Using Joi, how do I set a rule that states: if prop A is present, then prop b or C or D must be present?

Question

Using Joi, how can I require that, if a property is present then one of three different properties must also be present?

For example:

Take object obj:

const obj = {
    a: true,
    b: undefined, 
    c: true
    d: undefined
}

I can write:

const validate = (obj) =>{
    if(obj.a && (obj.b || obj.c || obj.d)){
        return true
    }
}

This way if I receive:

    const o1 = {
        a: true,
        b: undefined, 
        c: true,
        d: undefined
    }
    const o2 = {
        a: true,
        b: undefined, 
        c: undefined,
        d: undefined
    }
    const 03 {
        a: undefined,
        b: true, 
        c: undefined,
        d: undefined
    }
validate(o1)//true
validate(o2)//false
validate(03)//false

How do I accomplish this using Joi? I am able to enforce dependencies between two properties using the .when() method (see below); however, I am unsure how to accomplish the above. Is it even possible?

//from documentation
const schema = {
    a: Joi.valid('a', 'b', 'other'),
    other: Joi.string()
        .when('a', { is: 'other', then: Joi.required() }),
};

Hope to find an answer, as this is stumping me.

Answer 1

// If a === true, then b, c or d should be present
Joi.object({
  a: Joi.boolean(),
  b: Joi.boolean(),
  c: Joi.boolean(),
  d: Joi.boolean(),
}).when(".a", { is: true, then: Joi.object().or("b", "c", "d") });

// Validation Error: "value" must contain at least one of [b, c, d]
const invalidData = {
  a: true,
  b: undefined,
  c: undefined,
  d: undefined,
};

// Validation Passed
const validData = {
  a: true,
  b: undefined,
  c: true,
  d: undefined,
};

It works because the when allows to transform the object and add the or

You cant test here