How can I make jsonschema optional?

Question

{
  "$id": "https://example.com/person.schema.json",
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Person",
  "type": "object",
  "properties": {
    "firstName": {
      "type": "string",
      "description": "The person's first name."
    },
    "lastName": {
      "type": "string",
      "description": "The person's last name."
    },
    "age": {
      "description": "Age in years which must be equal to or greater than zero.",
      "type": "integer",
      "minimum": 0
    }
  }
  "required": ["firstName", "lastName", "age"]
}

(edited after @gregdennis's answer)

Given the above schema, a valid data would be

{
  "firstName": "John",
  "lastName": "Doe",
  "age": 21
}

But I want to make it "optional", as in I want to allow empty object

// should pass
{}

But not half schemas

// Shouldn't pass
{
    "firstName": "John"
}

Answer 1

What you have is already optional, so the empty object should pass. In order to make those properties required you need to put them in a required keyword.

"required": [ "first name", "last name", "age" ]

But just adding this keyword removes the ability to validate the empty object.

To fix that, wrap this in a oneOf asking with another schema that only accepts the empty object.

{
  "oneOf": [
    { "const": {} },
    {
      // your schema from above along with required
    }
  ]
}