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layout default
title Unevaluated keywords
parent Features
nav_order 8

Unevaluated properties and items

JSON Schema 2020-12-style unevaluatedProperties and unevaluatedItems apply to object keys and array positions that were not already “evaluated” by properties, patternProperties, additionalProperties, items, prefixItems, composition, and related keywords.

They are useful when you want forbid extra keys or constrain extra array elements after tuples or composition.

unevaluatedProperties

With explicit properties

type: object
properties:
  a:
    type: string
unevaluatedProperties: false
  • Valid: { a: hello }
  • Invalid: an extra key such as b.

With allOf

Annotations from branches of allOf can merge which names count as evaluated. Keys allowed by any subschema may be present; unevaluatedProperties: false then rejects any other keys.

allOf:
  - properties:
      a:
        type: string
  - unevaluatedProperties: false

a is allowed; b is not.

With anyOf

Successful branches contribute evaluated property names from their schemas. For example, if the instance matches the first branch, only keys allowed by that branch (and merged annotations) count as evaluated; extra keys can still fail unevaluatedProperties: false.

anyOf:
  - properties:
      a:
        type: string
  - properties:
      b:
        type: string
unevaluatedProperties: false
  • Valid: a: ok or b: ok (each branch’s allowed keys only).
  • Invalid: a: ok together with an extra unevaluated key such as c: extra.

unevaluatedItems

After prefixItems

Positions beyond the tuple are “unevaluated” unless covered by items.

type: array
prefixItems:
  - type: integer
unevaluatedItems:
  type: string
  • Valid: [1, foo, bar]
  • Invalid: a non-string after the first integer, e.g. [1, 2].

Without items or prefixItems

Every index is unevaluated, so the schema applies to all elements:

type: array
unevaluatedItems:
  type: integer
  • Valid: [1, 2]
  • Invalid: [1, hi] (second item is not an integer).

See also