How to allow any other key in Joi

The correct answer is actually to use object.unknown(true).

const schema = Joi.object().keys({
  a: Joi.string().required(),
  b: Joi.string().required()
}).unknown(true);

You can add unknown keys using object.pattern(regex, schema) this way IF you want to make sure these unknown keys are strings:

const schema = Joi.object().keys({
  a: Joi.string().required(),
  b: Joi.string().required()
}).pattern(/./, Joi.string());

For a general pass of all key types use object.unknown(true):

const schema = Joi.object().keys({
  a: Joi.string().required(),
  b: Joi.string().required()
}).unknown(true);

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