feat: undefinedOnPartialDeep type

[Emiyaaaaa]

Fixes #681

Hello Everyone~

I have a question about testing and want some suggest or help:

• This type is only useful when exactOptionalPropertyTypes enabled, but this option is disable in type-fest, so tests for this type must pass regardless of bugs.

Then I have an idea:

1. replace this line undefined with another any flag type like type FlagType = 1 by generic type. example undefinedOnPartialDeep<T, UnionType = FlagType>.
2. test if undefinedOnPartialDeep<{ a?: string }, FlagType> is assignable to type { a?: string | FlagType } or not.
3. export a stable new type like type NewStableType<T> = undefinedOnPartialDeep<T, undefined>, because 2nd generic type is just for testing, don't hope to export for user. (ignore type name, maybe is type UndefinedOnPartialDeep<T> = someOtherName<T, undefined>)

[sindresorhus]

// @kachkaev

[sindresorhus]

This type is only useful when exactOptionalPropertyTypes enabled, but this option is disable in type-fest, so tests for this type must pass regardless of bugs.

You could create a separate test file in a subfolder, give it a tsconfig file that extends the existing ones, but changes that flag, and then just run tsc --noEmit on that file.

[Emiyaaaaa]

This type is only useful when exactOptionalPropertyTypes enabled, but this option is disable in type-fest, so tests for this type must pass regardless of bugs.

You could create a separate test file in a subfolder, give it a tsconfig file that extends the existing ones, but changes that flag, and then just run tsc --noEmit on that file.

thanks! fixed in the lastest commit. but tsc seems just could test assignable, expectType is not working.

[sindresorhus]

@Emiyaaaaa Thank you 🙏

It would be amazing if you would be willing to help review some of the open pull requests too: https://github.com/sindresorhus/type-fest/pulls

[Emiyaaaaa]

@Emiyaaaaa Thank you 🙏

It would be amazing if you would be willing to help review some of the open pull requests too: https://github.com/sindresorhus/type-fest/pulls

Thanks for the invite, I'll try my best.