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Comment

Let's consider this Prettify type:

type Prettify<T extends object> = T extends Function
  ? T
  : { [P in keyof T]: T[P] };

There are two key things to note:

1. T has a constraint of object.
2. The mapped type { [P in keyof T]: T[P] } is homomorphic, so it should preserve the array structure in case of instantiations with arrays. So, for example, Prettify<[string, string]> returns [string, string].

Now, let's consider this instantiation of Prettify:

// Here, `T` is not constrained with `object`, so we take intersection with `object`
type Test<T> = Prettify<T & object>;

type T1 = Test<[string, string]>;
//   ^? type T1 = { [x: number]: string; 0: string; 1: string; length: 2; toString: () => string; toLocaleSt…

I'd expect T1 to be [string, string] but instead the array structure is lost.

Looks like this happens because of the object intersection:

type T1 = Prettify<[string, string] & object>;
//   ^? type T1 = { [x: number]: string; 0: string; 1: string; length: 2; toString: () => string; toLocaleSt…

The example above might be a bit contrived, but this is a common use case—we have object constraints at certain places and to satisfy those constraints we often have to take intersections with object.

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