[Typescript] 68. Medium - Fill

Fill, a common JavaScript function, now let us implement it with types. Fill<T, N, Start?, End?>, as you can see,Fill accepts four types of parameters, of which T and N are required parameters, and Start and End are optional parameters. The requirements for these parameters are: T must be a tuple, N can be any type of value, Start and End must be integers greater than or equal to 0.

type exp = Fill<[1, 2, 3], 0> // expected to be [0, 0, 0]

type PlusOne<T extends number, C extends unknown[] = []> = C['length'] extends T
  ? [...C, unknown]['length']
  : PlusOne<T, [...C, unknown]>
type Fill<
  T extends unknown[],
  N,
  Start extends number = 0,
  End extends number = T['length'],
  P extends number = 0                                                // set a pointer, init with 0
> = Start extends End                                                 // Stop when Start = End
  ? T
  : T extends [infer F, ...infer RT] 
    ? P extends Start                  
      ? [N, ...Fill<RT, N, PlusOne<Start>, End, PlusOne<P>>]          // Replace the value when Pointer = Start, increasing value by 1
      : [F, ...Fill<RT, N, Start, End, PlusOne<P>>]                   // No need to replace F, increase P
    : [];