BFE.devAI解答 - TypeScript 类型谜题
45. implement Slice<A, S, E>

type Slice<A extends any[], S extends number, E extends number = A['length']> = 
  A extends [...infer F, ...infer L] ? 
    (F['length'] extends S ? L : Slice<L, S, E>) extends [...infer FF, ...infer LL] ?
      (LL['length'] extends A['length'] ? [...F, ...FF] : Slice<F, S, E>) : never 
    : never 
  : never

Explanation:

We use the recursive approach to grab the sliced array.

First, we separate the array into two parts, F and L. F are the elements before the start index S, and L are the elements from the start index S.

If S is equal to F.length, it means we don't need to grab anything from F, so we can just return L.

If not, we need to keep searching in L for the correct E.

We use infer to get the type of FF (elements before LL) and LL (elements from E).

If LL.length is equal to the original array's length, it means we have grabbed all the elements we need, so we can simply concatenate F and FF to get the final result.

If not, we recursively search in F with the same S to grab more elements from the left side.

If we cannot match any condition, it means the input types are wrong and we return never.