How to check dependencies, if a value may be null in JOI?

I have four user inputs: reason1, date1, reason2, date2. All may be null / undefined. If date1 or reason1 is not empty, date1 and reason1 are now mandatory. Same for date2 and reason2. So the reasons are dependent of their dates and vice versa.

My tries so far:

var validationSchema = Joi.object.keys({
        date1: Joi.date().allow('').allow(null);
        date2: Joi.date().allow('').allow(null);
    })
    .with('reason1', 'date1')
    .with('reason2', 'date2')
;

wont work, because it's always true, due to .allow(null). Remember, the dates may be null, if the reason is also null.

So I decided to "hack" the solution via arrays: The length of an array may be 0 (both null), 2 (both containing data) or 1 (only one of them containing data)

var data = req.body;
data.checkReason1 = []; data.checkReason2 = [];
(data.reason1 ? data.checkReason1.push(data.reason1) : null);
(data.date1 ? data.checkReason1.push(data.date1) : null);
(data.reason2 ? data.checkReason2.push(data.reason2) : null);
(data.date2 ? data.checkReason2.push(data.date2) : null);
var validationSchema = {
    checkReason1: Joi.array().length(0).length(2);  
    checkReason2: Joi.array().length(0).length(2);
}

wont work either, because multiple .length() on an array wont work with Joi.

Do you have another solution?

via corax228