[docs] relation between JTDSchemaType and JTDDataType

What version of Ajv are you using? 8.4.0 Does the issue happen if you use the latest version? Yes

JTD Schema

export interface REQ {
	data: {
		text: string,
	}
};
export const Req: JTDSchemaType<REQ> = {
	properties: {
		data: {
			properties: {
				text: { type: 'string' },
			},
		},
	},
};

Your code

type ReqD = JTDDataType<typeof Req>
/* {
    data: {
        text: unknown;
    } & {};
} & {} */

type ReqD2 = JTDDataType<typeof Req.properties.data>
/*  {
    text: unknown;
} & {}; */

What results did you expect?

JTDDataType<typeof Req.properties.data>
// { text: string }

Are you going to resolve the issue?

If this is a bug, and I know where to start, sure.

[erikbrinkman]

Thanks for working through this mostly without bugging me, but feel free to tag me earlier @epoberezkin. There’s a few things to address.

First, for discriminated unions, JTD spec only allows strings. If you change your “ok” numbers to strings, the schema checks out. This isn’t normal javascript nonsense where types get coerced all the time.

Second, there is a SomeJTDSchemaType to check the general schema objects: https://github.com/ajv-validator/ajv/blob/be07d3d8c0aac8fffe68378d25bdd892c61ba2e5/lib/types/jtd-schema.ts#L8-L41

Finally, and unfortunately, there can’t be a nice correspondence between JTDSchemaType and JTDDataType and the answer has to do with unions. JTDSchemaType<string> isn’t { type: "string" } (even ignoring metadata), it’s { type: "string" | "timestamp" } due to the way that schemas are handled. From the outset JTDDataType was not designed to handle unions, because it’s not clear how they should be handled. For the same reason that JTDDataType<{ type: "timestamp" }> isn’t Date but Date | string. These ambiguities prevent a bijection between JTD schema- and data-space, and therefore prevent the kind of checking you desire.

[erikbrinkman]

closing due to inactivity, but feel free to reopen if this isn’t resolved @josh-hemphill