Add key in Joi object if key does not exist

Question

I have a joi schema like this

Joi.object({
  name: Joi.string(),
  language: Joi.string(),
  birthday: Joi.date(),
  age: Joi.number().integer()
})

The input can contain birthday and age at the same time or just one of the keys. If one of the keys does not exist, I want the missing key to be automatically added with the string "NULL" like this

//input
{
 "name": "Jane Doe",
 "language": "de",
 "birthday": "1960-06-27"
}
//modified to
{
 "name": "Jane Doe",
 "language": "de",
 "birthday": "1960-06-27",
 "age": "NULL"
}

Is there a way to automatically add a key with a default value, if the key is missing?

Answer 1

Use any.default([value]):

const schema = Joi.object({
  name: Joi.string(),
  language: Joi.string(),
  birthday: Joi.date(),
  age: Joi.number().integer().default(null)
});

console.log(schema.validate({
  "name": "Jane Doe",
  "language": "de",
  "birthday": "1960-06-27"
}));

Output:

{
  name: "Jane Doe",
  language: "de",
  birthday: Mon Jun 27 1960 01:00:00 GMT+0100 (Central European Standard Time),
  age: null
}

I would prefer null rather then a string "NULL" because both date and age are not of type string.