api.md

API Reference

• Ajv constructor and methods
• Options
• Validation errors

Ajv constructor and methods

new Ajv(options: object)

Create Ajv instance:

const ajv = new Ajv()

See Options

ajv.compile(schema: object): (data: any) => boolean | Promise<any>

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

Validating function returns a boolean value (or promise for async schemas that must have $async: true property - see Asynchronous validation). 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.

In typescript returned validation function can be a type guard if you pass type parameter:

interface Foo {
  foo: number
}

const FooSchema: JSONSchemaType<Foo> = {
  type: "object",
  properties: {foo: {type: "number"}},
  required: ["foo"],
  additionalProperties: false,
}

const validate = ajv.compile<Foo>(FooSchema) // type of validate extends `(data: any) => data is Foo`
const data: any = {foo: 1}
if (validate(data)) {
  // data is Foo here
  console.log(data.foo)
} else {
  console.log(validate.errors)
}

See more advanced example in the test.

ajv.compileAsync(schema: object, meta?: boolean): Promise<Function>

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.

Similarly to compile, it can return type guard in typescript.

See example in Asynchronous compilation.

ajv.validate(schemaOrRef: object | string, data: any): 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).

In typescript this method can act as a type guard (similarly to function retured by compile method - see example there).

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.

ajv.addSchema(schema: object | object[], key?: string): 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 return it instance for method chaining from all methods with the prefix add* and remove*:

const validate = new Ajv().addSchema(schema).addFormat(name, regex).getSchema(uri)

ajv.addMetaSchema(schema: object | object[], key?: string): 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.

ajv.validateSchema(schema: object): 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.

ajv.getSchema(key: string): undefined | ((data: any) => boolean | Promise<any>)

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.

ajv.removeSchema(schemaOrRef: object | string | RegExp): 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 optionally serialized) to remove schema from cache

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

ajv.addFormat(name: string, format: Format): Ajv

type Format =
  | true // to ignore this format (and pass validation)
  | string // will be converted to RegExp
  | RegExp
  | (data: string) => boolean
  | Object // format definition (see below and in types)

Add format to validate strings or numbers.

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

interface FormatDefinition { // actual type definition is more precise - see types.ts
  validate: string | RegExp | (data: number | string) => boolean | Promise<boolean>
  compare: (data1: string, data2: string): number // an optional 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](https://github.com/ajv-validator/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?: true // if `validate` is an asynchronous function
  type?: "string" | "number" // "string" is default. If data type is different, the validation will pass.
}

Formats can be also added via formats option.

ajv.addKeyword(definition: object):s Ajv

Add 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 may 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:

interface KeywordDefinition {
  // actual type definition is more precise - see types.ts
  keyword: string // keyword name
  type?: string | string[] // JSON data type(s) the keyword applies to. Default - all types.
  schemaType?: string | string[] // the required schema JSON type
  code?: Function // function to generate code, used for all pre-defined keywords
  validate?: Function // validating function
  compile?: Function // compiling function
  macro?: Function // macro function
  error?: object // error definition object - see types.ts
  schema?: false // used with "validate" keyword to not pass schema to function
  metaSchema?: object // meta-schema for keyword schema
  dependencies?: string[] // properties that must be present in the parent schema -
  // it will be checked during schema compilation
  implements?: string[] // keyword names to reserve that this keyword implements
  modifying?: true // MUST be passed if keyword modifies data
  valid?: boolean // to pre-define validation result, validation function result will be ignored -
  // this option MUST NOT be used with `macro` keywords.
  $data?: true // to support [\$data reference](./validation.md#data-reference) as the value of 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 `code` or `validate` function
  // (the latter can be used in addition to `compile` or `macro`).
  $dataError?: object // error definition object for invalid \$data schema - see types.ts
  async?: true // if the validation function is asynchronous
  // (whether it is returned from `compile` or passed in `validate` property).
  // It should return a promise that resolves with a value `true` or `false`.
  // This option is ignored in case of "macro" and "code" keywords.
  errors?: boolean | "full" // whether keyword returns errors.
  // If this property is not passed Ajv will determine
  // if the errors were set in case of failed validation.
}

compile, macro and code are mutually exclusive, only one should be used at a time. validate can be used separately or in addition to compile or macro 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 User defined keywords for more details.

ajv.getKeyword(keyword: string): object | boolean

Returns keyword definition, false if the keyword is unknown.

ajv.removeKeyword(keyword: string): Ajv

Removes added 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.

ajv.errorsText(errors?: object[], options?: object): 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

Option defaults:

// see types/index.ts for actual types
const defaultOptions = {
  // strict mode options
  strict: true,
  strictTypes: "log",
  strictTuples: "log",
  allowUnionTypes: false,
  allowMatchingProperties: false,
  validateFormats: true,
  // validation and reporting options:
  $data: false,
  allErrors: false,
  verbose: false,
  $comment: false,
  formats: {},
  keywords: {},
  schemas: {},
  logger: undefined,
  loadSchema: undefined, // function(uri: string): Promise {}
  // options to modify validated data:
  removeAdditional: false,
  useDefaults: false,
  coerceTypes: false,
  // advanced options:
  meta: true,
  validateSchema: true,
  addUsedSchema: true,
  inlineRefs: true,
  passContext: false,
  loopRequired: Infinity,
  loopEnum: Infinity,
  ownProperties: false,
  multipleOfPrecision: false,
  messages: true,
  code: {
    es5: false,
    lines: false,
    source: false,
    process: undefined, // (code: string) => string
    optimize: true,
  },
}

Strict mode options (NEW in v7)

• strict: By default Ajv executes in strict mode, that is designed to prevent any unexpected behaviours or silently ignored mistakes in schemas (see Strict Mode for more details). It does not change any validation results, but it makes some schemas invalid that would be otherwise valid according to JSON Schema specification. Option values:
  • true (default) - use strict mode and throw an exception when any strict mode restriction is violated.
  • "log" - log warning when any strict mode restriction is violated.
  • false - ignore all strict mode restrictions. Also ignores strictTypes restrictions unless it is explicitely passed.
• strictTypes: By default Ajv logs warning when "type" keyword is used in a way that may be incorrect or confusing to other people - see Strict types for more details. This option does not change validation results. Option values:
  • true - throw exception when any strictTypes restriction is violated.
  • "log" (default, unless option strict is false) - log warning when any strictTypes restriction is violated.
  • false - ignore all strictTypes restrictions violations.
• strictTuples: By default Ajv logs warning when "items" is array and "minItems" and "maxItems"/"additionalItems" not present or different from the number of items. See Strict mode for more details. This option does not change validation results. Option values:
  • true - throw exception.
  • "log" (default, unless option strict is false) - log warning.
  • false - ignore strictTuples restriction violations.
• allowUnionTypes: pass true to allow using multiple non-null types in "type" keyword (one of strictTypes restricitons). see Strict types
• allowMatchingProperties: pass true to allow overlap between "properties" and "patternProperties". Does not affect other strict mode restrictions. See Strict Mode.
• validateFormats: format validation. Option values:
  • true (default) - validate formats (see Formats). In strict mode unknown formats will throw exception during schema compilation (and fail validation in case format keyword value is $data reference).
  • false - do not validate any format keywords (TODO they will still collect annotations once supported).

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: 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
• formats: an object with format definitions. Keys and values will be passed to addFormat method. Pass true as format definition to ignore some formats.
• keywords: an array of keyword definitions or strings. Values will be passed to addKeyword method.
• 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:
  • logger instance - it should have methods log, warn and error. If any of these methods is missing an exception will be thrown.
  • false - logging is disabled.
• 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 Removing additional properties. 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 or undefined 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 (object literal is used).
  • "empty" - in addition to missing or undefined, use defaults for properties and items that are equal to null or "" (an empty string).
• 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).

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 be http://json-schema.org/draft-07/schema or absent (draft-07 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 - it improves performance.
  • false - to not inline referenced schemas (they will always be compiled as separate functions).
  • integer number - to limit the maximum number of keywords of the schema that will be inlined (to balance the total size of compiled functions and performance).
• passContext: pass validation context to compile and validate 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 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.
• loopEnum (NEW in v7): by default enum keyword is compiled into a single expression. In case of a very large number of allowed values it may result in a large validation function. Pass integer to set the number of values above which enum keyword will be validated in a loop.
• 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).
• messages: Include human-readable messages in errors. true by default. false can be passed when messages are generated outside of Ajv code (e.g. with ajv-i18n).
• code (new in v7): code generation options:

type CodeOptions = {
  es5?: boolean // to generate es5 code - by default code is es6, with "for-of" loops, "let" and "const"
  lines?: boolean // add line-breaks to code - to simplify debugging of generated functions
  source?: boolean // add `source` property (see Source below) to validating function.
  process?: (code: string, schema?: SchemaEnv) => string // an optional function to process generated code
  // before it is passed to Function constructor.
  // It can be used to either beautify or to transpile code.
  optimize?: boolean | number // code optimization flag or number of passes, 1 pass by default,
  // code optimizations reduce the size of the generated code (bytes, based on the tests) by over 10%,
  // the number of code tree nodes by nearly 17%.
  // You would almost never need more than one optimization pass, unless you have some really complex schemas -
  // the second pass in the tests (it has quite complex schemas) only improves optimization by less than 0.1%.
  // See [Code optimization](./codegen.md#code-optimization) for details.
}

type Source = {
  code: string // unlike func.toString() it includes assignments exernal to function scope
  scope: Scope // see Code generation (TODO)
}

Validation errors

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

Error objects

Each error is an object with the following properties:

interface ErrorObject {
  keyword: string // validation keyword.
  dataPath: string // JSON pointer to the part of the data that was validated (e.g., `"/prop/1/subProp"`).
  schemaPath: string // the path (JSON-pointer as a URI fragment) to the schema of the failing keyword.
  // the object with the additional information about error that can be used to generate error messages
  // (e.g., using [ajv-i18n](https://github.com/ajv-validator/ajv-i18n) package).
  // See below for parameters set by all keywords.
  params: object // type is defined by keyword value, see below
  propertyName?: string // set for errors in `propertyNames` keyword schema.
  // `dataPath` still points to the object in this case.
  message?: string // the standard error message (can be excluded with option `messages` set to false).
  schema?: any // the schema of the keyword (added with `verbose` option).
  parentSchema?: object // the schema containing the keyword (added with `verbose` option)
  data?: any // the data validated by the keyword (added with `verbose` option).
}

Error parameters

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

In typescript, the ErrorObject is a discriminated union that allows to determine the type of error parameters based on the value of keyword:

const ajv = new Ajv()
const validate = ajv.compile<MyData>(schema)
if (validate(data)) {
  // data is MyData here
  // ...
} else {
  // DefinedError is a type for all pre-defined keywords errors,
  // validate.errors has type ErrorObject[] - to allow user-defined keywords with any error parameters.
  // Users can extend DefinedError to include the keywords errors they defined.
  for (const err of validate.errors as DefinedError[]) {
    switch (err.keyword) {
      case "maximum":
        console.log(err.limit)
        break
      case "pattern":
        console.log(err.pattern)
        break
      // ...
    }
  }
}

Also see an example in this test

• maxItems, minItems, maxLength, minLength, maxProperties, minProperties:

type ErrorParams = {limit: number} // keyword value

• additionalItems:

// when `items` is an array of schemas and `additionalItems` is false:
type ErrorParams = {limit: number} // the maximum number of allowed items

• additionalProperties:

type ErrorParams = {additionalProperty: string}
// the property not defined in `properties` and `patternProperties` keywords

• dependencies:

type ErrorParams = {
  property: string // dependent property,
  missingProperty: string // required missing dependency - only the first one is reported
  deps: string // required dependencies, comma separated list as a string (TODO change to string[])
  depsCount: number // the number of required dependencies
}

• format:

type ErrorParams = {format: string} // keyword value

• maximum, minimum, exclusiveMaximum, exclusiveMinimum:

type ErrorParams = {
  limit: number // keyword value
  comparison: "<=" | ">=" | "<" | ">" // operation to compare the data to the limit,
  // with data on the left and the limit on the right
}

• multipleOf:

type ErrorParams = {multipleOf: number} // keyword value

• pattern:

type ErrorParams = {pattern: string} // keyword value

• required:

type ErrorParams = {missingProperty: string} // required property that is missing

• propertyNames:

type ErrorParams = {propertyName: string} // invalid property name

User-defined keywords can define other keyword parameters.

Error logging

A logger instance can be passed via logger option to Ajv constructor. The use of other logging packages is supported as long as the package or its associated wrapper exposes the required methods. If any of the required methods are missing an exception will be thrown.

• Required Methods: log, warn, error

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