NOT recommended in a secure CSP[1], as it has the potential to open the document to cross-site scripting (XSS) attacks.

In order to make use of Ajv without easing your CSP, you can pre-compile a schema using the CLI. This will transpile the schema JSON into a JavaScript file that exports a validate function that works simlarly to a schema compiled at runtime.

Note that pre-compilation of schemas is performed using ajv-pack and there are some limitations to the schema features it can compile. A successfully pre-compiled schema is equivalent to the same schema compiled at runtime.

Command line interface

CLI is available as a separate npm package ajv-cli. It supports:

Validation keywords

Ajv supports all validation keywords from draft-07 of JSON Schema standard:

With ajv-keywords package Ajv also supports validation keywords from JSON Schema extension proposals for JSON Schema standard:

See JSON Schema validation keywords for more details.

Annotation keywords

JSON Schema specification defines several annotation keywords that describe schema itself but do not perform any validation.

Please note: Ajv does not implement validation of the keywords examples, contentEncoding and contentMediaType but it reserves them. If you want to create a plugin that implements some of them, it should remove these keywords from the instance.


Ajv implements formats defined by JSON Schema specification and several other formats. It is recommended NOT to use “format” keyword implementations with untrusted data, as they use potentially unsafe regular expressions - see ReDoS attack.

Please note: if you need to use “format” keyword to validate untrusted data, you MUST assess their suitability and safety for your validation scenarios.

The following formats are implemented for string validation with “format” keyword:

Please note: JSON Schema draft-07 also defines formats iri, iri-reference, idn-hostname and idn-email for URLs, hostnames and emails with international characters. Ajv does not implement these formats. If you create Ajv plugin that implements them please make a PR to mention this plugin here.

There are two modes of format validation: fast and full. This mode affects formats date, time, date-time, uri, uri-reference, and email. See Options for details.

You can add additional formats and replace any of the formats above using addFormat method.

The option unknownFormats allows changing the default behaviour when an unknown format is encountered. In this case Ajv can either fail schema compilation (default) or ignore it (default in versions before 5.0.0). You also can allow specific format(s) that will be ignored. See Options for details.

You can find regular expressions used for format validation and the sources that were used in formats.js.

Combining schemas with $ref

You can structure your validation logic across multiple schema files and have schemas reference each other using $ref keyword.


var schema = {
  "$id": "",
  "type": "object",
  "properties": {
    "foo": { "$ref": "defs.json#/definitions/int" },
    "bar": { "$ref": "defs.json#/definitions/str" }

var defsSchema = {
  "$id": "",
  "definitions": {
    "int": { "type": "integer" },
    "str": { "type": "string" }

Now to compile your schema you can either pass all schemas to Ajv instance:

var ajv = new Ajv({schemas: [schema, defsSchema]});
var validate = ajv.getSchema('');

or use addSchema method:

var ajv = new Ajv;
var validate = ajv.addSchema(defsSchema)

See Options and addSchema method.

Please note:

$data reference

With $data option you can use values from the validated data as the values for the schema keywords. See proposal for more information about how it works.

$data reference is supported in the keywords: const, enum, format, maximum/minimum, exclusiveMaximum / exclusiveMinimum, maxLength / minLength, maxItems / minItems, maxProperties / minProperties, formatMaximum / formatMinimum, formatExclusiveMaximum / formatExclusiveMinimum, multipleOf, pattern, required, uniqueItems.

The value of “$data” should be a JSON-pointer to the data (the root is always the top level data object, even if the $data reference is inside a referenced subschema) or a relative JSON-pointer (it is relative to the current point in data; if the $data reference is inside a referenced subschema it cannot point to the data outside of the root level for this subschema).


This schema requires that the value in property smaller is less or equal than the value in the property larger:

var ajv = new Ajv({$data: true});

var schema = {
  "properties": {
    "smaller": {
      "type": "number",
      "maximum": { "$data": "1/larger" }
    "larger": { "type": "number" }

var validData = {
  smaller: 5,
  larger: 7

ajv.validate(schema, validData); // true

This schema requires that the properties have the same format as their field names:

var schema = {
  "additionalProperties": {
    "type": "string",
    "format": { "$data": "0#" }

var validData = {
  'date-time': '1963-06-19T08:30:06.283185Z',
  email: ''

$data reference is resolved safely - it won’t throw even if some property is undefined. If $data resolves to undefined the validation succeeds (with the exclusion of const keyword). If $data resolves to incorrect type (e.g. not “number” for maximum keyword) the validation fails.

$merge and $patch keywords

With the package ajv-merge-patch you can use the keywords $merge and $patch that allow extending JSON Schemas with patches using formats JSON Merge Patch (RFC 7396) and JSON Patch (RFC 6902).

To add keywords $merge and $patch to Ajv instance use this code:



Using $merge:

  "$merge": {
    "source": {
      "type": "object",
      "properties": { "p": { "type": "string" } },
      "additionalProperties": false
    "with": {
      "properties": { "q": { "type": "number" } }

Using $patch:

  "$patch": {
    "source": {
      "type": "object",
      "properties": { "p": { "type": "string" } },
      "additionalProperties": false
    "with": [
      { "op": "add", "path": "/properties/q", "value": { "type": "number" } }

The schemas above are equivalent to this schema:

  "type": "object",
  "properties": {
    "p": { "type": "string" },
    "q": { "type": "number" }
  "additionalProperties": false

The properties source and with in the keywords $merge and $patch can use absolute or relative $ref to point to other schemas previously added to the Ajv instance or to the fragments of the current schema.

See the package ajv-merge-patch for more information.

Defining custom keywords

The advantages of using custom keywords are:

If a keyword is used only for side-effects and its validation result is pre-defined, use option valid: true/false in keyword definition to simplify both generated code (no error handling in case of valid: true) and your keyword functions (no need to return any validation result).

The concerns you have to be aware of when extending JSON Schema standard with custom keywords are the portability and understanding of your schemas. You will have to support these custom keywords on other platforms and to properly document these keywords so that everybody can understand them in your schemas.

You can define custom keywords with addKeyword method. Keywords are defined on the ajv instance level - new instances will not have previously defined keywords.

Ajv allows defining keywords with:

Example. range and exclusiveRange keywords using compiled schema:

ajv.addKeyword('range', {
  type: 'number',
  compile: function (sch, parentSchema) {
    var min = sch[0];
    var max = sch[1];

    return parentSchema.exclusiveRange === true
            ? function (data) { return data > min && data < max; }
            : function (data) { return data >= min && data <= max; }

var schema = { "range": [2, 4], "exclusiveRange": true };
var validate = ajv.compile(schema);
console.log(validate(2.01)); // true
console.log(validate(3.99)); // true
console.log(validate(2)); // false
console.log(validate(4)); // false

Several custom keywords (typeof, instanceof, range and propertyNames) are defined in ajv-keywords package - they can be used for your schemas and as a starting point for your own custom keywords.

See Defining custom keywords for more details.

Asynchronous schema compilation

During asynchronous compilation remote references are loaded using supplied function. See compileAsync method and loadSchema option.


var ajv = new Ajv({ loadSchema: loadSchema });

ajv.compileAsync(schema).then(function (validate) {
  var valid = validate(data);
  // ...

function loadSchema(uri) {
  return request.json(uri).then(function (res) {
    if (res.statusCode >= 400)
      throw new Error('Loading error: ' + res.statusCode);
    return res.body;

Please note: Option missingRefs should NOT be set to "ignore" or "fail" for asynchronous compilation to work.

Asynchronous validation

Example in Node.js REPL:

You can define custom formats and keywords that perform validation asynchronously by accessing database or some other service. You should add async: true in the keyword or format definition (see addFormat, addKeyword and Defining custom keywords).

If your schema uses asynchronous formats/keywords or refers to some schema that contains them it should have "$async": true keyword so that Ajv can compile it correctly. If asynchronous format/keyword or reference to asynchronous schema is used in the schema without $async keyword Ajv will throw an exception during schema compilation.

Please note: all asynchronous subschemas that are referenced from the current or other schemas should have "$async": true keyword as well, otherwise the schema compilation will fail.

Validation function for an asynchronous custom format/keyword should return a promise that resolves with true or false (or rejects with new Ajv.ValidationError(errors) if you want to return custom errors from the keyword function).

Ajv compiles asynchronous schemas to es7 async functions that can optionally be transpiled with nodent. Async functions are supported in Node.js 7+ and all modern browsers. You can also supply any other transpiler as a function via processCode option. See Options.

The compiled validation function has $async: true property (if the schema is asynchronous), so you can differentiate these functions if you are using both synchronous and asynchronous schemas.

Validation result will be a promise that resolves with validated data or rejects with an exception Ajv.ValidationError that contains the array of validation errors in errors property.


var ajv = new Ajv;
// require('ajv-async')(ajv);

ajv.addKeyword('idExists', {
  async: true,
  type: 'number',
  validate: checkIdExists

function checkIdExists(schema, data) {
  return knex(schema.table)
  .where('id', data)
  .then(function (rows) {
    return !!rows.length; // true if record is found

var schema = {
  "$async": true,
  "properties": {
    "userId": {
      "type": "integer",
      "idExists": { "table": "users" }
    "postId": {
      "type": "integer",
      "idExists": { "table": "posts" }

var validate = ajv.compile(schema);

validate({ userId: 1, postId: 19 })
.then(function (data) {
  console.log('Data is valid', data); // { userId: 1, postId: 19 }
.catch(function (err) {
  if (!(err instanceof Ajv.ValidationError)) throw err;
  // data is invalid
  console.log('Validation errors:', err.errors);

Using transpilers with asynchronous validation functions.

ajv-async uses nodent to transpile async functions. To use another transpiler you should separately install it (or load its bundle in the browser).

Using nodent

var ajv = new Ajv;
// in the browser if you want to load ajv-async bundle separately you can:
// window.ajvAsync(ajv);
var validate = ajv.compile(schema); // transpiled es7 async function

Using other transpilers

var ajv = new Ajv({ processCode: transpileFunc });
var validate = ajv.compile(schema); // transpiled es7 async function

See Options.

Security considerations

JSON Schema, if properly used, can replace data sanitisation. It doesn’t replace other API security considerations. It also introduces additional security aspects to consider.

Security contact

To report a security vulnerability, please use the Tidelift security contact. Tidelift will coordinate the fix and disclosure. Please do NOT report security vulnerabilities via GitHub issues.

Untrusted schemas

Ajv treats JSON schemas as trusted as your application code. This security model is based on the most common use case, when the schemas are static and bundled together with the application.

If your schemas are received from untrusted sources (or generated from untrusted data) there are several scenarios you need to prevent:

It is difficult to predict all the scenarios, but at the very least it may help to limit the size of untrusted schemas (e.g. limit JSON string length) and also the maximum schema object depth (that can be high for relatively small JSON strings). You also may want to mitigate slow regular expressions in pattern and patternProperties keywords.

Regardless the measures you take, using untrusted schemas increases security risks.

Circular references in JavaScript objects

Ajv does not support schemas and validated data that have circular references in objects. See issue #802.

An attempt to compile such schemas or validate such data would cause stack overflow (or will not complete in case of asynchronous validation). Depending on the parser you use, untrusted data can lead to circular references.

Security risks of trusted schemas

Some keywords in JSON Schemas can lead to very slow validation for certain data. These keywords include (but may be not limited to):

Please note: The suggestions above to prevent slow validation would only work if you do NOT use allErrors: true in production code (using it would continue validation after validation errors).

You can validate your JSON schemas against this meta-schema to check that these recommendations are followed:

const isSchemaSecure = ajv.compile(require('ajv/lib/refs/json-schema-secure.json'));

const schema1 = {format: 'email'};
isSchemaSecure(schema1); // false

const schema2 = {format: 'email', maxLength: MAX_LENGTH};
isSchemaSecure(schema2); // true

Please note: following all these recommendation is not a guarantee that validation of untrusted data is safe - it can still lead to some undesirable results.

Content Security Policies (CSP)

See Ajv and Content Security Policies (CSP)

ReDoS attack

Certain regular expressions can lead to the exponential evaluation time even with relatively short strings.

Please assess the regular expressions you use in the schemas on their vulnerability to this attack - see safe-regex, for example.

Please note: some formats that Ajv implements use regular expressions that can be vulnerable to ReDoS attack, so if you use Ajv to validate data from untrusted sources it is strongly recommended to consider the following:

Whatever mitigation you choose, please assume all formats provided by Ajv as potentially unsafe and make your own assessment of their suitability for your validation scenarios.

Filtering data

With option removeAdditional (added by andyscott) you can filter data during the validation.

This option modifies original data.


var ajv = new Ajv({ removeAdditional: true });
var schema = {
  "additionalProperties": false,
  "properties": {
    "foo": { "type": "number" },
    "bar": {
      "additionalProperties": { "type": "number" },
      "properties": {
        "baz": { "type": "string" }

var data = {
  "foo": 0,
  "additional1": 1, // will be removed; `additionalProperties` == false
  "bar": {
    "baz": "abc",
    "additional2": 2 // will NOT be removed; `additionalProperties` != false

var validate = ajv.compile(schema);

console.log(validate(data)); // true
console.log(data); // { "foo": 0, "bar": { "baz": "abc", "additional2": 2 }

If removeAdditional option in the example above were "all" then both additional1 and additional2 properties would have been removed.

If the option were "failing" then property additional1 would have been removed regardless of its value and property additional2 would have been removed only if its value were failing the schema in the inner additionalProperties (so in the example above it would have stayed because it passes the schema, but any non-number would have been removed).

Please note: If you use removeAdditional option with additionalProperties keyword inside anyOf/oneOf keywords your validation can fail with this schema, for example:

  "type": "object",
  "oneOf": [
      "properties": {
        "foo": { "type": "string" }
      "required": [ "foo" ],
      "additionalProperties": false
      "properties": {
        "bar": { "type": "integer" }
      "required": [ "bar" ],
      "additionalProperties": false

The intention of the schema above is to allow objects with either the string property “foo” or the integer property “bar”, but not with both and not with any other properties.

With the option removeAdditional: true the validation will pass for the object { "foo": "abc"} but will fail for the object {"bar": 1}. It happens because while the first subschema in oneOf is validated, the property bar is removed because it is an additional property according to the standard (because it is not included in properties keyword in the same schema).

While this behaviour is unexpected (issues #129, #134), it is correct. To have the expected behaviour (both objects are allowed and additional properties are removed) the schema has to be refactored in this way:

  "type": "object",
  "properties": {
    "foo": { "type": "string" },
    "bar": { "type": "integer" }
  "additionalProperties": false,
  "oneOf": [
    { "required": [ "foo" ] },
    { "required": [ "bar" ] }

The schema above is also more efficient - it will compile into a faster function.

Assigning defaults

With option useDefaults Ajv will assign values from default keyword in the schemas of properties and items (when it is the array of schemas) to the missing properties and items.

With the option value "empty" properties and items equal to null or "" (empty string) will be considered missing and assigned defaults.

This option modifies original data.

Please note: the default value is inserted in the generated validation code as a literal, so the value inserted in the data will be the deep clone of the default in the schema.

Example 1 (default in properties):

var ajv = new Ajv({ useDefaults: true });
var schema = {
  "type": "object",
  "properties": {
    "foo": { "type": "number" },
    "bar": { "type": "string", "default": "baz" }
  "required": [ "foo", "bar" ]

var data = { "foo": 1 };

var validate = ajv.compile(schema);

console.log(validate(data)); // true
console.log(data); // { "foo": 1, "bar": "baz" }

Example 2 (default in items):

var schema = {
  "type": "array",
  "items": [
    { "type": "number" },
    { "type": "string", "default": "foo" }

var data = [ 1 ];

var validate = ajv.compile(schema);

console.log(validate(data)); // true
console.log(data); // [ 1, "foo" ]

default keywords in other cases are ignored:

The strictDefaults option customizes Ajv’s behavior for the defaults that Ajv ignores (true raises an error, and "log" outputs a warning).

Coercing data types

When you are validating user inputs all your data properties are usually strings. The option coerceTypes allows you to have your data types coerced to the types specified in your schema type keywords, both to pass the validation and to use the correctly typed data afterwards.

This option modifies original data.

Please note: if you pass a scalar value to the validating function its type will be coerced and it will pass the validation, but the value of the variable you pass won’t be updated because scalars are passed by value.

Example 1:

var ajv = new Ajv({ coerceTypes: true });
var schema = {
  "type": "object",
  "properties": {
    "foo": { "type": "number" },
    "bar": { "type": "boolean" }
  "required": [ "foo", "bar" ]

var data = { "foo": "1", "bar": "false" };

var validate = ajv.compile(schema);

console.log(validate(data)); // true
console.log(data); // { "foo": 1, "bar": false }

Example 2 (array coercions):

var ajv = new Ajv({ coerceTypes: 'array' });
var schema = {
  "properties": {
    "foo": { "type": "array", "items": { "type": "number" } },
    "bar": { "type": "boolean" }

var data = { "foo": "1", "bar": ["false"] };

var validate = ajv.compile(schema);

console.log(validate(data)); // true
console.log(data); // { "foo": [1], "bar": false }

The coercion rules, as you can see from the example, are different from JavaScript both to validate user input as expected and to have the coercion reversible (to correctly validate cases where different types are defined in subschemas of “anyOf” and other compound keywords).

See Coercion rules for details.


new Ajv(Object options) -> Object

Create Ajv instance.

.compile(Object schema) -> Function<Object data>

Generate validating function and cache the compiled schema for future use.

Validating function returns a boolean value. This function has properties errors and schema. Errors encountered during the last validation are assigned to errors property (it is assigned null if there was no errors). schema property contains the reference to the original schema.

The schema passed to this method will be validated against meta-schema unless validateSchema option is false. If schema is invalid, an error will be thrown. See options.

.compileAsync(Object schema [, Boolean meta] [, Function callback]) -> Promise

Asynchronous version of compile method that loads missing remote schemas using asynchronous function in options.loadSchema. This function returns a Promise that resolves to a validation function. An optional callback passed to compileAsync will be called with 2 parameters: error (or null) and validating function. The returned promise will reject (and the callback will be called with an error) when:

The function compiles schema and loads the first missing schema (or meta-schema) until all missing schemas are loaded.

You can asynchronously compile meta-schema by passing true as the second parameter.

See example in Asynchronous compilation.

.validate(Object schema|String key|String ref, data) -> Boolean

Validate data using passed schema (it will be compiled and cached).

Instead of the schema you can use the key that was previously passed to addSchema, the schema id if it was present in the schema or any previously resolved reference.

Validation errors will be available in the errors property of Ajv instance (null if there were no errors).

Please note: every time this method is called the errors are overwritten so you need to copy them to another variable if you want to use them later.

If the schema is asynchronous (has $async keyword on the top level) this method returns a Promise. See Asynchronous validation.

.addSchema(Array<Object>|Object schema [, String key]) -> Ajv

Add schema(s) to validator instance. This method does not compile schemas (but it still validates them). Because of that dependencies can be added in any order and circular dependencies are supported. It also prevents unnecessary compilation of schemas that are containers for other schemas but not used as a whole.

Array of schemas can be passed (schemas should have ids), the second parameter will be ignored.

Key can be passed that can be used to reference the schema and will be used as the schema id if there is no id inside the schema. If the key is not passed, the schema id will be used as the key.

Once the schema is added, it (and all the references inside it) can be referenced in other schemas and used to validate data.

Although addSchema does not compile schemas, explicit compilation is not required - the schema will be compiled when it is used first time.

By default the schema is validated against meta-schema before it is added, and if the schema does not pass validation the exception is thrown. This behaviour is controlled by validateSchema option.

Please note: Ajv uses the method chaining syntax for all methods with the prefix add* and remove*. This allows you to do nice things like the following.

var validate = new Ajv().addSchema(schema).addFormat(name, regex).getSchema(uri);
.addMetaSchema(Array<Object>|Object schema [, String key]) -> Ajv

Adds meta schema(s) that can be used to validate other schemas. That function should be used instead of addSchema because there may be instance options that would compile a meta schema incorrectly (at the moment it is removeAdditional option).

There is no need to explicitly add draft-07 meta schema ( - it is added by default, unless option meta is set to false. You only need to use it if you have a changed meta-schema that you want to use to validate your schemas. See validateSchema.

.validateSchema(Object schema) -> Boolean

Validates schema. This method should be used to validate schemas rather than validate due to the inconsistency of uri format in JSON Schema standard.

By default this method is called automatically when the schema is added, so you rarely need to use it directly.

If schema doesn’t have $schema property, it is validated against draft 6 meta-schema (option meta should not be false).

If schema has $schema property, then the schema with this id (that should be previously added) is used to validate passed schema.

Errors will be available at ajv.errors.

.getSchema(String key) -> Function<Object data>

Retrieve compiled schema previously added with addSchema by the key passed to addSchema or by its full reference (id). The returned validating function has schema property with the reference to the original schema.

.removeSchema([Object schema|String key|String ref|RegExp pattern]) -> Ajv

Remove added/cached schema. Even if schema is referenced by other schemas it can be safely removed as dependent schemas have local references.

Schema can be removed using:

If no parameter is passed all schemas but meta-schemas will be removed and the cache will be cleared.

.addFormat(String name, String|RegExp|Function|Object format) -> Ajv

Add custom format to validate strings or numbers. It can also be used to replace pre-defined formats for Ajv instance.

Strings are converted to RegExp.

Function should return validation result as true or false.

If object is passed it should have properties validate, compare and async:

Custom formats can be also added via formats option.

.addKeyword(String keyword, Object definition) -> Ajv

Add custom validation keyword to Ajv instance.

Keyword should be different from all standard JSON Schema keywords and different from previously defined keywords. There is no way to redefine keywords or to remove keyword definition from the instance.

Keyword must start with a letter, _ or $, and may continue with letters, numbers, _, $, or -. It is recommended to use an application-specific prefix for keywords to avoid current and future name collisions.

Example Keywords:

Keyword definition is an object with the following properties:

compile, macro and inline are mutually exclusive, only one should be used at a time. validate can be used separately or in addition to them to support $data reference.

Please note: If the keyword is validating data type that is different from the type(s) in its definition, the validation function will not be called (and expanded macro will not be used), so there is no need to check for data type inside validation function or inside schema returned by macro function (unless you want to enforce a specific type and for some reason do not want to use a separate type keyword for that). In the same way as standard keywords work, if the keyword does not apply to the data type being validated, the validation of this keyword will succeed.

See Defining custom keywords for more details.

.getKeyword(String keyword) -> Object|Boolean

Returns custom keyword definition, true for pre-defined keywords and false if the keyword is unknown.

.removeKeyword(String keyword) -> Ajv

Removes custom or pre-defined keyword so you can redefine them.

While this method can be used to extend pre-defined keywords, it can also be used to completely change their meaning - it may lead to unexpected results.

Please note: schemas compiled before the keyword is removed will continue to work without changes. To recompile schemas use removeSchema method and compile them again.

.errorsText([Array<Object> errors [, Object options]]) -> String

Returns the text with all errors in a String.

Options can have properties separator (string used to separate errors, “, “ by default) and dataVar (the variable name that dataPaths are prefixed with, “data” by default).



  // validation and reporting options:
  $data:            false,
  allErrors:        false,
  verbose:          false,
  $comment:         false, // NEW in Ajv version 6.0
  jsonPointers:     false,
  uniqueItems:      true,
  unicode:          true,
  nullable:         false,
  format:           'fast',
  formats:          {},
  unknownFormats:   true,
  schemas:          {},
  logger:           undefined,
  // referenced schema options:
  schemaId:         '$id',
  missingRefs:      true,
  extendRefs:       'ignore', // recommended 'fail'
  loadSchema:       undefined, // function(uri: string): Promise {}
  // options to modify validated data:
  removeAdditional: false,
  useDefaults:      false,
  coerceTypes:      false,
  // strict mode options
  strictDefaults:   false,
  strictKeywords:   false,
  strictNumbers:    false,
  // asynchronous validation options:
  transpile:        undefined, // requires ajv-async package
  // advanced options:
  meta:             true,
  validateSchema:   true,
  addUsedSchema:    true,
  inlineRefs:       true,
  passContext:      false,
  loopRequired:     Infinity,
  ownProperties:    false,
  multipleOfPrecision: false,
  errorDataPath:    'object', // deprecated
  messages:         true,
  sourceCode:       false,
  processCode:      undefined, // function (str: string, schema: object): string {}
  cache:            new Cache,
  serialize:        undefined
Validation and reporting options
Referenced schema options
Options to modify validated data
Strict mode options
Asynchronous validation options
Advanced options

Validation errors

In case of validation failure, Ajv assigns the array of errors to errors property of validation function (or to errors property of Ajv instance when validate or validateSchema methods were called). In case of asynchronous validation, the returned promise is rejected with exception Ajv.ValidationError that has errors property.

Error objects

Each error is an object with the following properties:

Please note: propertyNames keyword schema validation errors have an additional property propertyName, dataPath points to the object. After schema validation for each property name, if it is invalid an additional error is added with the property keyword equal to "propertyNames".

Error parameters

Properties of params object in errors depend on the keyword that failed validation.

Error logging

Using the logger option when initiallizing Ajv will allow you to define custom logging. Here you can build upon the exisiting logging. The use of other logging packages is supported as long as the package or its associated wrapper exposes the required methods. If any of the required methods are missing an exception will be thrown.

var otherLogger = new OtherLogger();
var ajv = new Ajv({
  logger: {
    log: console.log.bind(console),
    warn: function warn() {
      otherLogger.logWarn.apply(otherLogger, arguments);
    error: function error() {
      otherLogger.logError.apply(otherLogger, arguments);
      console.error.apply(console, arguments);


Ajv can be extended with plugins that add custom keywords, formats or functions to process generated code. When such plugin is published as npm package it is recommended that it follows these conventions:

If you have published a useful plugin please submit a PR to add it to the next section.

Some packages using Ajv


npm install
git submodule update --init
npm test


All validation functions are generated using doT templates in dot folder. Templates are precompiled so doT is not a run-time dependency.

npm run build - compiles templates to dotjs folder.

npm run watch - automatically compiles templates when files in dot folder change

Please see Contributing guidelines

Changes history


Please note: Changes in version 7.0.0-beta

Version 6.0.0.

Code of conduct

Please review and follow the Code of conduct.

Please report any unacceptable behaviour to - it will be reviewed by the project team.

Open-source software support

Ajv is a part of Tidelift subscription - it provides a centralised support to open-source software users, in addition to the support provided by software maintainers.