Unlimited Array.prototype.flat depth

[PKrasinski]

Fixes #55685

The old Array.prototype.flat definition supports depth only up to 21. Over 21 return wrong type.
This solution allow call flat method with unlimited depth parameter.

const foo = [ [ ['string' ] ] ]
const flatFoo = foo.flat(1000000) // string[]

Performance (checked with this code) is the same or better in some cases.

The only limitation of this solution is that for arrays deeper then 999 levels unknown[] type is returned.

[
  [
    [
       // 997 levels
    ]
  ]
].flat(1000) // unknown[]

[PKrasinski]

@microsoft-github-policy-service agree

[MartinJohns]

Introducing new global types with generic names and increasing the complexity of the types enormously for a highly questionable use case. :-/

[PKrasinski]

@MartinJohns Yep, probably you are right. I was just looking for the way how to type safe recurrent functions so I checked how it looks like in flat method declaration and I was a bit surprised that solution is using hard-coded array and returns wrong type when the depth is higher. So I decided to write my own implementation just for fun 😄 I could avoid new global types in this solution. I also know that probably I solved the problem that nobody had.

[DanielRosenwasser]

I think for reasons discussed here and elsewhere, I would encourage people who need this to include their own local augmentations to the global scope.