The fastest JSON Schema Validator

Data validation

JSON Schema draft-2019-09

The default export of Ajv provides support of JSON-Schema draft-07, without any of draft-2019-09 features:

import Ajv from "ajv"
// const Ajv = require("ajv").default
const ajv = new Ajv()

To use Ajv with the support of all JSON Schema draft-2019-09 features you need to use a different export:

import Ajv from "ajv/dist/2019"
// const Ajv2019 = require("ajv/dist/2019").default
const ajv = new Ajv2019()

Optionally, you can add draft-07 meta-schema, to use both draft-07 and draft-2019-09 schemas in one Ajv instance:

const draft7MetaSchema = require("ajv/dist/refs/json-schema-draft-07.json")
ajv.addMetaSchema(draft7MetaSchema)

Draft-2019-09 support is provided via a separate export in order to avoid increasing the bundle and generated code size for draft-07 users.

With this import Ajv supports the following features:

Please note: Supporting dynamic recursive references and unevaluatedProperties/Items adds additional generated code even to the validation functions where these features are not used (when possible, Ajv determines which properties/items are “unevaluated” at compilation time, but support for dynamic references always adds additional generated code). If you are not using these features in your schemas it is recommended to use default Ajv export with JSON-Schema draft-07 support.

You can also use individual draft-2019-09 features to Ajv with the advanced options dynamicRef, next and unevaluated. These options are changing how the code is generated for draft-07 keywords to support the new features of draft-2019-09, but they do not add the new keywords - they should be added separately. The code examples below shows how to enable individual draft-2019-09 features:

import Ajv from "ajv"
// const Ajv = require("ajv").default

// add support for unevaluatedProperties and unevaluatedItems without other 2019-09 features
const ajv = new Ajv({unevaluated: true})
import unevaluatedVocabulary from "ajv/dist/vocabularies/unevaluated"
// const unevaluatedVocabulary = require("ajv/dist/vocabularies/unevaluated").default
ajv.addVocabulary(unevaluatedVocabulary)

// add support for dependentRequired, dependentSchemas, maxContains and minContains
const ajv = new Ajv({next: true})
import nextVocabulary from "ajv/dist/vocabularies/next"
// const nextVocabulary = require("ajv/dist/vocabularies/next").default
ajv.addVocabulary(nextVocabulary)

// add support for dynamic recursive references
const ajv = new Ajv({dynamicRef: true})
import dynamicVocabulary from "ajv/dist/vocabularies/dynamic"
// const dynamicVocabulary = require("ajv/dist/vocabularies/dynamic").default
ajv.addVocabulary(dynamicVocabulary)

If you want to have support of all these features you should import Ajv from "ajv/dist/2019" as shown above.

Validation basics

JSON Schema validation keywords

Ajv supports all validation keywords from draft-07 of JSON Schema standard - see JSON Schema validation keywords for more details.

ajv-keywords package provides additional validation keywords that can be used with Ajv.

Metadata keywords

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

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

Formats

From version 7 Ajv does not include formats defined by JSON Schema specification - these and several other formats are provided by ajv-formats plugin.

To add all formats from this plugin:

import Ajv from "ajv"
import addFormats from "ajv-formats"

const ajv = new Ajv()
addFormats(ajv)

See ajv-formats documentation for further details.

It is recommended NOT to use “format” keyword implementations with untrusted data, as they may use potentially unsafe regular expressions (even though known issues are fixed) - 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 defined in ajv-formats for string validation with “format” keyword:

Please note: JSON Schema draft-07 also defines formats iri, iri-reference, idn-hostname and idn-email for URLs, hostnames and emails with international characters. These formats are available in ajv-formats-draft2019 plugin.

You can add and replace any formats using addFormat method.

Modular schemas

Combining schemas with $ref

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

Example:

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

const 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:

const ajv = new Ajv({schemas: [schema, defsSchema]})
const validate = ajv.getSchema("http://example.com/schemas/schema.json")

or use addSchema method:

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

See Options and addSchema method.

Please note:

Extending recursive schemas

While statically defined $ref keyword allows to split schemas to multiple files, it is difficult to extend recursive schemas - the recursive reference(s) in the original schema points to the original schema, and not to the extended one. So in JSON Schema draft-07 the only available solution to extend the recursive schema was to redefine all sections of the original schema that have recursion.

It was particularly repetitive when extending meta-schema, as it has many recursive references, but even in a schema with a single recursive reference extending it was very verbose.

JSON Schema draft-2019-09 and the upcoming draft defined the mechanism for dynamic recursion using keywords $recursiveRef/$recursiveAnchor (draft-2019-09) or $dynamicRef/$dynamicAnchor (the next JSON Schema draft) that is somewhat similar to “open recursion” in functional programming.

Consider this recursive schema with static recursion:

const treeSchema = {
  $id: "https://example.com/tree",
  type: "object",
  required: ["data"],
  properties: {
    data: true,
    children: {
      type: "array",
      items: {$ref: "#"},
    },
  },
}

The only way to extend this schema to prohibit additional properties is by adding additionalProperties keyword right in the schema - this approach can be impossible if you do not control the source of the original schema. Ajv also provided the additional keywords in ajv-merge-patch package to extend schemas by treating them as plain JSON data. While this approach may work for you, it is non-standard and therefore not portable.

The new keywords for dynamic recursive references allow extending this schema without modifying it:

const treeSchema = {
  $id: "https://example.com/tree",
  $recursiveAnchor: true,
  type: "object",
  required: ["data"],
  properties: {
    data: true,
    children: {
      type: "array",
      items: {$recursiveRef: "#"},
    },
  },
}

const strictTreeSchema = {
  $id: "https://example.com/strict-tree",
  $recursiveAnchor: true,
  $ref: "tree",
  unevaluatedProperties: false,
}

import Ajv2019 from "ajv/dist/2019"
// const Ajv2019 = require("ajv/dist/2019").default
const ajv = new Ajv2019({
  schemas: [treeSchema, strictTreeSchema],
})
const validate = ajv.getSchema("https://example.com/strict-tree")

See dynamic-refs test for the example using $dynamicAnchor/$dynamicRef.

At the moment Ajv implements the spec for dynamic recursive references with these limitations:

Ajv also does not support dynamic references in asynchronous schemas (Ajv extension) - it is assumed that the referenced schema is synchronous, and there is no validation-time check for it.

$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:

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

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

const validData = {
  smaller: 5,
  larger: 7,
}

ajv.validate(schema, validData) // true

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

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

const validData = {
  "date-time": "1963-06-19T08:30:06.283185Z",
  email: "joe.bloggs@example.com",
}

$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.

User-defined keywords

The advantages of defining keywords are:

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

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

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

Ajv allows defining keywords with:

Example. range and exclusiveRange keywords using compiled schema:

ajv.addKeyword({
  keyword: "range",
  type: "number",
  schemaType: "array",
  implements: "exclusiveRange",
  compile: ([min, max], parentSchema) =>
    parentSchema.exclusiveRange === true
      ? (data) => data > min && data < max
      : (data) => data >= min && data <= max,
})

const schema = {range: [2, 4], exclusiveRange: true}
const 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 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 keywords.

See User-defined keywords for more details.

Asynchronous schema compilation

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

Example:

const ajv = new Ajv({loadSchema: loadSchema})

ajv.compileAsync(schema).then(function (validate) {
  const 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://runkit.com/esp/ajv-asynchronous-validation

You can define 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 User-defined 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 format/keyword should return a promise that resolves with true or false (or rejects with new Ajv.ValidationError(errors) if you want to return errors from the keyword function).

Ajv compiles asynchronous schemas to async functions. Async functions are supported in Node.js 7+ and all modern browsers. You can supply a 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:

const ajv = new Ajv()

ajv.addKeyword({
  keyword: "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
    })
}

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

const 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

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

See Options.

Modifying data during validation

Removing additional properties

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

This option modifies original data.

Example:

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

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

const 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):

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

const data = {foo: 1}

const validate = ajv.compile(schema)

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

Example 2 (default in items):

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

const data = [1]

const validate = ajv.compile(schema)

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

With useDefaults option default keywords throws exception during schema compilation when used in:

The strict mode option can change the behaviour for these unsupported defaults (strict: false to ignore them, "log" to log a warning).

See Strict mode.

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:

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

const data = {foo: "1", bar: "false"}

const validate = ajv.compile(schema)

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

Example 2 (array coercions):

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

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

const 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.