Ability to check optional type

[Shinigami92]

Is there a way to check if a target is string | undefined?

I have the current structure:

// value := [string, string, string?]
check(value, 'value').exists();
check(value, 'value').is.an.array();
check(value.length, 'value').is.not.empty();

check(value[0], 'value[0]').exists();
check(value[0], 'value[0]').is.a.string();

check(value[1], 'value[1]').exists();
check(value[1], 'value[1]').is.a.string();

// Maybe with .optional ?
check(value[2], 'value[2]').is.optional.a.string();

Currently when I use check(value[2], 'value[2]').is.a.string();
I get Check failed: value[2] (undefined) was not a String,
but I want to check if it is a string or undefined

[pnann]

I've been thinking about this, and appreciate the feature request -- this is definitely a common case in public APIs.

One alternative is to support chaining (e.g. check(a).exists().and.is.a.string()), give me a bit of time to think it through.

[Shinigami92]

I think chaining with and would be nice.
This could be the right way as it is the same as BDD and is used in chai or similar libraries.