failover() support for default on errors

[WesTyler]

Describe the problem you are trying to fix (provide as much context as possible)

From the discussion here:
per @genediazjr

I'm thinking of an API for Joi that would allow a default value similar to any.default() but only if the original value is invalid. The value key on the error object will contain the set infInvalid value instead of the original value. Or we can put it on separate key whichever would be better.

Which API (or modification of the current API) do you suggest to solve that problem ?

I would like to propose adding an optional config param to .default():

Joi.string().default('name', { invalid: true })

Are you ready to work on a pull request if your suggestion is accepted ?

Yes.

[WesTyler]

@Marsup I see that you added this to the Priorities project. If you're open to this addition, I can go ahead and work on it.

[Marsup]

It's a P3 but sure. Remember there's already a 2nd argument for default.
Also should default apply to invalid, undefined values, or both ? This API seems confusing to me.

[WesTyler]

Remember there's already a 2nd argument for default

👍

should default apply to invalid, undefined values, or both

My thought was apply only to empty/undefined values as it currently does, unless you have set the invalid: true option in which case it will apply on empty/undefined as well as invalid.

[Marsup]

What if you only want invalid ?

[WesTyler]

I guess that's something we need to decide on, then. My understanding of the original request was "invalid in addition to empty".

So if we need to consider "empty", "invalid", and "empty or invalid" as individual use cases then yeah this is a bit confusing.