Fill

问题描述

Fill一个常见的 JavaScript 函数，现在我们用类型来实现它。 Fill<T, N, Start?, End?>可以看到，Fill它接受四种类型的参数，其中T和N为必选参数， 和Start为End可选参数。这几个参数的要求是：T必须是tuple，N可以是任意类型的值，Start并且End必须是大于等于 0 的整数。

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

为了模拟真实的功能，测试可能包含一些边界条件，希望你能喜欢它:)

// ============= Test Cases =============
import type { Equal, Expect } from './test-utils'
​
type cases = [
  Expect<Equal<Fill<[], 0>, []>>,
  Expect<Equal<Fill<[], 0, 0, 3>, []>>,
  Expect<Equal<Fill<[1, 2, 3], 0, 0, 0>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], 0, 2, 2>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], 0>, [0, 0, 0]>>,
  Expect<Equal<Fill<[1, 2, 3], true>, [true, true, true]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 0, 1>, [true, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 1, 3>, [1, true, true]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 10, 0>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 10, 20>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 0, 10>, [true, true, true]>>,
]
​
​
// ============= Your Code Here =============
type Fill<
  T extends unknown[],
  N,
  Start extends number = 0,
  End extends number = T['length'],
  Count extends any[] = [],
  Flag extends boolean = Count['length'] extends Start ? true : false
> = Count['length'] extends End
  ? T
  : T extends [infer R, ...infer U]
    ? Flag extends false
      ? [R, ...Fill<U, N, Start, End, [...Count, 0]>]
      : [N, ...Fill<U, N, Start, End, [...Count, 0], Flag>]
    : T
​

看看大佬的带思路版的答案，带思路版的答案。