alternatives

Generates a type that will match one of the provided alternative schemas via the try()
method. If no schemas are added, the type will not match any value except for undefined.

Supports the same methods of the any() type.

alternatives.try(schema, schema2, ...| [schema, schema2, ...])

Try accepts schemas using single or multiple arguments, or as an array.

var alt = Joi.alternatives().try(Joi.number(), Joi.string());
alt.validate('a', function (err, value) { });

Using .try() is optional and allows for a shorter version:

var alt = Joi.alternatives(Joi.number(), Joi.string())