As well as various methods which specify the type of value you expect (Joi.number(), Joi.boolean(), Joi.object(), Joi.string() etc etc etc) Joi comes with a number of methods that allow for more specific validation. Here are some examples!
.allow(val)- whitelists the val. Could be used with Joi.number() to accept certain non-number values.valid(val)- Similar to .valid, but will ONLY accept values passed to .valid.invalid(val)- Joi will reject any value passed to invalid..required()- Joi will fail to validate if a .required() key doesn't receive a value.