var joi = require('joi'),
    convert = require('joi-to-json-schema'),
    joiSchema = joi.object({
      'name': joi.string().required().regex(/^\w+$/),
      'description': joi.string().optional().default('no description provided'),
      'a': joi.boolean().required().default(false),
      'b': joi.alternatives().when('a', {
        is: true,
        then: joi.string().default('a is true'),
        otherwise: joi.number().default(0)
      })
    });

convert(joiSchema);

{ type: 'object',
  properties: 
   { name: { type: 'string', pattern: '^\\w+$' },
     description: { default: 'no description provided', type: 'string' },
     a: { type: 'boolean', default: false },
     b: { oneOf: [ { default: 'a is true', type: 'string' }, { type: 'number', default: 0 } ] } },
  additionalProperties: false,
  required: [ 'name', 'a' ] }

 /**
  * Converts the supplied joi validation object into a JSON schema object,
  * optionally applying a transformation.
  *
  * @param {JoiValidation} joi
  * @param {TransformFunction} [transformer=null]
  * @returns {JSONSchema}
  */
 export default function convert(joi,transformer=null) {
   // ...
 };

 /**
  * Joi Validation Object
  * @typedef {object} JoiValidation
  */

 /**
  * Transformation Function - applied just before `convert()` returns and called as `function(object):object`
  * @typedef {function} TransformFunction
  */

 /**
  * JSON Schema Object
  * @typedef {object} JSONSchema
  */