Add additionalProperties decorator

[Devon-White]

Clear and concise description of the problem

Related to discussion: #4316

It would be helpful to have a decorator that would allow me to specify models/Unions to not accept additionalProperties.

If the property is not set, JSON schema will default to allowing additionalProperties, which could impact proper validation.

As discussed in the linked discussion, this is visibly an issue when using tools like Monaco editor.

I would expect it to look similar to such:

@additionalProperties(false)
union Test {
  A,
  B,
  C
}

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[Devon-White]

The more I think about it, I actually think it would be beneficial that additionalProperties are set to false by default upon compile a TypeSpec model.

I personally don't understand why JSON schemas by default will treat a object to have additionalProperties if not defined.

Maybe i'm not seeing something, but this feels counter-intuitive for the sake a validation? I would imagine it should always be treated as false by default and you have the option to enable it.

This all said, perhaps TypeSpec should by default generate with additionalProperties set to false unless the proposed decorator is passed and set to true, or you use a spread operator?

[jeremiahlee]

+1 for the need of this

additionalProperties: false is a hard requirement for OpenAPI schemas used by many financial institutions. More data than expected is a liability. I think false should be the default and the decorator should be used when wanting to set additionalProperties to true.

[wanlwanl]

Related issues:

• Allow configuration of unevaluatedProperties in JSON Schema emitter, and possibly change the default behaviour of all models. #3549
• Allow customized generating of additionalProperties in openapi 3 emitter #3241

[adamscybot]

I just want to add here (in fear of it getting lost) my comment over on #3549 (comment).

I believe additionalProperties defaulting to false would break semantics of TypeSpec and if anything were to be defaulted, it would instead be unevaluatedProperties, which works well alongside TypeSpec and can be used to achieve the same thing.

[wanlwanl]

Thank you for your reminder. You are correct that this is possible in OpenAPI 3.1. Unfortunately, OpenAPI 3.0 does not support the unevaluatedProperties keyword. This feature is part of JSON Schema 2019-09 and later versions, while OpenAPI 3.0 uses a subset of JSON Schema Draft 4, which does not include unevaluatedProperties. If you have any further questions or need additional assistance, please feel free to let us know!

[theogravity]

Can we get a decorator for OAI 3.0 users?