Hi,

Apparently, Joi is returning a reference to the original default object declared in a schema, allowing schema mutation by validation results consumers, which can result in nasty, hard to find bugs.

var schema = Joi.object().default({ key: 'original_value' });

var firstResult = Joi.validate(undefined, schema);
console.log(firstResult);   // <- { error: null, value: { key: 'original_value' } }

firstResult.value.key = 'mutated_value';

var secondResult = Joi.validate(undefined, schema);
console.log(secondResult);  // <- { error: null, value: { key: 'mutated_value' } }

As shown in the code snippet, various calls to validate() that cause a default value to be returned, return the original object, instead of a copy of it.

This can be confirmed by checking that the schema._flags.default property is indeed mutated after the firstResult.value.key = 'mutated_value'; statement.

I'm not sure if this is by design (performance reasons?) or if it's a bug, but either way it allows a validation schema to be changed by any code that consumes the resulting object.

I believe validate() should always return a copy of the default object, therefore preventing unintended changes to the original schema.