How to get value of another property with Joi?

How can I define property of resulting value if I have optional schema property like:

passport: customJoi.string().optional().strip(true),
passportSeries: ? // return passport ? passport.slice(0,4) : undefined,

I tried to use when and custom but it seems that they work only if passportSeries in original object are defined and it looks like I don't have access to passport property anyway.

Solution

You can make use of the ref method in joi, which allows you to refers to another field in the object:

Joi.object({
  passport: Joi.string().optional().strip(true),
  passportSeries: Joi.ref('passport', {
    adjust: (val) => val.slice(0, 4),
  }),
});

By default, Joi.ref('field') refers to the exact value of the field, which is usually helpful for cases like password and confirm_password. However, you can make use of the second argument options.adjust that allows you to adjust the value before matching.

However, I think the question is a bit ambiguous here. If your purpose is to create another property based on an existing property, you can make use of forbidden method and default method instead:

Joi.object({
  passport: Joi.string().optional().strip(true),
  passportSeries: Joi.forbidden().default(
    Joi.ref('passport', { adjust: (val) => val.slice(0, 4) })
  ),
});

Basically, you mark the property passportSeries as forbidden(), which means this key cannot exists in your original object. However, with the use of default() you can set a value based on another value with the help of ref() method.