AJV - additionalProperties: false but with exceptions

I am using ajv to validate my mongodb schemas.

{
  type: "object",
  properties: { goal: { type: "string" }, cost: { type: "number" } },
  required: ["goal", "cost"],
  additionalProperties: false,
}

The above is the ajv schema for a document in my collection. The property additionalProperties: false is configured, so no additional properties are allowed.

The question is, how do I specify a set of exceptional additional properties that are allowed?

Any property in your schema that is not specified as required is basically optional: it will be validated when exists, but will be ignored if missing when data is being validated.

See the example below of validating object with "optional" maybeValue property:

const Ajv = require("ajv");
const ajv = new Ajv();

const schema = {
  type: "object",
  properties: { 
    goal: { type: "string" },
    cost: { type: "number" }, 
    maybeValue: { type: "string" },
  },
  required: ["goal", "cost"], // maybeValue is "optional"
  additionalProperties: false,
};

const validate = ajv.compile(schema);

const valid = {
  goal: 'win',
  cost: 10,
};

// this is a valid schema also maybeValue is missing since it is not required
validate(valid);

const notValid = {
  goal: 'win',
  cost: 10,
  maybeValue: 10
};

// this is a not a valid schema since maybeValue is not a string
validate(notValid);