Schema does not enforce requirement of array items object

In the 1.2 and 2.0 specifications, it says this regarding the items property: Required if type is "array". The problem is I can create a property like this without the JSON schema validator throwing an error:

"ids": {
  "type": "array"
}

[handrews]

Not sure if this is still an active concern. if anyone is still curious as to why JSON Schema works this way, it has to do with the difference between assertions, annotations, and data model keywords (which is discussed more clearly in the soon-to-be-published draft-07).

items is a data model keyword. It applies a subschema to part of an instance based on the data model (in particular, if the instance is an array, it applies the subschema to ever element of an array and ANDs the results- contrast with contains which applies the subschema and ORs the results) Note the “if it’s an array, then apply the subschema” approach, with the implicit “else don’t do anything for this keyword” clause.

type is an assertion. It evaluates a constraint against the instance and produces a boolean result.

The third category, annotations, include keywords like title and default.

Data model keywords themselves never cause validation to fail. They apply subschemas which have assertions, and it is the assertions in the subschemas which cause the pass or fail.

This seems weird for use cases like OpenAPI. But consider using JSON Schema to scan arbitrary data for objectionable language with a regex. You don’t know or care what data types you’ll get. Objects, arrays, strings, numbers, booleans, nulls, nested structures of arbitrary complexity, etc. You just want to apply your regex using pattern to every string in the document.

If item behaved as an assertion, this would be very hard. You could do it with anyOf but it would be very complicated. Preserving the data model vs assertion split makes it easy to do this sort of “validation” with JSON Schema.

FWIW, JSON Schema does in fact treat a lack of items the same as "items": {}, which only works if items is not an assertion of array-ness.

[handrews]

We decided not to fix this in the other issue since the schema is not intended to catch all errors, and this is against old OAS versions that aren’t being worked on anymore anyway. I think this is safe to close.