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JSON-Schema validation keywords

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.

Keywords

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.

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 boolean value. These keyword cannot be used without maximum (minimum). If this keyword value is equal to true, the data should not be equal to the value in maximum (minimum) keyword to be valid.

Examples

  1. schema: { "maximum": 5 }

    valid: 4, 5, any non-number ("abc", [], {}, null, true)

    invalid: 6, 7

  2. schema: { "minimum": 5 }

    valid: 5, 6, any non-number ("abc", [], {}, null, true)

    invalid: 4, 4.5

  3. schema: { "minimum": 5, "exclusiveMinimum": true }

    valid: 6, 7, any non-number ("abc", [], {}, null, true)

    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: { "multipleOf": 5 }

    valid: 5, 10, any non-number ("abc", [], {}, null, true)

    invalid: 1, 4

  2. schema: { "multipleOf": 2.5 }

    valid: 2.5, 5, 7.5, any non-number ("abc", [], {}, null, true)

    invalid: 1, 4

Keywords for strings

maxLength / minLength

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: { "maxLength": 5 }

    valid: "abc", "abcde", any non-string (1, [], {}, null, true)

    invalid: "abcdef"

  2. schema: { "minLength": 2 }

    valid: "ab", "😀😀", any non-string (1, [], {}, null, true)

    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 Regex(value) to create the regular expression that will be used to test data.

Example

schema: { "pattern": "[abc]+" }

valid: "a", "abcd", "cde", any non-string (1, [], {}, null, true)

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 defines these formats: date, date-time, uri, email, hostname, ipv4, ipv6, regex.

Example

schema: { "format": "ipv4" }

valid: "192.168.0.1", any non-string (1, [], {}, null, true)

invalid: "abc"

formatMaximum / formatMinimum and formatExclusiveMaximum / formatExclusiveMinimum (v5 proposal)

The value of keyword formatMaximum (formatMinimum) should be a string. This value is the maximum (minimum) allowed value for the data to be valid as determined by format keyword.

Ajv defines comparison rules for formats "date", "time" and `"date-time".

The value of keyword formatExclusiveMaximum (formatExclusiveMinimum) should be a boolean value. These keyword cannot be used without formatMaximum (formatMinimum). If this keyword value is equal to true, the data to be valid should not be equal to the value in formatMaximum (formatMinimum) keyword.

Example

schema:

{
    "format": "date",
    "formatMaximum": "2016-02-06",
    "formatExclusiveMaximum": true
}

valid: 2015-12-31, "2016-02-05", any non-string

invalid: "2016-02-06", "2016-02-07", "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: { "maxItems": 3 }

valid: [], [1], ["1", 2, "3"], any non-array ("abc", 1, {}, null, true)

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: { "uniqueItems": true }

valid: [], [1], ["1", 2, "3"], any non-array ("abc", 1, {}, null, true)

invalid: [1, 2, 1], [{ "a": 1, "b": 2 }, { "b": 2, "a": 1 }]

items

The value of the keyword should be an object or an array of objects.

If the keyword value is an object, then for the data array to be valid each item of the array should be valid according to the schema in this value. In this case the "additionalItems" keyword is ignored.

If the keyword value is an array, then items with indeces less than the number of items in the keyword should be valid according to the schemas with the same indeces. Whether additional items are valid will depend on "additionalItems" keyword.

Examples

  1. schema: { "items": { "type": "integer" } }

    valid: [1,2,3], [], any non-array (1, "abc", {}, null, true)

    invalid: [1,"abc"]

  2. schema:

    {
    "items": [
        { "type": "integer" },
        { "type": "string" }
    ]
}

    valid: [1], [1, "abc"], [1, "abc", 2], [], any non-array (1, "abc", {}, null, true)

    invalid: ["abc", 1], ["abc"]

additionalItems

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 is ignored regardless of its value.

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 indeces greater or equal than "items" keyword value length) are valid according to the schema in "assitionalItems" keyword.

Examples

  1. schema: { "additionalItems": { "type": "integer" } }

    any data is valid against such schema - "additionalItems" is ignored.

  2. schema:

    {
    "items": { "type": "integer" },
    "additionalItems": { "type": "string" }
}

    valid: [], [1, 2], any non-array ("additionalItems" is ignored)

    invalid: [1, "abc"], (any array with some items other than integers)

  3. schema:

    {
    "items": [
        { "type": "integer" },
        { "type": "integer" }
    ],
    "additionalItems": true
}

    valid: [], [1, 2], [1, 2, 3], [1, 2, "abc"], any non-array

    invalid: ["abc"], [1, "abc", 3]

  4. schema:

    {
    "items": [
        { "type": "integer" },
        { "type": "integer" }
    ],
    "additionalItems": { "type": "string" }
}

    valid: [], [1, 2], [1, 2, "abc"], any non-array

    invalid: ["abc"], [1, 2, 3]

contains (v5 proposal)

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: { "contains": { "type": "integer" } }

valid: [1], [1, "foo"], any array with at least one integer, any non-array

invalid: [], ["foo", "bar"], any array without integers

The same can be expressed using only draft 4 keywords but it is quite verbose. The schema from the example above is equivalent to:

{
    "not": {
        "type": "array",
        "items": {
            "not": { "type": "integer" }
        }
    }
}

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: { "maxProperties": 2 }

valid: {}, {"a": 1}, {"a": "1", "b": 2}, any non-object

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: { "required": ["a", "b"] }

valid: {"a": 1, "b": 2}, {"a": 1, "b": 2, "c": 3}, any non-object

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.

Please note: properties keyword does not require that the properties mentioned in it are present in the object (see examples).

Example

schema:

{
    "properties": {
        "foo": { "type": "string" },
        "bar": {
            "type": "number",
            "minimum": 2
        }
    }
}

valid: {}, {"foo": "a"}, {"foo": "a", "bar": 2}, any non-object

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.

Please note: patternProperties keyword does not require that properties matching patterns are present in the object (see examples).

Example

schema:

{
    "patternProperties": {
        "^fo.*$": { "type": "string" },
        "^ba.*$": { "type": "number" }
    }
}

valid: {}, {"foo": "a"}, {"foo": "a", "bar": 1}, any non-object

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:

    {
    "properties": {
        "foo": { "type": "number" }
    },
    "patternProperties": {
        "^.*r$": { "type": "number" }
    },
    "additionalProperties": false
}

    valid: {}, {"foo": 1}, {"foo": 1, "bar": 2}, any non-object

    invalid: {"a": 3}, {"foo": 1, "baz": 3}

  2. schema:

    {
    "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"}, any non-object

    invalid: {"a": 3}, {"foo": 1, "baz": 3}

dependencies

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") or a JSON schema ("schema dependency").

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):

    {
    "dependencies": {
        "foo": ["bar", "baz"]
    }
}

    valid: {"foo": 1, "bar": 2, "baz": 3}, {}, {"a": 1}, any non-object

    invalid: {"foo": 1}, {"foo": 1, "bar": 2}, {"foo": 1, "baz": 3}

  2. schema (schema dependency):

    {
    "dependencies": {
        "foo": {
            "properties": {
                "bar": { "type": "number" }
            } 
        }
    }
}

    valid: {}, {"foo": 1}, {"foo": 1, "bar": 2}, {"a": 1}, any non-object

    invalid: {"foo": 1, "bar": "a"}

patternGroups (v5 proposal)

The value of this keyword should be a map where keys should be regular expressions and the values should be objects with the following properties:

  • schema (required) - should be a JSON schema. 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).
  • maximum / minimum (optional) - should be integers. For data object to be valid the number of properties that match regular expression(s) should be within limits set by minimum(s) and maximum(s).

Example

schema:

{
    "patternGroups": {
        "^[a-z]+$": {
            "minimum": 1,
            "schema": { "type": "string" }
        },
        "^[0-9]+$": {
            "minimum": 1,
            "schema": { "type": "integer" }
        }
    }
}

valid: { "foo": "bar", "1": "2" }, any non-object

invalid: {}, { "foo": "bar" }, { "1": "2" }

patternRequired (v5 proposal)

The value of this keyword should be an array of strings, each string being a regular expression. For data object to be valid each regular expression in this array should match at least one property name in the data object.

If the array contains multiple regular expressions, more than one expression can match the same property name.

Examples

  1. schema: { "patternRequired": [ "f.*o" ] }

    valid: { "foo": 1 }, { "-fo-": 1 }, { "foo": 1, "bar": 2 }, any non-object

    invalid: {}, { "bar": 2 }, { "Foo": 1 },

  2. schema: { "patternRequired": [ "f.*o", "b.*r" ] }

    valid: { "foo": 1, "bar": 2 }, { "foobar": 3 }, any non-object

    invalid: {}, { "foo": 1 }, { "bar": 2 }

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 the array

constant (v5 proposal)

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: { "constant": "foo" }

valid: "foo"

invalid: any other value

The same can be achieved with enum keyword using the array with one item. But constant keyword is more that just a syntax sugar for enum. In combination with the $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:

{
    "properties": {
        "foo": { "type": "number" },
        "bar": { "constant": { "$data": "1/foo" } }
    }
}

valid: { "foo": 1, "bar": 1 }, {}

invalid: { "foo": 1 }, { "bar": 1 }, { "foo": 1, "bar": 2 }

not

The value of the keyword should be a JSON schema. The data is valid if it is invalid according to this schema.

Examples

  1. schema: { "not": { "minimum": 3 } }

    valid: 1, 2

    invalid: 3, 4, any non-number

  2. schema:

    {
    "not": {
        "items": {
            "not": { "type": "string" }
        }
    }
}

    valid: ["a"], [1, "a"], any array containing at least one string

    invalid: [], [1], any non-array, any array not containing strings

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:

{
    "oneOf": [
        { "maximum": 3 },
        { "type": "integer" }
    ]
}

valid: 1.5, 2.5, 4, 5, any non-number

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:

{
    "anyOf": [
        { "maximum": 3 },
        { "type": "integer" }
    ]
}

valid: 1.5, 2, 2.5, 3, 4, 5, any non-number

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:

{
    "allOf": [
        { "maximum": 3 },
        { "type": "integer" }
    ]
}

valid: 2, 3

invalid: 1.5, 2.5, 4, 4.5, 5, 5.5, any non-number

switch (v5 proposal)

The value of the keyword is the array of if/then clauses. Each clause is the object with the following properties:

  • if (optional) - the value is JSON-schema
  • then (required) - the value is JSON-schema or boolean
  • continue (optional) - the value is boolean

The validation process is dynamic; all clauses are executed sequentially in the following way:

  1. if:
    1. if property is JSON-schema according to which the data is:
      1. valid => go to step 2.
      2. invalid => go to the NEXT clause, if this was the last clause the validation of switch SUCCEEDS.
    2. if property is absent => go to step 2.
  2. then:
    1. then property is true or it is JSON-schema according to which the data is valid => go to step 3.
    2. then property is false or it is JSON-schema according to which the data is invalid => the validation of switch FAILS.
  3. continue:
    1. continue property is true => go to the NEXT clause, if this was the last clause the validation of switch SUCCEEDS.
    2. continue property is false or absent => validation of switch SUCCEEDS.

Examples

  1. schema:

    {
    "switch": [
        {
            "if": { "properties": { "power": { "minimum": 9000 } } },
            "then": { "required": [ "disbelief" ] },
            "continue": true
        },
        { "then": { "required": [ "confidence" ] } }
    ]
}

    valid:

    • { "power": 9000, "disbelief": true, "confidence": true }
    • { "confidence": true }
    • { "power": 1000, "confidence": true }

    invalid:

    • { "power": 9000 } (disbelief & confidence are required)
    • { "power": 9000, "disbelief": true } (confidence is always required)
    • { "power": 1000 }
    • {}

  2. schema:

    {
    "type": "integer",
    "switch": [
        { "if": { "not": { "minimum": 1 } }, "then": false },
        { "if": { "maximum": 10 }, "then": true },
        { "if": { "maximum": 100 }, "then": { "multipleOf": 10 } },
        { "if": { "maximum": 1000 }, "then": { "multipleOf": 100 } },
        { "then": false }
    ]
}

    valid: 1, 5, 10, 20, 50, 100, 200, 500, 1000

    invalid:

    • -1, 0 (<1)
    • 2000 (>1000)
    • 11, 57, 123 (any number with more than one non-zero digit)
    • non-integers