Ajv: Another JSON Schema Validator

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

Ajv v7 beta is released

Ajv version 7 (beta) is released with these changes:

• support of JSON Schema draft-2019-09 features: unevaluatedProperties and unevaluatedItems, dynamic recursive references and other additional keywords.
• to reduce the mistakes in JSON schemas and unexpected validation results, strict mode is added - it prohibits ignored or ambiguous JSON Schema elements.
• to make code injection from untrusted schemas impossible, code generation is fully re-written to be safe and to allow code optimization (compiled schema code size is reduced by more than 10%).
• to simplify Ajv extensions, the new keyword API that is used by pre-defined keywords is available to user-defined keywords - it is much easier to define any keywords now, especially with subschemas. ajv-keywords package was updated to use the new API (in v4.0.0-beta.0)
• schemas are compiled to ES6 code (ES5 code generation is also supported with an option).
• to improve reliability and maintainability the code is migrated to TypeScript.

Please note:

• the support for JSON-Schema draft-04 is removed - if you have schemas using "id" attributes you have to replace them with "$id" (or continue using version 6 that will be supported until 02/28/2021).
• all formats are separated to ajv-formats package - they have to be explicitely added if you use them.

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:

• GitHub sponsors page (GitHub will match it)
• Ajv Open Collective️

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:

ajv.addMetaSchema(require('ajv/lib/refs/json-schema-draft-06.json'));

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.addMetaSchema(require('ajv/lib/refs/json-schema-draft-04.json'));

Contents

• Performance
• Features
• Getting started
• Frequently Asked Questions
• Using in browser
  • Ajv and Content Security Policies (CSP)
• Command line interface
• Validation
  • Keywords
  • Annotation keywords
  • Formats
  • Combining schemas with $ref
  • $data reference
  • NEW: $merge and $patch keywords
  • Defining custom keywords
  • Asynchronous schema compilation
  • Asynchronous validation
• Security considerations
  • Security contact
  • Untrusted schemas
  • Circular references in objects
  • Trusted schemas
  • ReDoS attack
• Modifying data during validation
  • Filtering data
  • Assigning defaults
  • Coercing data types
• API
  • Methods
  • Options
  • Validation errors
• Plugins
• Related packages
• Some packages using Ajv
• Tests, Contributing, Changes history
• Support, Code of conduct, License

Performance

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:

• json-schema-benchmark - 50% faster than the second place
• jsck benchmark - 20-190% faster
• z-schema benchmark
• themis benchmark

Performance of different validators by json-schema-benchmark:

Features

• Ajv implements full JSON Schema draft-06/07 and draft-04 standards:
  • all validation keywords (see JSON Schema validation keywords)
  • full support of remote refs (remote schemas have to be added with addSchema or compiled to be available)
  • support of circular references between schemas
  • correct string lengths for strings with unicode pairs (can be turned off)
  • formats defined by JSON Schema draft-07 standard and custom formats (can be turned off)
  • validates schemas against meta-schema
• supports browsers and Node.js 0.10-14.x
• asynchronous loading of referenced schemas during compilation
• "All errors" validation mode with option allErrors
• error messages with parameters describing error reasons to allow creating custom error messages
• i18n error messages support with ajv-i18n package
• filtering data from additional properties
• assigning defaults to missing properties and items
• coercing data to the types specified in type keywords
• custom keywords
• draft-06/07 keywords const, contains, propertyNames and if/then/else
• draft-06 boolean schemas (true/false as a schema to always pass/fail).
• keywords switch, patternRequired, formatMaximum / formatMinimum and formatExclusiveMaximum / formatExclusiveMinimum from JSON Schema extension proposals with ajv-keywords package
• $data reference to use values from the validated data as values for the schema keywords
• asynchronous validation of custom formats and keywords

Install

npm install ajv

Getting started

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

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);
// ...

or

// ...
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:

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:

• 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-07 (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, JSON5, YAML, 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-07 of JSON Schema standard:

• type
• for numbers - maximum, minimum, exclusiveMaximum, exclusiveMinimum, multipleOf
• for strings - maxLength, minLength, pattern, format
• for arrays - maxItems, minItems, uniqueItems, items, additionalItems, contains
• for objects - maxProperties, minProperties, required, properties, patternProperties, additionalProperties, dependencies, propertyNames
• for all types - enum, const
• compound keywords - not, oneOf, anyOf, allOf, if/then/else

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

• patternRequired - like required but with patterns that some property should match.
• formatMaximum, formatMinimum, formatExclusiveMaximum, formatExclusiveMinimum - setting limits for date, time, etc.

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.

• title and description: information about the data represented by that schema
• $comment (NEW in draft-07): information for developers. With option $comment Ajv logs or passes the comment string to the user-supplied function. See Options.
• default: a default value of the data instance, see Assigning defaults.
• examples (NEW in draft-06): an array of data instances. Ajv does not check the validity of these instances against the schema.
• readOnly and writeOnly (NEW in draft-07): marks data-instance as read-only or write-only in relation to the source of the data (database, api, etc.).
• contentEncoding: RFC 2045, e.g., "base64".
• contentMediaType: RFC 2046, e.g., "image/png".

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.

Formats

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:

• 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.
• uri-reference: URI reference, including full and relative URIs.
• uri-template: URI template according to RFC6570
• url (deprecated): URL record.
• 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.

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.

Example:

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

var defsSchema = {
  "$id": "http://example.com/schemas/defs.json",
  "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('http://example.com/schemas/schema.json');

or use addSchema method:

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

See Options and addSchema method.

Please note:

• $ref is resolved as the uri-reference using schema $id as the base URI (see the example).
• References can be recursive (and mutually recursive) to implement the schemas for different data structures (such as linked lists, trees, graphs, etc.).
• You don't have to host your schema files at the URIs that you use as schema $id. These URIs are only used to identify the schemas, and according to JSON Schema specification validators should not expect to be able to download the schemas from these URIs.
• The actual location of the schema file in the file system is not used.
• You can pass the identifier of the schema as the second parameter of addSchema method or as a property name in schemas option. This identifier can be used instead of (or in addition to) schema $id.
• You cannot have the same $id (or the schema identifier) used for more than one schema - the exception will be thrown.
• You can implement dynamic resolution of the referenced schemas using compileAsync method. In this way you can store schemas in any system (files, web, database, etc.) and reference them without explicitly adding to Ajv instance. See Asynchronous schema compilation.

$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).

Examples.

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: '[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:

require('ajv-merge-patch')(ajv);

Examples.

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.

Example:

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

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.

Example:

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)
  .select('id')
  .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;
require('ajv-async')(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
validate(data).then(successFunc).catch(errorFunc);

Using other transpilers

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

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:

• compiling schemas can cause stack overflow (if they are too deep)
• compiling schemas can be slow (e.g. #557)
• validating certain data can be slow

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):

• pattern and format for large strings - in some cases using maxLength can help mitigate it, but certain regular expressions can lead to exponential validation time even with relatively short strings (see ReDoS attack).
• patternProperties for large property names - use propertyNames to mitigate, but some regular expressions can have exponential evaluation time as well.
• uniqueItems for large non-scalar arrays - use maxItems to mitigate

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:

• making assessment of "format" implementations in Ajv.
• using format: 'fast' option that simplifies some of the regular expressions (although it does not guarantee that they are safe).
• replacing format implementations provided by Ajv with your own implementations of "format" keyword that either uses different regular expressions or another approach to format validation. Please see addFormat method.
• disabling format validation by ignoring "format" keyword with option format: false

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.

Example:

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:

• 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

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.

API

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:

• missing schema can't be loaded (loadSchema returns a Promise that rejects).
• a schema containing a missing reference is loaded, but the reference cannot be resolved.
• schema (or some loaded/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]) -> 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 (http://json-schema.org/draft-07/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:

• 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) -> 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:

• 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/ajv-validator/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) -> 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:

• "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
• dependencies: an optional list of properties that must be present in the parent schema - it will be checked during schema compilation
• modifying: true MUST be passed if keyword modifies data
• statements: true can be passed in case inline keyword generates statements (as opposed to expression)
• 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 or string "full" 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) -> 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).

Options

Defaults:

{
  // 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

• $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).
• $comment (NEW in Ajv version 6.0): log or pass the value of $comment keyword to a function. Option values:
  • false (default): ignore $comment keyword.
  • true: log the keyword value to console.
  • function: pass the keyword value, its schema path and root schema to the specified function
• jsonPointers: set dataPath property 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.
• nullable: support keyword "nullable" from Open API 3 specification.
• format: formats validation mode. Option values:
  • "fast" (default) - simplified and fast validation (see Formats for details of which formats are available and affected by this option).
  • "full" - more restrictive and slow validation. 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.
  • false - ignore all format keywords.
• formats: an object with custom formats. Keys and values will be passed to addFormat method.
• keywords: an object with custom keywords. Keys and values will be passed to addKeyword 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. In case you pass the array the schemas must have IDs in them. When the object is passed the method addSchema(value, key) will be called for each schema in this object.
• logger: sets the logging method. Default is the global console object that should have methods log, warn and error. See Error logging. Option values:
  • custom logger - it should have methods log, warn and error. If any of these methods is missing an exception will be thrown.
  • false - logging is disabled.

Referenced schema options

• schemaId: this option defines which keywords are used as schema URI. Option value:
  • "$id" (default) - only use $id keyword as schema URI (as specified in JSON Schema draft-06/07), 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).
  • "auto" - 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 succe