What I'm trying to do
I want to use the JTDSchemaType to create a schema for messages I will pass through a websocket. The messages have a type field used with a discriminator in the schema. Some of the messages require no additional information, such as a "ping" message, but I am getting an error when creating the JTDSchemaType unless there are additional properties.

What version of Ajv are you using? Does the issue happen if you use the latest version?
v8.11

Ajv options object

{}

Your code

import Ajv, { JTDSchemaType } from "ajv/dist/jtd";

const ajv = new Ajv();

type Message = { type: "ping" } // causes error unless an additional property is added
            |  { type: "error"; message: string };

const schema: JTDSchemaType<Message> = {
  discriminator: "type",
  mapping: {
    ping: {
      properties: {}, // compile error because ping is type 'never'
    },
    error: {
      properties: {
        message: { type: "string" },
      },
    },
  },
};

What results did you expect?
I expect to be able to create a schema for a tagged union that has no properties except the tag. I can't compile this because typescript expects the mapping of ping to be type never.

Are you going to resolve the issue?
I'm not sure how, but I'm happy to help.