How to use maths operation with Joi.ref() for validating object using Joi?

Question

I want to validate object using Joi which inovle use of Joi.ref() with multiplication operation.

var object = {
    a: 5,
    b: 6
}

// this is wrong as Joi.ref('a')*2 is now allowed in max()
var schema = Joi.object({
    a: Joi.number().integer(),
    b: Joi.number().integer().min(1).max(Joi.ref('a')*2)
})

Joi.ref('a')*2 is not allowed. So how can I validate object such that b<=2*a?

Answer 1

Using adjust option

var schema = Joi.object({
    a: Joi.number().integer(),
    b: Joi.number().integer().min(1).max(Joi.ref('a', {
      adjust: (value) => value * 2
    }))
})

stackblitz