Using Joi, require one of two fields to be non empty

An alternative way of using Joi.when() that worked for me:

var schema = Joi.object().keys({
  a: Joi.string().allow(''),
  b: Joi.when('a', { is: '', then: Joi.string(), otherwise: Joi.string().allow('') })
}).or('a', 'b')

.or('a', 'b') prevents a AND b being null (as opposed to '').

Code below worked for me. I used alternatives because .or is really testing for the existence of keys and what you really wanted was an alternative where you would allow one key or the other to be empty.

var console = require("consoleit");
var Joi = require('joi');

var schema = Joi.alternatives().try(
  Joi.object().keys({
    a: Joi.string().allow(''),
    b: Joi.string()
    }),
  Joi.object().keys({
    a: Joi.string(),
    b: Joi.string().allow('')
    })
);

var tests = [
  // both empty - should fail
  {a: '', b: ''},
  // one not empty - should pass but is FAILING
  {a: 'aa', b: ''},
  // both not empty - should pass
  {a: 'aa', b: 'bb'},
  // one not empty, other key missing - should pass
  {a: 'aa'}
];

for(var i = 0; i < tests.length; i++) {
  console.log(i, Joi.validate(tests[i], schema)['error']);
}

If you want to express the dependency between 2 fields without having to repeat all other parts of the object, you could use when:

var schema = Joi.object().keys({
  a: Joi.string().allow(''),
  b: Joi.string().allow('').when('a', { is: '', then: Joi.string() })
}).or('a', 'b');