JTDSchemaType has Typescript error with valid JTD

What version of Ajv are you using? Does the issue happen if you use the latest version?

"ajv": "^8.8.2",

** Code and error **

import Ajv, { JTDSchemaType } from 'ajv/dist/jtd'
const ajv = new Ajv()

// then 

export interface JumpSpec<P = unknown, T = unknown> {
  type: string
  uri: string
  param?: P
  mountAt?: string
  defaults?: T[]           // this array will caus the problem
}

const spec: JTDSchemaType<JumpSpec> = {
  properties: {
    type: { type: 'string' },
    uri: { type: 'string' },
  },
  optionalProperties: {
    param: {},
    mountAt: { type: 'string' },
    defaults: { elements: {} },    // <-- error 
  },
}

Typescript objects to ‘elements’ with : (property) elements: {} Type '{}' is not assignable to type 'never'

Looking at the spec: https://datatracker.ietf.org/doc/html/rfc8927#section-2

I think elements can take the empty form from the CDDL:

   ; All JTD schemas are JSON objects taking on one of eight forms
   ; listed here.
   schema = (
     ref //
     type //
     enum //
     elements //
     properties //
     values //
     discriminator //
     empty //
   )
...

   ; elements describes the "elements" schema form.
   elements = ( elements: { schema }, shared )

Which makes me think it’s a bug 😃

Thanks for your awesome validation library!

[erikbrinkman]

What you’re writing makes sense, but is actually nonsense 😅 . Typescript doesn’t allow parametrized types to actually be used, you need to make them “concrete”. In this example, both P and T have default definitions, so you can think of it like:

export interface DefaultJumpSpec2 {
  type: string;
  uri: string;
  param?: Record<string, unknown>;
  mountAt?: string;
  defaults?: Record<string, unknown>[];
}

in JTD, the correct schema for Record<string, unknown> is { values: {} }, thus if you fix your schema to be:

const spec: JTDSchemaType<JumpSpec2> = {
  properties: {
    type: { type: 'string' },
    uri: { type: 'string' },
  },
  optionalProperties: {
    param: {
        values: {}
    },
    mountAt: { type: 'string' },
    defaults: { elements: { values: {} } },
  },
}

it then compiles fine.

Note: this was called out in the error which is saying that {} is missing the key values to be compatible with JTDSchemaType<Record<string, unknown>>.

By not using JTDSchemaType you’re implicitly using JTDDataType under the hood, which should be parsing param and defaults as unknowns instead of as records. That’s fine, but different from the interface you specified.

It’s probably also worth noting that JTD doesn’t have a way to specify Record<string, never> which seems like it was maybe what you intended. For that you’ll need a more powerful verifier (like JSON Schema) to guarantee that there are no keys.

[MichaelJCole]

@epoberezkin @erikbrinkman Thank you, this is super helpful!