Mapping values in json-schema

Question

So JSON schema is defined here and I'm particularly using fluent-json-schema implementation by Fastify team.

I used Joi for validation in an Express app that I'm porting to Fastify.

I use a lot of validation for checkboxes and choice radio inputs. I would like to know how to assign the two boolean values to arbitrary string values

// So this is possible in Joi but not in `fluent-json-schema`
S.boolean().truthy('on').falsy('off').default(false)

Not only for booleans, it would be nice to have the possibility to map values as this is really handy in front and back architectures.