Add rule "optional"

[Bruno C. Couto]

This is a simple rule but that can add a lot of flexibility to the library.

It should be a composite rule. It must get a validation as input and should pass when this validation passes or when the validated value is not defined (null or undefined).

const validation = v8n()
  .optional(
    v8n().number().positive()
  );

validation.test(-1); // false
validation.test(1); // true
validation.test(null); // true

Note: its implementation should use check instead of test to provide better error handling.

[Sebastian Barfurth]

Lots of validation nesting recently. Could there a nicer way to implement something like this? Maybe another argument on the test functions?

[Bruno C. Couto]

This rule is very straightforward and looks very clean to me. Also, we can benefit from the already implemented error handling system which works very well with rules.

I don't think we need to change our API now just to support this kind of feature. This would be a challenge to make it work for all kind of validations. It would be also hard to make it work with the already in use error handling system.

[Sebastian Barfurth]

Is this no basically the same as:

v8n()
  .passesAnyOf(
    v8n().null(),
    v8n().number().positive()
  )
  .test(...);

So this would just be a shortcut for that, yes?

[Bruno C. Couto]

almost that,

actually, we also need to check for undefined:

v8n()
  .passesAnyOf(
    v8n().null(),
    v8n().undefined(),
    v8n().number().positive()
  )

The optional rule would be a kind of a shorthand for that.

[Sebastian Barfurth]

I guess it's a fine shortcut then 👍

[Bruno C. Couto]

Can you make a PR for this? 😁