interface ObjType {
  firstName: string;
  lastName: string;
  nestedObject: {
    middleName: string;
  }
}

// no errors
Joi.object<ObjType, true>({
  firstName: Joi.string(),
  lastName: Joi.string(),
  nestedObject: Joi.object<ObjType['nestedObject'], true>({
    middleName: Joi.string()
  })
})

// error -> 'lastName' is missing
Joi.object<ObjType, true>({
  firstName: Joi.string(),
  nestedObject: Joi.object<ObjType['nestedObject'], true>({
    middleName: Joi.string()
  })
})

// error -> 'lastName' has incorrect shema type
Joi.object<ObjType, true>({
  firstName: Joi.string(),
  lastName: Joi.number(),
  nestedObject: Joi.object<ObjType['nestedObject'], true>({
    middleName: Joi.string()
  })
})