[Request] Add detail to .describe for function defaults

[WesTyler]

What are you trying to achieve or the steps to reproduce ?

The .default method already supports providing a function value. I would like to expand the information returned by .describe() when the default is a function instead of a string.

const randomNumber = function () {
    return Math.random();
};
randomNumber.description = 'Generate random number';

const schema = Joi.any().default(randomNumber);
schema.validate(); // { error: null, value: 0.8081045034806544 }
schema.validate(); // { error: null, value: 0.42643964379927235 }

Which result you had ?

Using the above schema:

schema.describe();
/*
{ type: 'any', flags: { default: 'Generate random number' } }
*/

What did you expect ?

I would like to add details so that describe instead returns:

schema.describe();
/*
{ type: 'any', flags: { default: {
    description: 'Generate random number',
    function: randomNumber
} } }
*/

This would allow for this contrived example to work:

schema.describe().flags.default.function(); // 0.6658680107310528

(Full disclosure, I would like to allow for dynamic defaults in Felicity, which is built upon .describe())

Any feedback on this would be appreciated, and I'm 100% willing to add this support if approved.

[WesTyler]

If the above implementation is not acceptable, but the general goal of the request (having access to the default function in the describe), I'm open to suggestions.

One thing I hadn't decided on is this behavior for stringDefault:

const makeAString = function() { return 'made a string'; };
makeAString.decription = 'Makes strings';

const stringDefault = Joi.any().default('just a string');
const funcDefault = Joi.any().default(makeAString);

funcDefault.describe();
/*
{ type: 'any', flags: { default: {
    description: 'Makes strings',
    function: makeAString
} } }
*/

stringDefault.describe();
/*
{ type: 'any', flags: { default: {
    description: 'just a string'
} } }
--OR--
{ type: 'any', flags: { default: 'just a string' } }
*/

[AdriVanHoudt]

If this entails a simple change @

joi/lib/any.js

Line 645 in 37b8dc6

description.flags[flag] = this._flags[flag].description;

I don't see why not

[WesTyler]

That's exactly where I was looking :)

else if (typeof this._flags[flag] === 'function') {
    description.flags[flag] = {
        description: this._flags[flag].description,
        function   : this._flags[flag]
    }
}

[Redacted due to developer cough me cough error]

Going to be pushing up a PR for this sometime this afternoon.