Schema with function type for Typescript Object validation

[rodelcabs]

Hi, I'm working on a project, an SDK that can mainly integrated with React App, but we provided the vanilla JS feature and this is where AJV comes in, We want to validate the type of props/parameters that will be passed when the sdk is bundled into JS.

Main Problem: How to define a schema for a function property?

I've looked enough for some time to find a solution or sample for an object with function parameters, all I see is adding a keyword called "instanceof" and verifying if the property is an instance of a Function

import Ajv from 'ajv';
const ajv = new Ajv();
ajv.addKeyword({
    keyword: 'instanceof',
    type: 'object',
    validate: (schema: any, data: any) => {
        return data instanceof Function;
    }
});

It just doesn't work. I tried to use the AJV's example here and just added a function property to test on a minimal code:

interface MyData {
  foo: number
  bar?: string,
  callback: (someArg: any) => void
}

const schema: JSONSchemaType<MyData> = {
  type: "object",
  properties: {
    foo: {type: "integer"},
    bar: {type: "string", nullable: true},
    callback: { instanceof: "Function", type: 'object' }
  },
  required: ["foo"],
  additionalProperties: false
}

const validate = ajv.compile(schema)

const data = {
  foo: 1,
  bar: "abc",
  callback: (arg) => { console.log(arg) }
}

if (validate(data)) {
  // data is MyData here
  console.log(data.foo)
} else {
  console.log(validate.errors)
}

and it just returns

[
  {
    instancePath: '/callback',
    schemaPath: '#/properties/callback/type',
    keyword: 'type',
    params: { type: 'object' },
    message: 'must be object'
  }
]

Can someone help me please?