Nodejs - Joi Check if string is in a given list

I'm using Joi package for server side Validation.
I want to check if a given string is in a given list or if it is not in a given list.(define black list or white list for values)
sth like an "in" or "notIn" function.how can I do that?

var schema = Joi.object().keys({
    firstname: Joi.string().in(['a','b']),
    lastname : Joi.string().notIn(['c','d']),
});

from Nodejs - Joi Check if string is in a given list