Support `prefixItems` for tuple validation

[isti115]

From the JSON Schema 2020-12 Release Notes:

The items and additionalItems keywords have been replaced with prefixItems and items where prefixItems has the same functionality as the array-of-schemas for of the old items and the new items keyword has the same functionality as the old additionalItems keyword.

For an example with more explanation:
https://json-schema.org/understanding-json-schema/reference/array.html#tuple-validation

As per #1 (which has a broken link by the way), here is the respective file from the official test suite:
https://github.com/json-schema-org/JSON-Schema-Test-Suite/blob/main/tests/draft2020-12/prefixItems.json

In case there is interest in resolving this issue, I would be willing to look into the code (I've already checked the relevant parts of parsing and generation) and try to provide a pull request for it.

[bcherny]

This is a pretty unfortunate change in the spec. It means that the meaning of items is dependent on the version of JSON-Schema you use. But since a JSON-Schema is not required to specify its version, this leaves the interpretation of items ambiguous in practice 🫤.

A middle of the road approach to support this without breaking others' code may be to expose the new behavior behind a switch, say --json-schema-version=2020-12. We would check the option in the normalizer, and normalize prefixItems to items and items to additionalItems if needed.

Want to give this a shot?

[TheoAnastasiadis]

I thought I'd have a go at it.

Added a CLI argument as "--usePrefixItems" which defaults to false, I did not think specifying JSON schema spec version would be helpful at this point since only this version would be supported.

The main addition is a new normalizer rule; renaming the schema keys.

schema.additionalItems = schema.items
schema.items = schema.prefixItems
delete schema.prefixItems

I created a simple unit test as well. I will let it sink in cause maybe more test cases might be needed. I had difficulty understanding the E2E testing setup, so I did not add any new E2E tests, maybe if somebody would very briefly walk me through I am going to do so. Tests are passing, for now.

[xcanchal-ut]

Any news on this topic?

[isti115]

Back here two years later.

So, upon reading the actual specification, not just the pretty online docs, I have discovered that in 10.3.1. Keywords for Applying Subschemas to Arrays it states the following:

Note that the behavior of "items" without "prefixItems" is identical to that of the schema form of "items" in prior drafts. When "prefixItems" is present, the behavior of "items" is identical to the former "additionalItems" keyword.

Based on this, I think that the lack of version specification in schema files can be overcome by simply checking for the presence of the prefixItems key and adjusting the behavior based on that without ambiguity.

Adding the following quick and dirty rule seems to work well enough for me:

rules.set('Support the `prefixItems` key from draft 2020-12', schema => {
  if (!isArrayType(schema)) {
    return
  }

  if (schema.hasOwnProperty('prefixItems')) {
    if (schema.hasOwnProperty('items')) {
      schema.additionalItems = schema.items as any
      delete schema.items
    }

    schema.items = schema.prefixItems as any
    delete schema.prefixItems
  }
})

If I manage to find some time for it, I'll properly update the types, write tests and submit a PR.

---

In the meantime anyone can test the solution by applying it through patch-package: json-schema-to-typescript+15.0.4.patch