docs/json-schema.md
# JSON Schema
In a simple way, JSON Schema is an object with validation keywords.
The keywords and their values define what rules the data should satisfy to be valid.
[[toc]]
## JSON Schema versions
### draft-07 <Badge text="default" />
This version is provided as default export:
<code-group>
<code-block title="JavaScript">
```javascript
const Ajv = require("ajv")
const ajv = new Ajv()
```
</code-block>
<code-block title="TypeScript">
```typescript
import Ajv from "ajv"
const ajv = new Ajv()
```
</code-block>
</code-group>
::: tip draft-07 has better performance
Unless you need the new features of later versions, you would have more efficient generated code with this draft.
:::
### draft-2019-09 <Badge text="NEW" />
Ajv supports all new keywords of JSON Schema draft-2019-09:
- [unevaluatedProperties](#unevaluatedproperties)
- [unevaluatedItems](#unevaluateditems)
- [dependentRequired](#dependentrequired)
- [dependentSchemas](#dependentschemas)
- [maxContains/minContains](#maxcontains--mincontains)
- [$recursiveAnchor/$recursiveRef](./guide/combining-schemas.md#extending-recursive-schemas)
To use draft-2019-09 schemas you need to import a different Ajv class:
<code-group>
<code-block title="JavaScript">
```javascript
const Ajv2019 = require("ajv/dist/2019")
const ajv = new Ajv2019()
```
</code-block>
<code-block title="TypeScript">
```typescript
import Ajv2019 from "ajv/dist/2019"
const ajv = new Ajv2019()
```
</code-block>
</code-group>
You can use draft-07 schemas with this Ajv instance as well, draft-2019-09 is backwards compatible. If your schemas use `$schema` keyword, you need to add draft-07 meta-schema to Ajv instance:
<code-group>
<code-block title="JavaScript">
```javascript
const draft7MetaSchema = require("ajv/dist/refs/json-schema-draft-07.json")
ajv.addMetaSchema(draft7MetaSchema)
```
</code-block>
<code-block title="TypeScript">
```typescript
import * as draft7MetaSchema from "ajv/dist/refs/json-schema-draft-07.json"
ajv.addMetaSchema(draft7MetaSchema)
```
</code-block>
</code-group>
### draft-2020-12 <Badge text="BREAKING" type="warning" />
::: warning draft-2020-12 is not backwards compatible
You cannot use draft-2020-12 and previous JSON Schema versions in the same Ajv instance.
:::
Ajv supports all keywords of JSON Schema draft-2020-12:
- [prefixItems](#prefixItems) that replaced array form of items keyword
- changed [items](#items-in-draft-2020-12) keyword that combined parts of functionality of items and additionalItems
- [$dynamicAnchor/$dynamicRef](./guide/combining-schemas.md#extending-recursive-schemas)
To use draft-2020-12 schemas you need to import a different Ajv class:
<code-group>
<code-block title="JavaScript">
```javascript
const Ajv2020 = require("ajv/dist/2020")
const ajv = new Ajv2020()
```
</code-block>
<code-block title="TypeScript">
```typescript
import Ajv2020 from "ajv/dist/2020"
const ajv = new Ajv2020()
```
</code-block>
</code-group>
### draft-06
You can use JSON Schema draft-06 schemas with Ajv v7/8. If your schemas use `$schema` keyword, you need to add draft-06 meta-schema to Ajv instance. This example shows how to support both draft-06 and draft-07 schemas:
<code-group>
<code-block title="JavaScript">
```javascript
const Ajv = require("ajv")
const draft6MetaSchema = require("ajv/dist/refs/json-schema-draft-06.json")
const ajv = new Ajv()
ajv.addMetaSchema(draft6MetaSchema)
```
</code-block>
<code-block title="TypeScript">
```typescript
import Ajv from "ajv"
import * as draft6MetaSchema from "ajv/dist/refs/json-schema-draft-06.json"
const ajv = new Ajv()
ajv.addMetaSchema(draft6MetaSchema)
```
</code-block>
</code-group>
### draft-04
You can use JSON Schema draft-04 schemas with Ajv from v8.5.0 and the additional package [ajv-draft-04](https://github.com/ajv-validator/ajv-draft-04) (both ajv and ajv-draft-04 should be installed).
<code-group>
<code-block title="JavaScript">
```javascript
const Ajv = require("ajv-draft-04")
const ajv = new Ajv()
```
</code-block>
<code-block title="TypeScript">
```typescript
import Ajv from "ajv-draft-04"
const ajv = new Ajv()
```
</code-block>
</code-group>
::: warning Ajv cannot combine multiple JSON Schema versions
You can only use this import with JSON Schema draft-04, you cannot combine multiple JSON Schema versions in this ajv instance.
:::
## OpenAPI support
Ajv supports these additional [OpenAPI specification](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.1.0.md) keywords:
- [nullable](#nullable) - to avoid using `type` keyword with array of types.
- [discriminator](#discriminator) - to optimize validation and error reporting of tagged unions
## JSON data type
### `type`
`type` keyword requires that the data is of certain type (or some of types). Its value can be a string (the allowed type) or an array of strings (multiple allowed types).
Type can be: `number`, `integer`, `string`, `boolean`, `array`, `object` or `null`.
**Examples**
1. _schema_: `{type: "number"}`
_valid_: `1`, `1.5`
_invalid_: `"abc"`, `"1"`, `[]`, `{}`, `null`, `true`
2) _schema_: `{type: "integer"}`
_valid_: `1`, `2`
_invalid_: `"abc"`, `"1"`, `1.5`, `[]`, `{}`, `null`, `true`
3. _schema_: `{type: ["number", "string"]}`
_valid_: `1`, `1.5`, `"abc"`, `"1"`
_invalid_: `[]`, `{}`, `null`, `true`
All examples above are JSON Schemas that only require data to be of certain type to be valid.
Most other keywords apply only to a particular type of data. If the data is of different type, the keyword will not apply and the data will be considered valid.
In v7 Ajv introduced [Strict types](./strict-mode.md#strict-types) mode that makes these mistakes less likely by requiring that types are constrained with type keyword whenever another keyword that applies to specific type is used.
### nullable <Badge text="OpenAPI" />
This keyword can be used to allow `null` value in addition to the defined `type`.
Ajv supports it by default, without additional options. These two schemas are equivalent, but the first one is better supported by some tools and is also compatible with `strictTypes` option (see [Strict types](./strict-mode.md#strict-types))
```json
{
"type": "string",
"nullable": true
}
```
and
```json
{
"type": ["string", "null"]
}
```
::: warning nullable does not extend enum and const
If you use [enum](#enum) or [const](#const) keywords, `"nullable": true` would not extend the list of allowed values - `null` value has to be explicitly added to `enum` (and `const` would fail, unless it is `"const": null`)
This is different from how `nullable` is defined in [JSON Type Definition](./json-type-definition.md), where `"nullable": true` allows `null` value in addition to any data defined by the schema.
:::
## Keywords for numbers
### `maximum` / `minimum` and `exclusiveMaximum` / `exclusiveMinimum`
The value of keyword `maximum` (`minimum`) should be a number. This value is the maximum (minimum) allowed value for the data to be valid.
The value of keyword `exclusiveMaximum` (`exclusiveMinimum`) should be a number. This value is the exclusive maximum (minimum) allowed value for the data to be valid (the data equal to this keyword value is invalid).
::: warning NO support for boolean keyword values
Boolean values for keywords `exclusiveMaximum` (`exclusiveMinimum`) are not supported.
:::
**Examples**
1. _schema_: `{type: "number", maximum: 5}`
_valid_: `4`, `5`
_invalid_: `6`, `7`
2. _schema_: `{type: "number", minimum: 5}`
_valid_: `5`, `6`
_invalid_: `4`, `4.5`
3. _schema_: `{type: "number", exclusiveMinimum: 5}`
_valid_: `6`, `7`
_invalid_: `4.5`, `5`
### `multipleOf`
The value of the keyword should be a number. The data to be valid should be a multiple of the keyword value (i.e. the result of division of the data on the value should be integer).
**Examples**
1. _schema_: `{type: "number", multipleOf: 5}`
_valid_: `5`, `10`
_invalid_: `1`, `4`
2) _schema_: `{type: "number", multipleOf: 2.5}`
_valid_: `2.5`, `5`, `7.5`
_invalid_: `1`, `4`
## Keywords for strings
### `maxLength` / `minLength`
::: warning Grapheme clusters will count as multiple characters
Certain charsets have characters that are made up of multiple Unicode code points. These [grapheme clusters](https://www.unicode.org/reports/tr29/tr29-17.html#Grapheme_Cluster_Boundaries) are counted as multiple in length calculations.
:::
The value of the keywords should be a number. The data to be valid should have length satisfying this rule. Unicode pairs are counted as a single character.
**Examples**
1. _schema_: `{type: "string", maxLength: 5}`
_valid_: `"abc"`, `"abcde"`
_invalid_: `"abcdef"`
2) _schema_: `{type: "string", minLength: 2}`
_valid_: `"ab"`, `"😀😀"`
_invalid_: `"a"`, `"😀"`
### `pattern`
The value of the keyword should be a string. The data to be valid should match the regular expression defined by the keyword value.
Ajv uses `new RegExp(value, "u")` to create the regular expression that will be used to test data.
**Example**
_schema_: `{type: "string", pattern: "[abc]+"}`
_valid_: `"a"`, `"abcd"`, `"cde"`
_invalid_: `"def"`, `""`
### `format`
The value of the keyword should be a string. The data to be valid should match the format with this name.
Ajv does not include any formats, they can be added with [ajv-formats](https://github.com/ajv-validator/ajv-formats) plugin.
**Example**
_schema_: `{type: "string", format: "ipv4"}`
_valid_: `"192.168.0.1"`
_invalid_: `"abc"`
## Keywords for arrays
### `maxItems` / `minItems`
The value of the keywords should be a number. The data array to be valid should not have more (less) items than the keyword value.
**Example**
_schema_: `{type: "array", maxItems: 3}`
_valid_: `[]`, `[1]`, `["1", 2, "3"]`
_invalid_: `[1, 2, 3, 4]`
### `uniqueItems`
The value of the keyword should be a boolean. If the keyword value is `true`, the data array to be valid should have unique items.
**Example**
_schema_: `{type: "array", uniqueItems: true}`
_valid_: `[]`, `[1]`, `["1", 2, "3"]`
_invalid_: `[1, 2, 1]`, `[{a: 1, b: 2}, {b: 2, a: 1}]`
### `items`
#### `items` in draft-04, -06, -07 and -2019-09
::: warning items keyword changed in JSON Schema draft-2020-12
This section describes `items` keyword in all JSON Schema versions prior to draft-2020-12.
:::
The value of the keyword should be a schema or an array of schemas.
If the keyword value is a schema, then for the data array to be valid each item of the array should be valid according to the schema. In this case the `additionalItems` keyword is ignored.
If the keyword value is an array, then items with indices less than the number of items in the keyword should be valid according to the schemas with the same indices. Whether additional items are valid will depend on `additionalItems` keyword.
**Examples**
1. _schema_: `{type: "array", items: {type: "integer"}}`
_valid_: `[1,2,3]`, `[]`
_invalid_: `[1,"abc"]`
2. _schema_:
```javascript
{
type: "array",
items: [{type: "integer"}, {type: "string"}]
}
```
_valid_: `[1]`, `[1, "abc"]`, `[1, "abc", 2]`, `[]`
_invalid_: `["abc", 1]`, `["abc"]`
The schema in example 2 will log warning by default (see `strictTuples` option), because it defines unconstrained tuple. To define a tuple with exactly 2 elements use `minItems` and `additionalItems` keywords (see example 1 in `additionalItems`).
#### `items` in draft-2020-12 <Badge text="NEW" />
::: warning items keyword changed in JSON Schema draft-2020-12
This section describes `items` keyword in JSON draft-2020-12.
:::
The value of the keyword must be a schema.
For the data array to be valid:
- if [prefixItems](#prefixItems) keyword is not used in the schema, then each item of the array must be valid according to the schema in `items`.
- if [prefixItems](#prefixItems) keyword is used in the schema, then each item with the index starting from the size of `prefixItems` schema must be valid according to the schema in `items`
**Examples**
1. _schema_: `{type: "array", items: {type: "integer"}}`
_valid_: `[1,2,3]`, `[]`
_invalid_: `[1,"abc"]`
2. _schema_:
```javascript
{
type: "array",
prefixItems: [{type: "integer"}, {type: "integer"}],
minItems: 2
items: false
}
```
_valid_: `[1, 2]`
_invalid_: `[]`, `[1]`, `[1, 2, 3]`, `[1, "abc"]` (any wrong number of items or wrong type)
3. _schema_:
```javascript
{
type: "array",
prefixItems: [{type: "integer"}, {type: "integer"}],
items: {type: "string"}
}
```
_valid_: `[]`, `[1, 2]`, `[1, 2, "abc"]`
_invalid_: `["abc"]`, `[1, 2, 3]`
```
_valid_: `[1]`, `[1, "abc"]`, `[1, "abc", 2]`, `[]`
_invalid_: `["abc", 1]`, `["abc"]`
The schema in example 3 will log warning by default (see `strictTuples` option), because it defines unconstrained tuple. To define a tuple with exactly 2 elements use `minItems` and `items` keywords (see example 2).
### `prefixItems` <Badge text="NEW: draft 2020-12" />
The value of the keyword must be an array of schemas.
For the data array to be valid, the items with indices less than the number of schemas in this keyword must be valid according to the schemas with the same indices. Whether additional items are valid will depend on `items` keyword.
**Examples**
_schema_:
```javascript
{
type: "array",
prefixItems: [{type: "integer"}, {type: "string"}]
}
```
_valid_: `[1]`, `[1, "abc"]`, `[1, "abc", 2]`, `[]`
_invalid_: `["abc", 1]`, `["abc"]`
The schema in example will log warning by default (see `strictTuples` option), because it defines unconstrained tuple. To define a tuple with exactly 2 elements use [minItems](#minitems) and [items](#items-in-draft-2020-12) keywords (see example 2 in [items](#items-in-draft-2020-12)).
### `additionalItems`
::: warning additionalItems is not supported in JSON Schema draft-2020-12
To create and equivalent schema in draft-2020-12 use keywords [prefixItems](#prefixItems) and the new [items](#items-in-draft-2020-12) keyword
:::
The value of the keyword should be a boolean or an object.
If `items` keyword is not present or it is an object, `additionalItems` keyword should be ignored regardless of its value. By default Ajv will throw exception in this case - see [Strict mode](./strict-mode.md)
If `items` keyword is an array and data array has not more items than the length of `items` keyword value, `additionalItems` keyword is also ignored.
If the length of data array is bigger than the length of "items" keyword value than the result of the validation depends on the value of `additionalItems` keyword:
- `false`: data is invalid
- `true`: data is valid
- an object: data is valid if all additional items (i.e. items with indices greater or equal than "items" keyword value length) are valid according to the schema in "additionalItems" keyword.
The schemas in examples 2-3 log warning by default, use option `strictTuples: false` to allow)
**Examples**
1. _schema_:
```javascript
{
type: "array",
items: [{type: "integer"}, {type: "integer"}],
minItems: 2
additionalItems: false
}
```
_valid_: `[1, 2]`
_invalid_: `[]`, `[1]`, `[1, 2, 3]`, `[1, "abc"]` (any wrong number of items or wrong type)
2. _schema_:
```javascript
{
type: "array",
items: [{type: "integer"}, {type: "integer"}],
additionalItems: true
}
```
_valid_: `[]`, `[1, 2]`, `[1, 2, 3]`, `[1, 2, "abc"]`
_invalid_: `["abc"]`, `[1, "abc", 3]`
3. _schema_:
```javascript
{
type: "array",
items: [{type: "integer"}, {type: "integer"}],
additionalItems: {type: "string"}
}
```
_valid_: `[]`, `[1, 2]`, `[1, 2, "abc"]`
_invalid_: `["abc"]`, `[1, 2, 3]`
### `contains`
The value of the keyword is a JSON Schema. The array is valid if it contains at least one item that is valid according to this schema.
**Example**
_schema_: `{type: "array", contains: {type: "integer"}}`
_valid_: `[1]`, `[1, "foo"]`, any array with at least one integer
_invalid_: `[]`, `["foo", "bar"]`, any array without integers
### `maxContains` / `minContains` <Badge text="NEW: draft 2019-09" />
The value of these keywords should be an integer.
Without `contains` keyword they are ignored (logs error or throws exception in ajv [strict mode](./strict-mode.md)).
The array is valid if it contains at least `minContains` items and no more than `maxContains` items that are valid against the schema in `contains` keyword.
**Example**
_schema_:
```javascript
{
type: "array",
contains: {type: "integer"},
minContains: 2,
maxContains: 3
}
```
_valid_: `[1, 2]`, `[1, 2, 3, "foo"]`, any array with 2 or 3 integers
_invalid_: `[]`, `[1, "foo"]`, `[1, 2, 3, 4]`, any array with fewer than 2 or more than 3 integers
### `unevaluatedItems` <Badge text="NEW: draft 2019-09" />
The value of this keyword is a JSON Schema (can be a boolean).
This schema will be applied to all array items that were not evaluated by other keywords for items (`items`, `additionalItems` and `contains`) in the current schema and all sub-schemas that were valid for this data instance. It includes:
- all subschemas schemas in `allOf` and `$ref` keywords
- valid sub-schemas in `oneOf` and `anyOf` keywords
- sub-schema in `if` keyword
- sub-schemas in `then` or `else` keywords that were applied based on the validation result by `if` keyword.
The only scenario when this keyword would be applied to some items is when `items` keyword value is an array of schemas and `additionalItems` was not present (or did not apply, in case it was present in some invalid subschema).
Some user-defined keywords can also make items "evaluated".
**Example**
_schema_:
```javascript
{
type: "array",
items: [
{type: "number"},
{type: "number"}
],
unevaluatedItems: false,
anyOf: [
{items: [true, true, {type: "number"}]},
{items: [true, true, {type: "boolean"}]}
]
}
```
_valid_: `[1, 2, 3]`, `[1, 2, true]`
_invalid_:
- `[1, 2]` - the third item is not present
- `[1, 2, "3"]` - the third item is "unevaluated"
See [tests](https://github.com/json-schema-org/JSON-Schema-Test-Suite/blob/master/tests/draft2019-09/unevaluatedItems.json) for `unevaluatedItems` keyword for other examples.
## Keywords for objects
### `maxProperties` / `minProperties`
The value of the keywords should be a number. The data object to be valid should have not more (less) properties than the keyword value.
**Example**
_schema_: `{type: "object", maxProperties: 2 }`
_valid_: `{}`, `{a: 1}`, `{a: "1", b: 2}`
_invalid_: `{a: 1, b: 2, c: 3}`
### `required`
The value of the keyword should be an array of unique strings. The data object to be valid should contain all properties with names equal to the elements in the keyword value.
**Example**
_schema_: `{type: "object", required: ["a", "b"]}`
_valid_: `{a: 1, b: 2}`, `{a: 1, b: 2, c: 3}`
_invalid_: `{}`, `{a: 1}`, `{c: 3, d: 4}`
### `properties`
The value of the keyword should be a map with keys equal to data object properties. Each value in the map should be a JSON Schema. For data object to be valid the corresponding values in data object properties should be valid according to these schemas.
::: warning Properties are not required
`properties` keyword does not require that the properties mentioned in it are present in the object (see examples).
:::
**Example**
_schema_:
```javascript
{
type: "object",
properties: {
foo: {type: "string"},
bar: {
type: "number",
minimum: 2
}
}
}
```
_valid_: `{}`, `{foo: "a"}`, `{foo: "a", bar: 2}`
_invalid_: `{foo: 1}`, `{foo: "a", bar: 1}`
### `patternProperties`
The value of this keyword should be a map where keys should be regular expressions and the values should be JSON Schemas. For data object to be valid the values in data object properties that match regular expression(s) should be valid according to the corresponding schema(s).
When the value in data object property matches multiple regular expressions it should be valid according to all the schemas for all matched regular expressions.
::: warning Unexpected validation results
1. `patternProperties` keyword does not require that properties matching patterns are present in the object (see examples).
2. By default, Ajv does not allow schemas where patterns in `patternProperties` match any property name in `properties` keyword - that leads to unexpected validation results. It can be allowed with option `allowMatchingProperties`. See [Strict mode](./strict-mode.md)
:::
**Example**
_schema_:
```javascript
{
type: "object",
patternProperties: {
"^fo.*$": {type: "string"},
"^ba.*$": {type: "number"}
}
}
```
_valid_: `{}`, `{foo: "a"}`, `{foo: "a", bar: 1}`
_invalid_: `{foo: 1}`, `{foo: "a", bar: "b"}`
### `additionalProperties`
The value of the keyword should be either a boolean or a JSON Schema.
If the value is `true` the keyword is ignored.
If the value is `false` the data object to be valid should not have "additional properties" (i.e. properties other than those used in "properties" keyword and those that match patterns in "patternProperties" keyword).
If the value is a schema for the data object to be valid the values in all "additional properties" should be valid according to this schema.
**Examples**
1. _schema_:
```javascript
{
type: "object",
properties: {
foo: {type: "number"}
},
patternProperties: {
"^.*r$": {type: "number"}
},
additionalProperties: false
}
```
_valid_: `{}`, `{foo: 1}`, `{foo: 1, bar: 2}`
_invalid_: `{a: 3}`, `{foo: 1, baz: 3}`
2. _schema_:
```javascript
{
type: "object",
properties: {
foo: {type: "number"}
},
patternProperties: {
"^.*r$": {type: "number"}
},
additionalProperties: {type: "string"}
}
```
_valid_: `{}`, `{a: "b"}`, `{foo: 1}`, `{foo: 1, bar: 2}`, `{foo: 1, bar: 2, a: "b"}`
_invalid_: `{a: 3}`, `{foo: 1, baz: 3}`
3. _schema_:
```javascript
{
type: "object",
properties: {
foo: {type: "number"}
},
additionalProperties: false,
anyOf: [
{
properties: {
bar: {type: "number"}
}
},
{
properties: {
baz: {type: "number"}
}
}
]
}
```
_valid_: `{}`, `{foo: 1}`
_invalid_: `{bar: 2}`, `{baz: 3}`, `{foo: 1, bar: 2}`, etc.
### `dependencies` <Badge text="deprecated in draft 2019-09" type="warning" />
This keyword is deprecated. The same functionality is available with keywords `dependentRequired` and `dependentSchemas`.
The value of the keyword is a map with keys equal to data object properties. Each value in the map should be either an array of unique property names ("property dependency" - see [`dependentRequired`](#`dependentrequired`) keyword) or a JSON Schema ("schema dependency" - see [`dependentSchemas`](#`dependentschemas`) keyword).
For property dependency, if the data object contains a property that is a key in the keyword value, then to be valid the data object should also contain all properties from the array of properties.
For schema dependency, if the data object contains a property that is a key in the keyword value, then to be valid the data object itself (NOT the property value) should be valid according to the schema.
**Examples**
1. _schema (property dependency)_:
```javascript
{
type: "object",
dependencies: {
foo: ["bar", "baz"]
}
}
```
_valid_: `{foo: 1, bar: 2, baz: 3}`, `{}`, `{a: 1}`
_invalid_: `{foo: 1}`, `{foo: 1, bar: 2}`, `{foo: 1, baz: 3}`
2. _schema (schema dependency)_:
```javascript
{
type: "object",
dependencies: {
foo: {
properties: {
bar: {type: "number"}
}
}
}
}
```
_valid_: `{}`, `{foo: 1}`, `{foo: 1, bar: 2}`, `{a: 1}`
_invalid_: `{foo: 1, bar: "a"}`
### `dependentRequired` <Badge text="NEW: draft 2019-09" />
The value of this keyword should be a map with keys equal to data object properties. Each value in the map should be an array of unique property names.
If the data object contains a property that is a key in the keyword value, then to be valid the data object should also contain all properties from the corresponding array of properties in this keyword.
**Example**
_schema_:
```javascript
{
type: "object",
dependentRequired: {
foo: ["bar", "baz"]
}
}
```
_valid_: `{foo: 1, bar: 2, baz: 3}`, `{}`, `{a: 1}`
_invalid_: `{foo: 1}`, `{foo: 1, bar: 2}`, `{foo: 1, baz: 3}`
### `dependentSchemas` <Badge text="NEW: draft 2019-09" />
The value of the keyword should be a map with keys equal to data object properties. Each value in the map should be a JSON Schema.
If the data object contains a property that is a key in the keyword value, then to be valid the data object itself (NOT the property value) should be valid according to the corresponding schema in this keyword.
**Example**
_schema_:
```javascript
{
type: "object",
dependentSchemas: {
foo: {
properties: {
bar: {type: "number"}
}
}
}
}
```
_valid_: `{}`, `{foo: 1}`, `{foo: 1, bar: 2}`, `{a: 1}`
_invalid_: `{foo: 1, bar: "a"}`
### `propertyNames`
The value of this keyword is a JSON Schema.
For data object to be valid each property name in this object should be valid according to this schema.
**Example**
_schema_ (requires `email` format from [ajv-formats](https://github.com/ajv-validator/ajv-formats)):
```javascript
{
type: "object",
propertyNames: {
format: "email"
}
}
```
_valid_: `{"foo@bar.com": "any", "bar@bar.com": "any"}`
_invalid_: `{foo: "any value"}`
### `unevaluatedProperties` <Badge text="NEW: draft 2019-09" />
The value of this keyword is a JSON Schema (can be a boolean).
This schema will be applied to all properties that were not evaluated by other keywords for properties (`properties`, `patternProperties` and `additionalProperties`) in the current schema and all sub-schemas that were valid for this data instance. It includes:
- all subschemas schemas in `allOf` and `$ref` keywords
- valid sub-schemas in `oneOf` and `anyOf` keywords
- sub-schema in `if` keyword
- sub-schemas in `then` or `else` keywords that were applied based on the validation result by `if` keyword.
Some user-defined keywords can also make properties "evaluated".
**Example**
_schema_:
```javascript
{
type: "object",
required: ["foo"],
properties: {foo: {type: "number"}},
unevaluatedProperties: false,
anyOf: [
{
required: ["bar"],
properties: {bar: {type: "number"}}
}
{
required: ["baz"],
properties: {baz: {type: "number"}}
}
]
}
```
_valid_: `{foo: 1, bar: 2}`, `{foo: 1, baz: 2}`, `{foo: 1, bar: 2, baz: 3}`
_invalid_:
- `{foo: 1}` - neither `bar` nor `baz` are present
- `{foo: 1, bar: 2, boo: 3}` - `boo` is unevaluated
- `{foo: 1, bar: 2, baz: "3"}` - not valid against the 2nd subschema, so `baz` is "unevaluated".
See [tests](https://github.com/json-schema-org/JSON-Schema-Test-Suite/blob/master/tests/draft2019-09/unevaluatedProperties.json) for `unevaluatedProperties` keyword for other examples.
### discriminator <Badge text="NEW: OpenAPI" />
Ajv has a limited support for `discriminator` keyword: to optimize validation, error handling, and [modifying data](./guide/modifying-data.md) with [oneOf](#oneof) keyword.
Its value should be an object with a property `propertyName` - the name of the property used to discriminate between union members.
When using discriminator keyword only one subschema in `oneOf` will be used, determined by the value of discriminator property.
::: warning Use option discriminator
To use `discriminator` keyword you have to use option `discriminator: true` with Ajv constructor - it is not enabled by default.
:::
**Example**
_schema_:
```javascript
{
type: "object",
discriminator: {propertyName: "foo"},
required: ["foo"],
oneOf: [
{
properties: {
foo: {const: "x"},
a: {type: "string"},
},
required: ["a"],
},
{
properties: {
foo: {enum: ["y", "z"]},
b: {type: "string"},
},
required: ["b"],
},
],
}
```
_valid_: `{foo: "x", a: "any"}`, `{foo: "y", b: "any"}`, `{foo: "z", b: "any"}`
_invalid_:
- `{}`, `{foo: 1}` - discriminator tag must be string
- `{foo: "bar"}` - discriminator tag value must be in oneOf subschema
- `{foo: "x", b: "b"}`, `{foo: "y", a: "a"}` - invalid object
From the perspective of validation result `discriminator` is defined as no-op (that is, removing discriminator will not change the validity of the data), but errors reported in case of invalid data will be different.
There are following requirements and limitations of using `discriminator` keyword:
- `mapping` in discriminator object is not supported.
- [oneOf](#oneof) keyword must be present in the same schema.
- discriminator property should be [required](#required) either on the top level, as in the example, or in all `oneOf` subschemas.
- each `oneOf` subschema must have [properties](#properties) keyword with discriminator property. The subschemas should be either inlined or included as direct references (only `$ref` keyword without any extra keywords is allowed).
- schema for discriminator property in each `oneOf` subschema must be [const](#const) or [enum](#enum), with unique values across all subschemas.
Not meeting any of these requirements would fail schema compilation.
## Keywords for all types
### `enum`
The value of the keyword should be an array of unique items of any types. The data is valid if it is deeply equal to one of items in the array.
**Example**
_schema_: `{enum: [2, "foo", {foo: "bar" }, [1, 2, 3]]}`
_valid_: `2`, `"foo"`, `{foo: "bar"}`, `[1, 2, 3]`
_invalid_: `1`, `"bar"`, `{foo: "baz"}`, `[1, 2, 3, 4]`, any value not in enum
### `const`
The value of this keyword can be anything. The data is valid if it is deeply equal to the value of the keyword.
**Example**
_schema_: `{const: "foo"}`
_valid_: `"foo"`
_invalid_: any other value
The same can be achieved with `enum` keyword using the array with one item. But `const` keyword is more than just a syntax sugar for `enum`. In combination with the [$data reference](./guide/combining-schemas.md#data-reference) it allows to define equality relations between different parts of the data. This cannot be achieved with `enum` keyword even with `$data` reference because `$data` cannot be used in place of one item - it can only be used in place of the whole array in `enum` keyword.
**Example**
_schema_:
```javascript
{
type: "object",
properties: {
foo: {type: "number"},
bar: {const: {$data: "1/foo"}}
}
}
```
_valid_: `{foo: 1, bar: 1}`, `{}`
_invalid_: `{foo: 1}`, `{bar: 1}`, `{foo: 1, bar: 2}`
## Compound keywords
### `not`
The value of the keyword should be a JSON Schema. The data is valid if it is invalid according to this schema.
**Example**
_schema_: `{type: "number", not: {minimum: 3}}`
_valid_: `1`, `2`
_invalid_: `3`, `4`
### `oneOf`
The value of the keyword should be an array of JSON Schemas. The data is valid if it matches exactly one JSON Schema from this array. Validators have to validate data against all schemas to establish validity according to this keyword.
**Example**
_schema_:
```javascript
{
type: "number",
oneOf: [{maximum: 3}, {type: "integer"}]
}
```
_valid_: `1.5`, `2.5`, `4`, `5`
_invalid_: `2`, `3`, `4.5`, `5.5`
### `anyOf`
The value of the keyword should be an array of JSON Schemas. The data is valid if it is valid according to one or more JSON Schemas in this array. Validators only need to validate data against schemas in order until the first schema matches (or until all schemas have been tried). For this reason validating against this keyword is faster than against "oneOf" keyword in most cases.
**Example**
_schema_:
```javascript
{
type: "number",
anyOf: [{maximum: 3}, {type: "integer"}]
}
```
_valid_: `1.5`, `2`, `2.5`, `3`, `4`, `5`
_invalid_: `4.5`, `5.5`
### `allOf`
The value of the keyword should be an array of JSON Schemas. The data is valid if it is valid according to all JSON Schemas in this array.
**Example**
_schema_:
```javascript
{
type: "number",
allOf: [{maximum: 3}, {type: "integer"}]
}
```
_valid_: `2`, `3`
_invalid_: `1.5`, `2.5`, `4`, `4.5`, `5`, `5.5`
### `if`/`then`/`else`
These keywords allow to implement conditional validation. Their values should be valid JSON Schemas (object or boolean).
If `if` keyword is absent, the validation succeeds.
If the data is valid against the sub-schema in `if` keyword, then the validation result is equal to the result of data validation against the sub-schema in `then` keyword (if `then` is absent, the validation succeeds).
If the data is invalid against the sub-schema in `if` keyword, then the validation result is equal to the result of data validation against the sub-schema in `else` keyword (if `else` is absent, the validation succeeds).
**Examples**
1. _schema_:
```javascript
{
type: "object",
if: {properties: {foo: {minimum: 10}}},
then: {required: ["bar"]},
else: {required: ["baz"]}
}
```
_valid_:
- `{foo: 10, bar: true }`
- `{}`
- `{foo: 1, baz: true }`
_invalid_:
- `{foo: 10}` (`bar` is required)
- `{foo: 10, baz: true }` (`bar` is required)
- `{foo: 1}` (`baz` is required)
2) _schema_:
```javascript
{
type: "integer",
minimum: 1,
maximum: 1000,
if: {minimum: 100},
then: {multipleOf: 100},
else: {
if: {minimum: 10},
then: {multipleOf: 10}
}
}
```
_valid_: `1`, `5`, `10`, `20`, `50`, `100`, `200`, `500`, `1000`
_invalid_:
- `-1`, `0` (<1)
- `2000` (>1000)
- `11`, `57`, `123` (any integer with more than one non-zero digit)
- non-integers
## Metadata keywords
JSON Schema specification defines several metadata keywords that describe the schema itself but do not perform any validation.
- `title` and `description`: information about the data represented by that schema
- `$comment`: information for developers. With option `$comment` Ajv logs or passes the comment string to the user-supplied function. See [Options](./api.md#options).
- `default`: a default value of the data instance, see [Assigning defaults](./guide/modifying-data.md#assigning-defaults).
- `examples`: an array of data instances. Ajv does not check the validity of these instances against the schema.
- `readOnly` and `writeOnly`: marks data-instance as read-only or write-only in relation to the source of the data (database, api, etc.).
- `contentEncoding`: [RFC 2045](https://tools.ietf.org/html/rfc2045#section-6.1), e.g., "base64".
- `contentMediaType`: [RFC 2046](https://datatracker.ietf.org/doc/rfc2046/), e.g., "image/png".
::: warning Ignored keywords
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.
:::