Joi.object({
  type: Joi.string().valid('number', 'string', 'boolean'),
  requireDefault: Joi.boolean().default(false),
  defaultValue: Joi.when('type', { is: 'number', then: Joi.number().default(1) })
    .when('type', { is: 'string', then: Joi.string().default('string') })
    .when('type', { is: 'boolean', then: Joi.boolean().default(true) })
    .try(Joi.strip())
});

// solution 1, best choice but got another complexity
Joi.object({
  type: Joi.string().valid('number', 'string', 'boolean'),
  requireDefault: Joi.boolean().default(false),
  defaultValue: Joi.when('requireDefault', {
    is: true,
    then: Joi.when('..type', { is: 'number', then: Joi.number().default(1) })
      .when('..type', { is: 'string', then: Joi.string().default('string') })
      .when('..type', { is: 'boolean', then: Joi.boolean().default(true) }),
    otherwise: Joi.strip()
  })
});

// solution 2, readability is getting low 
// it also changes the base schema to 'alternatives' and breaks Joi.reach(...) functionality
Joi.object({
  type: Joi.string().valid('number', 'string', 'boolean'),
  requireDefault: Joi.boolean().default(false)
})
  .when(Joi.object({ type: 'number', requireDefault: true }).options({ presence: 'required', allowUnknown: true }), {
    then: { defaultValue: Joi.number().default(1) }
  })
  .when(Joi.object({ type: 'string', requireDefault: true }).options({ presence: 'required', allowUnknown: true }), {
    then: { defaultValue: Joi.string().default('string') }
  })
  .when(Joi.object({ type: 'boolean', requireDefault: true }).options({ presence: 'required', allowUnknown: true }), {
    then: { defaultValue: Joi.boolean().default(true) },
    otherwise: { defaultValue: Joi.strip() }
  });

// suggest:
// if we have self-returning reference e.g. Joi.ref('.'),
// we can reduce complexity without changing base schema
Joi.object({
  type: Joi.string().valid('number', 'string', 'boolean'),
  requireDefault: Joi.boolean().default(false),
  defaultValue: Joi.when('.', {
    is: Joi.object({ type: 'number', requireDefault: true }).options({ presence: 'required', allowUnknown: true }),
    then: Joi.number().default(1)
  })
    .when('.', {
      is: Joi.object({ type: 'number', requireDefault: true }).options({ presence: 'required', allowUnknown: true }),
      then: Joi.string().default('string')
    })
    .when('.', {
      is: Joi.object({ type: 'number', requireDefault: true }).options({ presence: 'required', allowUnknown: true }),
      then: Joi.boolean().default(true)
    })
    .try(Joi.strip())
});