PartialDeep: Add allowUndefinedInArrays option

[PicoI2]

Add option allowUndefinedInArrays to PartialDeep, default to true to respect backward compatibility.

PartialDeep with recurseIntoArrays option works like this

import type {PartialDeep} from 'type-fest';

interface Settings {
	languages: string[];
}

const partialSettings: PartialDeep<Settings, {recurseIntoArrays: true}> = {
	languages: [undefined]
};

But it can be annoying that undefined type could be allowed in an array of string.
So with this new option allowUndefinedInArrays: false, the languages array is still optional, but type undefined is not allowed anymore into the languages array.

[som-sm]

Will this create confusion with tuples, because allowUndefinedInArrays wouldn't do anything with tuples.

For eg,

// @exactOptionalPropertyTypes: false

type Test = PartialDeep<
  // ^? { a?: [string?] }
  {a: [string]},
  {recurseIntoArrays: true; allowUndefinedInArrays: false}
>;

const test: Test = {a: [undefined]};

In the example above, undefined is allowed even though allowUndefinedInArrays is set to false. And this is because allowUndefinedInArrays doesn't seem to do anything for tuples, for tuples, allowance of undefined is decided by exactOptionalPropertyTypes setting.

@PicoI2 WDYT?

[som-sm]

IMO, there's no concept of optionality with non-tuple arrays. A string[] is also in a way optional, because it either holds strings or nothing, so (string | undefined)[] isn't IMO the optional version of string[].

Although, adding the optional modifier (?) in a mapped type and instantiating it with a non-tuple array like T[], does produce (T | undefined)[], like Partial<string[]> gives (string | undefined)[], but this feels incorrect to me.

So, IMO, allowUndefinedInArrays in arrays should be the default behaviour.

[PicoI2]

type Test = PartialDeep<
  // ^? { a?: [string?] }
  {a: [string]},
  {recurseIntoArrays: true; allowUndefinedInArrays: false}
>;

const test: Test = {a: [undefined]};

You are right, I did not saw this case. But as the option is allowUndefinedInArraysand not allowUndefinedInArraysAndTuples it could give a hint. Or maybe we could found another name for the option to be more explicit.

IMO, there's no concept of optionality with non-tuple arrays. A string[] is also in a way optional, because it either holds strings or nothing, so (string | undefined)[] isn't IMO the optional version of string[].

Although, adding the optional modifier (?) in a mapped type and instantiating it with a non-tuple array like T[], does produce (T | undefined)[], like Partial<string[]> gives (string | undefined)[], but this feels incorrect to me.

So, IMO, allowUndefinedInArrays in arrays should be the default behaviour.

I totally agree, but making allowUndefinedInArrays default value to false would be a breaking change, and nobody likes breaking changes. IMO, the default value should be true, until the release of the version 5 of type-fest, where it could be noticed in the migration guide.

[som-sm]

I totally agree, but making allowUndefinedInArrays default value to false would be a breaking change, and nobody likes breaking changes. IMO, the default value should be true, until the release of the version 5 of type-fest, where it could be noticed in the migration guide.

Yeah true, setting the default value to false would indeed be a breaking change. However, my rationale was that most users would likely be using the recurseIntoArrays option to make the items within the array optional, rather than adding undefined to the array itself, so it wouldn't be noticeable in majority of the cases.

But yeah, to be on the safer side we can keep it as an option, and in v5 we can either totally remove the option or flip the default value.

You are right, I did not saw this case. But as the option is allowUndefinedInArraysand not allowUndefinedInArraysAndTuples it could give a hint. Or maybe we could found another name for the option to be more explicit.

How about addUndefinedToNonTupleArrays or allowUndefinedInNonTupleArrays?