4518 - Fill

[x820745674]

type Fill<
  T extends unknown[],
  N,
  Start extends number = 0,
  End extends number = T['length'],
  L extends 1[] = [],
  IsReplace extends boolean = false
> = Start extends End 
    ? T
    :T extends  [infer Head, ...infer Rest]
    ? L['length'] extends Start
    ? [N, ...Fill<Rest, N, Start, End, [1,...L], true>]
    : L['length'] extends End
    ? [Head, ...Fill<Rest, N, Start, End, [1,...L], false>]
    : [(IsReplace extends true? N : Head), ...Fill<Rest, N, Start, End, [1, ...L], IsReplace>]
    : []