Joi require at least one of a group of keys when a specific key exists

Question

Given the example:

Joi.object({
  a: Joi.string(),
  b: Joi.string(),
  c: Joi.string(),
})

I want that when key A is filled, keys B and C are required at least one of them:

{ a: 'foo', b: 'bar' } //success

{ a: 'foo', c: 'bar' } //success

{ a: 'foo' } #error

Answer 1

You can require a minimum count of keys on the object.

Joi.object({
  a: Joi.string(),
  b: Joi.string(),
  c: Joi.string(),
}).min(1)