A few seconds.


Another JSON Schema Validator

Ajv logo

Ajv: Another JSON Schema Validator

The fastest JSON Schema validator for node.js and browser with draft 6 support.

Build Status npm version npm downloads Code Climate Coverage Status Greenkeeper badge Gitter

Using version 5

JSON-Schema draft-06 is published.

Ajv version 5.0.0 that supports draft-06 is released. It may require either migrating your schemas or updating your code (to continue using draft-04 and v5 schemas).



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:

Performace of different validators by json-schema-benchmark:



Currently Ajv is the only validator that passes all the tests from JSON Schema Test Suite (according to json-schema-benchmark, apart from the test that requires that 1.0 is not an integer that is impossible to satisfy in JavaScript).


npm install ajv

Getting started

Try it in the node REPL: https://tonicdev.com/npm/ajv

The fastest validation call:

var Ajv = require('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);
// ...


// ...
ajv.addSchema(schema, 'mySchema');
var valid = ajv.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 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 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

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 or 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).

Command line interface

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

  • compiling JSON-schemas to test their validity
  • BETA: generating standalone module exporting a validation function to be used without Ajv (using ajv-pack)
  • migrate schemas to draft-06 (using json-schema-migrate)
  • validating data file(s) against JSON-schema
  • testing expected validity of data against JSON-schema
  • referenced schemas
  • custom meta-schemas
  • files in JSON and JavaScript format
  • all Ajv options
  • reporting changes in data after validation in JSON-patch format

Validation keywords

Ajv supports all validation keywords from draft 4 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.


The following formats are supported for string validation with "format" keyword:

  • date: full-date according to RFC3339.
  • time: time with optional time-zone.
  • date-time: date-time from the same source (time-zone is mandatory). date, time and date-time validate ranges in full mode and only regexp in fast mode (see options).
  • uri: full uri with optional protocol.
  • url: URL record.
  • uri-template: URI template according to RFC6570
  • email: email address.
  • hostname: host name according to RFC1034.
  • ipv4: IP address v4.
  • ipv6: IP address v6.
  • regex: tests whether a string is a valid regular expression by passing it to RegExp constructor.
  • uuid: Universally Unique IDentifier according to RFC4122.
  • json-pointer: JSON-pointer according to RFC6901.
  • relative-json-pointer: relative JSON-pointer according to this draft.

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

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

The option unknownFormats allows to change the behaviour in case an unknown format is encountered - Ajv can either fail schema compilation (default) or ignore it (default in versions before 5.0.0). You also can whitelist specific format(s) to be ignored. See Options for details.

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

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

var validData = {
  smaller: 5,
  larger: 7

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: [email protected]'

$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:

  • allow creating validation scenarios that cannot be expressed using JSON-Schema
  • simplify your schemas
  • help bringing a bigger part of the validation logic to your schemas
  • make your schemas more expressive, less verbose and closer to your application domain
  • implement custom data processors that modify your data (modifying option MUST be used in keyword definition) and/or create side effects while the data is being validated

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:

  • validation function
  • compilation function
  • macro function
  • inline compilation function that should return code (as string) that will be inlined in the currently compiled schema.

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 REPL: https://tonicdev.com/esp/ajv-asynchronous-validation

You can define custom formats and keywords that perform validation asyncronously by accessing database or some service. You should add async: true in the keyword or format defnition (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 either es7 async functions that can optionally be transpiled with nodent or with regenerator or to generator functions that can be optionally transpiled with regenerator as well. You can also supply any other transpiler as a function. 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 syncronous and asynchronous schemas.

If you are using generators, the compiled validation function can be either wrapped with co (default) or returned as generator function, that can be used directly, e.g. in koa 1.0. co is a small library, it is included in Ajv (both as npm dependency and in the browser bundle).

Async functions are currently supported in Chrome 55, Firefox 52, Node 7 (with --harmony-async-await) and MS Edge 13 (with flag).

Generator functions are currently supported in Chrome, Firefox and node.js.

If you are using Ajv in other browsers or in older versions of node.js you should use one of available transpiling options. All provided async modes use global Promise class. If your platform does not have Promise you should use a polyfill that defines it.

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


 * Default mode is non-transpiled generator function wrapped with `co`.
 * Using package ajv-async (https://github.com/epoberezkin/ajv-async)
 * you can auto-detect the best async mode.
 * In this case, without "async" and "transpile" options
 * (or with option {async: true})
 * Ajv will choose the first supported/installed option in this order:
 * 1. native async function
 * 2. native generator function wrapped with co
 * 3. es7 async functions transpiled with nodent
 * 4. es7 async functions transpiled with regenerator

var setupAsync = require('ajv-async');
var ajv = setupAsync(new 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 asyncronous validation functions.

To use a transpiler you should separately install it (or load its bundle in the browser).

Ajv npm package includes minified browser bundles of regenerator and nodent in dist folder.

Using nodent

var setupAsync = require('ajv-async');
var ajv = new Ajv({ /* async: 'es7', */ transpile: 'nodent' });
var validate = ajv.compile(schema); // transpiled es7 async function

npm install nodent or use nodent.min.js from dist folder of npm package.

Using regenerator

var setupAsync = require('ajv-async');
var ajv = new Ajv({ /* async: 'es7', */ transpile: 'regenerator' });
var validate = ajv.compile(schema); // transpiled es7 async function

npm install regenerator or use regenerator.min.js from dist folder of npm package.

Using other transpilers

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

See Options.

Comparison of async modes

mode transpile
es7 async
- 0.75 -
- 1.0 -
es7.nodent 1.35 1.1 215Kb
es7.regenerator 1.0 2.7 1109Kb
regenerator 1.0 3.2 1109Kb

* Relative performance in node v.7, smaller is better.

nodent has several advantages:

  • much smaller browser bundle than regenerator
  • almost the same performance of generated code as native generators in nodejs and the latest Chrome
  • much better performace than native generators in other browsers
  • works in IE 9 (regenerator does not)

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.

This option modifies original data.

Please note: by default the default value is inserted in the generated validation code as a literal (starting from v4.0), so the value inserted in the data will be the deep clone of the default in the schema.

If you need to insert the default value in the data by reference pass the option useDefaults: "shared".

Inserting defaults by reference can be faster (in case you have an object in default) and it allows to have dynamic values in defaults, e.g. timestamp, without recompiling the schema. The side effect is that modifying the default value in any validated data instance will change the default in the schema and in other validated data instances. See example 3 below.

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" ]

Example 3 (inserting "defaults" by reference):

var ajv = new Ajv({ useDefaults: 'shared' });

var schema = {
  properties: {
    foo: {
      default: { bar: 1 }

var validate = ajv.compile(schema);

var data = {};
console.log(validate(data)); // true
console.log(data); // { foo: { bar: 1 } }

data.foo.bar = 2;

var data2 = {};
console.log(validate(data2)); // true
console.log(data2); // { foo: { bar: 2 } }

default keywords in other cases are ignored:

  • not in properties or items subschemas
  • in schemas inside anyOf, oneOf and not (see #42)
  • in if subschema of switch keyword
  • in schemas generated by custom macro keywords

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.

All the instance methods below are bound to the instance, so they can be used without the instance.

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

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

Validating function returns boolean and has properties errors with the errors from the last validation (null if there were no errors) and schema with the reference to the original schema.

Unless the option validateSchema is false, the schema will be validated against meta-schema and if schema is invalid the error will be thrown. See options.

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

Asyncronous 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 callback if passed will be called with an error) when:

  • missing schema can't be loaded (loadSchema returns the Promise that rejects).
  • the schema containing missing reference is loaded, but the reference cannot be resolved.
  • schema (or some referenced schema) is invalid.

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])

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.

.addMetaSchema(Array<Object>|Object schema [, String key])

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 6 meta schema (http://json-schema.org/draft-06/schema and http://json-schema.org/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). Returned validating function has schema property with the reference to the original schema.

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

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:

  • key passed to addSchema
  • it's full reference (id)
  • RegExp that should match schema id or key (meta-schemas won't be removed)
  • actual schema object that will be stable-stringified to remove schema from cache

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)

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:

  • validate: a string, RegExp or a function as described above.
  • compare: an optional comparison function that accepts two strings and compares them according to the format meaning. This function is used with keywords formatMaximum/formatMinimum (defined in ajv-keywords package). It should return 1 if the first value is bigger than the second value, -1 if it is smaller and 0 if it is equal.
  • async: an optional true value if validate is an asynchronous function; in this case it should return a promise that resolves with a value true or false.
  • type: an optional type of data that the format applies to. It can be "string" (default) or "number" (see https://github.com/epoberezkin/ajv/issues/291#issuecomment-259923858). If the type of data is different, the validation will pass.

Custom formats can be also added via formats option.

.addKeyword(String keyword, Object definition)

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:

  • "xyz-example": valid, and uses prefix for the xyz project to avoid name collisions.
  • "example": valid, but not recommended as it could collide with future versions of JSON schema etc.
  • "3-example": invalid as numbers are not allowed to be the first character in a keyword

Keyword definition is an object with the following properties:

  • type: optional string or array of strings with data type(s) that the keyword applies to. If not present, the keyword will apply to all types.
  • validate: validating function
  • compile: compiling function
  • macro: macro function
  • inline: compiling function that returns code (as string)
  • schema: an optional false value used with "validate" keyword to not pass schema
  • metaSchema: an optional meta-schema for keyword schema
  • modifying: true MUST be passed if keyword modifies data
  • valid: pass true/false to pre-define validation result, the result returned from validation function will be ignored. This option cannot be used with macro keywords.
  • $data: an optional true value to support $data reference as the value of custom keyword. The reference will be resolved at validation time. If the keyword has meta-schema it would be extended to allow $data and it will be used to validate the resolved value. Supporting $data reference requires that keyword has validating function (as the only option or in addition to compile, macro or inline function).
  • async: an optional true value if the validation function is asynchronous (whether it is compiled or passed in validate property); in this case it should return a promise that resolves with a value true or false. This option is ignored in case of "macro" and "inline" keywords.
  • errors: an optional boolean indicating whether keyword returns errors. If this property is not set Ajv will determine if the errors were set in case of failed validation.

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)

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:
  $data:            false,
  allErrors:        false,
  verbose:          false,
  jsonPointers:     false,
  uniqueItems:      true,
  unicode:          true,
  format:           'fast',
  formats:          {},
  unknownFormats:   true,
  schemas:          {},
  // referenced schema options:
  schemaId:         undefined // recommended '$id'
  missingRefs:      true,
  extendRefs:       'ignore', // recommended 'fail'
  loadSchema:       undefined, // function(uri: string): Promise {}
  // options to modify validated data:
  removeAdditional: false,
  useDefaults:      false,
  coerceTypes:      false,
  // asynchronous validation options:
  async:            'co*',
  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',
  messages:         true,
  sourceCode:       false,
  processCode:      undefined, // function (str: string): string {}
  cache:            new Cache,
  serialize:        undefined
Validation and reporting options
  • $data: support $data references). Draft 6 meta-schema that is added by default will be extended to allow them. If you want to use another meta-schema you need to use $dataMetaSchema method to add support for $data reference. See API.
  • allErrors: check all rules collecting all errors. Default is to return after the first error.
  • verbose: include the reference to the part of the schema (schema and parentSchema) and validated data in errors (false by default).
  • jsonPointers: set dataPath propery of errors using JSON Pointers instead of JavaScript property access notation.
  • uniqueItems: validate uniqueItems keyword (true by default).
  • unicode: calculate correct length of strings with unicode pairs (true by default). Pass false to use .length of strings that is faster, but gives "incorrect" lengths of strings with unicode pairs - each unicode pair is counted as two characters.
  • format: formats validation mode ('fast' by default). Pass 'full' for more correct and slow validation or false not to validate formats at all. E.g., 25:00:00 and 2015/14/33 will be invalid time and date in 'full' mode but it will be valid in 'fast' mode.
  • formats: an object with custom formats. Keys and values will be passed to addFormat method.
  • unknownFormats: handling of unknown formats. Option values:
    • true (default) - if an unknown format is encountered the exception is thrown during schema compilation. If format keyword value is $data reference and it is unknown the validation will fail.
    • [String] - an array of unknown format names that will be ignored. This option can be used to allow usage of third party schemas with format(s) for which you don't have definitions, but still fail if another unknown format is used. If format keyword value is $data reference and it is not in this array the validation will fail.
    • "ignore" - to log warning during schema compilation and always pass validation (the default behaviour in versions before 5.0.0). This option is not recommended, as it allows to mistype format name and it won't be validated without any error message. This behaviour is required by JSON-schema specification.
  • schemas: an array or object of schemas that will be added to the instance. If the order is important, pass array. In this case schemas must have IDs in them. Otherwise the object can be passed - addSchema(value, key) will be called for each schema in this object.
Referenced schema options
  • schemaId: this option defines which keywords are used as schema URI. Option value:
    • "$id" (recommended) - only use $id keyword as schema URI (as specified in JSON-Schema draft-06), ignore id keyword (if it is present a warning will be logged).
    • "id" - only use id keyword as schema URI (as specified in JSON-Schema draft-04), ignore $id keyword (if it is present a warning will be logged).
    • undefined (default) - use both $id and id keywords as schema URI. If both are present (in the same schema object) and different the exception will be thrown during schema compilation.
  • missingRefs: handling of missing referenced schemas. Option values:
    • true (default) - if the reference cannot be resolved during compilation the exception is thrown. The thrown error has properties missingRef (with hash fragment) and missingSchema (without it). Both properties are resolved relative to the current base id (usually schema id, unless it was substituted).
    • "ignore" - to log error during compilation and always pass validation.
    • "fail" - to log error and successfully compile schema but fail validation if this rule is checked.
  • extendRefs: validation of other keywords when $ref is present in the schema. Option values:
    • "ignore" (default) - when $ref is used other keywords are ignored (as per JSON Reference standard). A warning will be logged during the schema compilation.
    • "fail" (recommended) - if other validation keywords are used together with $ref the exception will be thrown when the schema is compiled. This option is recomended to make sure schema has no keywords that are ignored, which can be confusing.
    • true - validate all keywords in the schemas with $ref (the default behaviour in versions before 5.0.0).
  • loadSchema: asynchronous function that will be used to load remote schemas when compileAsync method is used and some reference is missing (option missingRefs should NOT be 'fail' or 'ignore'). This function should accept remote schema uri as a parameter and return a Promise that resolves to a schema. See example in Asynchronous compilation.
Options to modify validated data
  • removeAdditional: remove additional properties - see example in Filtering data. This option is not used if schema is added with addMetaSchema method. Option values:
    • false (default) - not to remove additional properties
    • "all" - all additional properties are removed, regardless of additionalProperties keyword in schema (and no validation is made for them).
    • true - only additional properties with additionalProperties keyword equal to false are removed.
    • "failing" - additional properties that fail schema validation will be removed (where additionalProperties keyword is false or schema).
  • useDefaults: replace missing properties and items with the values from corresponding default keywords. Default behaviour is to ignore default keywords. This option is not used if schema is added with addMetaSchema method. See examples in Assigning defaults. Option values:
    • false (default) - do not use defaults
    • true - insert defaults by value (safer and slower, object literal is used).
    • "shared" - insert defaults by reference (faster). If the default is an object, it will be shared by all instances of validated data. If you modify the inserted default in the validated data, it will be modified in the schema as well.
  • coerceTypes: change data type of data to match type keyword. See the example in Coercing data types and coercion rules. Option values:
    • false (default) - no type coercion.
    • true - coerce scalar data types.
    • "array" - in addition to coercions between scalar types, coerce scalar data to an array with one element and vice versa (as required by the schema).
Asynchronous validation options
  • async: determines how Ajv compiles asynchronous schemas (see Asynchronous validation) to functions. Option values:

    • "*" / "co*" (default) - compile to generator function ("co*" - wrapped with co.wrap). If generators are not supported and you don't provide processCode option (or transpile option if you use ajv-async package), the exception will be thrown when async schema is compiled.
    • "es7" - compile to es7 async function. Unless your platform supports them you need to provide processCode/transpile option. According to compatibility table) async functions are supported by:
      • Firefox 52,
      • Chrome 55,
      • Node.js 7 (with --harmony-async-await),
      • MS Edge 13 (with flag).
    • undefined/true - auto-detect async mode. It requires ajv-async package. If transpile option is not passed ajv-async will choose the first of supported/installed async/transpile modes in this order:
      • "es7" (native async functions),
      • "co*" (native generators with co.wrap),
      • "es7"/"nodent",
      • "co*"/"regenerator" during the creation of the Ajv instance.

    If none of the options is available the exception will be thrown.

  • transpile: Requires ajv-async package. It determines whether Ajv transpiles compiled asynchronous validation function. Option values:
    • "nodent" - transpile with nodent. If nodent is not installed, the exception will be thrown. nodent can only transpile es7 async functions; it will enforce this mode.
    • "regenerator" - transpile with regenerator. If regenerator is not installed, the exception will be thrown.
    • a function - this function should accept the code of validation function as a string and return transpiled code. This option allows you to use any other transpiler you prefer. If you are passing a function, you can simply pass it to processCode option without using ajv-async.
Advanced options
  • meta: add meta-schema so it can be used by other schemas (true by default). If an object is passed, it will be used as the default meta-schema for schemas that have no $schema keyword. This default meta-schema MUST have $schema keyword.
  • validateSchema: validate added/compiled schemas against meta-schema (true by default). $schema property in the schema can either be http://json-schema.org/schema or http://json-schema.org/draft-04/schema or absent (draft-4 meta-schema will be used) or can be a reference to the schema previously added with addMetaSchema method. Option values:
    • true (default) - if the validation fails, throw the exception.
    • "log" - if the validation fails, log error.
    • false - skip schema validation.
  • addUsedSchema: by default methods compile and validate add schemas to the instance if they have $id (or id) property that doesn't start with "#". If $id is present and it is not unique the exception will be thrown. Set this option to false to skip adding schemas to the instance and the $id uniqueness check when these methods are used. This option does not affect addSchema method.
  • inlineRefs: Affects compilation of referenced schemas. Option values:
    • true (default) - the referenced schemas that don't have refs in them are inlined, regardless of their size - that substantially improves performance at the cost of the bigger size of compiled schema functions.
    • false - to not inline referenced schemas (they will be compiled as separate functions).
    • integer number - to limit the maximum number of keywords of the schema that will be inlined.
  • passContext: pass validation context to custom keyword functions. If this option is true and you pass some context to the compiled validation function with validate.call(context, data), the context will be available as this in your custom keywords. By default this is Ajv instance.
  • loopRequired: by default required keyword is compiled into a single expression (or a sequence of statements in allErrors mode). In case of a very large number of properties in this keyword it may result in a very big validation function. Pass integer to set the number of properties above which required keyword will be validated in a loop - smaller validation function size but also worse performance.
  • ownProperties: by default Ajv iterates over all enumerable object properties; when this option is true only own enumerable object properties (i.e. found directly on the object rather than on its prototype) are iterated. Contributed by @mbroadst.
  • multipleOfPrecision: by default multipleOf keyword is validated by comparing the result of division with parseInt() of that result. It works for dividers that are bigger than 1. For small dividers such as 0.01 the result of the division is usually not integer (even when it should be integer, see issue #84). If you need to use fractional dividers set this option to some positive integer N to have multipleOf validated using this formula: Math.abs(Math.round(division) - division) < 1e-N (it is slower but allows for float arithmetics deviations).
  • errorDataPath: set dataPath to point to 'object' (default) or to 'property' when validating keywords required, additionalProperties and dependencies.
  • messages: Include human-readable messages in errors. true by default. false can be passed when custom messages are used (e.g. with ajv-i18n).
  • sourceCode: add sourceCode property to validating function (for debugging; this code can be different from the result of toString call).
  • processCode: an optional function to process generated code before it is passed to Function constructor. It can be used to either beautify (the validating function is generated without line-breaks) or to transpile code. Starting from version 5.0.0 this option replaced options:
    • beautify that formatted the generated function using js-beautify. If you want to beautify the generated code pass require('js-beautify').js_beautify.
    • transpile that transpiled asynchronous validation function. You can still use transpile option with ajv-async package. See Asynchronous validation for more information.
  • cache: an optional instance of cache to store compiled schemas using stable-stringified schema as a key. For example, set-associative cache sacjs can be used. If not passed then a simple hash is used which is good enough for the common use case (a limited number of statically defined schemas). Cache should have methods put(key, value), get(key), del(key) and clear().
  • serialize: an optional function to serialize schema to cache key. Pass false to use schema itself as a key (e.g., if WeakMap used as a cache). By default json-stable-stringify is used.

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 in case validate or validateSchema methods were called). In case of asynchronous validation the returned promise is rejected with the exception of the class Ajv.ValidationError that has .errors property.

Error objects

Each error is an object with the following properties:

  • keyword: validation keyword.
  • dataPath: the path to the part of the data that was validated. By default dataPath uses JavaScript property access notation (e.g., ".prop[1].subProp"). When the option jsonPointers is true (see Options) dataPath will be set using JSON pointer standard (e.g., "/prop/1/subProp").
  • schemaPath: the path (JSON-pointer as a URI fragment) to the schema of the keyword that failed validation.
  • params: the object with the additional information about error that can be used to create custom error messages (e.g., using ajv-i18n package). See below for parameters set by all keywords.
  • message: the standard error message (can be excluded with option messages set to false).
  • schema: the schema of the keyword (added with verbose option).
  • parentSchema: the schema containing the keyword (added with verbose option)
  • data: the data validated by the keyword (added with verbose option).

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.

  • maxItems, minItems, maxLength, minLength, maxProperties, minProperties - property limit (number, the schema of the keyword).
  • additionalItems - property limit (the maximum number of allowed items in case when items keyword is an array of schemas and additionalItems is false).
  • additionalProperties - property additionalProperty (the property not used in properties and patternProperties keywords).
  • dependencies - properties:
    • property (dependent property),
    • missingProperty (required missing dependency - only the first one is reported currently)
    • deps (required dependencies, comma separated list as a string),
    • depsCount (the number of required dependedncies).
  • format - property format (the schema of the keyword).
  • maximum, minimum - properties:
    • limit (number, the schema of the keyword),
    • exclusive (boolean, the schema of exclusiveMaximum or exclusiveMinimum),
    • comparison (string, comparison operation to compare the data to the limit, with the data on the left and the limit on the right; can be "<", "<=", ">", ">=")
  • multipleOf - property multipleOf (the schema of the keyword)
  • pattern - property pattern (the schema of the keyword)
  • required - property missingProperty (required property that is missing).
  • propertyNames - property propertyName (an invalid property name).
  • patternRequired (in ajv-keywords) - property missingPattern (required pattern that did not match any property).
  • type - property type (required type(s), a string, can be a comma-separated list)
  • uniqueItems - properties i and j (indices of duplicate items).
  • enum - property allowedValues pointing to the array of values (the schema of the keyword).
  • $ref - property ref with the referenced schema URI.
  • custom keywords (in case keyword definition doesn't create errors) - property keyword (the keyword name).
  • ajv-cli - command line interface for Ajv
  • ajv-i18n - internationalised error messages
  • ajv-merge-patch - keywords $merge and $patch.
  • ajv-keywords - several custom keywords that can be used with Ajv (typeof, instanceof, range, propertyNames)
  • ajv-errors - custom error messages for Ajv

Some packages using Ajv

Pincer is a project which aims to provide best library discovery tools for developers. We're growing day by day. We have only npm platform for now but we will add the others as much as we can.