Make literals work with null

[KristjanTammekivi]

Closes #685

[samchon]

As I have said, null is not a value categorized in atomic, but a type of object.

Therefore, this PR and related issue need to consider about that.

#685 (comment)

[KristjanTammekivi]

Can you explain why that matters in the context of this issue/pr?

[samchon]

I repeat #685 issue's comment.

typia.literals() function has been designed to get list of constant literal values.

• boolean
• number
• bigint
• string

The null value what you want is categorized in object type.` It's the problem.

[KristjanTammekivi]

No problem then, null is a literal
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/null#description

The value null is written with a literal: null

Also typeof null is considered a bug that was never fixed https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof#typeof_null

And even if it wasn't a literal I would say that that it's an unnecessary self-imposed constraint. I don't think undefined is a literal (although typeof undefined === 'undefined' so going by that standard it might be), but I would argue that literals should work with that as well. Let me talk about my use case.

We use Joi for validation and want to provide a list of valid values for it. Because literals doesn't support null we have to instead do something like the following

type ValidValues = 'A' | 'B' | null;
const validValues = [...literals<Exclude<ValidValues, null>>(), null]

And this is why I want null to be included

[samchon]

Wow, the null value to be considered an object type was a bug.

Got it. Your PR would be published as a next version v4.0.5.

Thanks for contribution.