Construct Tuple

题目

构造一个给定长度的元组。

例如

type result = ConstructTuple<2>; // 期望得到 [unknown, unkonwn]

解答

type ConstructTuple<
  L extends number,
  U extends unknown[] = []
> = U["length"] extends L ? U : ConstructTuple<L, [...U, unknown]>;

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> 这题非常有意思，如果不追求完美的话，非常简单，我们只需要用 `infer` + 递归就可以了

type ConstructTuple<
  L extends number,
  U extends unknown[] = []
> = U["length"] extends L ? U : ConstructTuple<L, [...U, unknown]>;

但是 TS 中递归最多只能 1000 次，因此入参到了 1000 就会挂掉，测试用例最后有一个边界 case 就是 1000，那就说明有其他方法实现

既然没有办法递归这么多次，那么我们可以换个思路，怎么把递归次数降下来

我们想想 9999 = 9 * 100 + 99 * 10 + 9 * 10 + 9,那么我们是不是可以通过这样的方法来递归呢

每次取数字的第一位，返回该长度的数组，例如 23,第一次就返回 [unknown, unknown],

递归第二个数字的时候，我们需要把上一次的数组长度 * 10，怎么实现呢

[...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T];

我们可以接受数组 T，展开 10 次，这样就实现了，那么本次的数字（3）就是在它后面加几个 unknown

[...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, unknown, unknown, unknown],

这样第一次返回的 T 是 [unknown, unknown],第二次解构后，就会得到 20 个，再加上 3 个本次的，length 就达到了，递归的次数也大大的缩减了

那么我们就需要构造 0-9 数字对应的展开搭配，就有了下面的答案，其中 N<Count>[keyof N & F] 中 [keyof N & F] 就是数字，例如 9 就是 N<Count>[9],也就对应 N['9']

// 本题答案
type ConstructTuple<
  L extends number,
  O extends string = `${L}`,
  Count extends unknown[] = []
> = O extends `${infer F}${infer R}`
  ? ConstructTuple<L, R, N<Count>[keyof N & F]>
  : Count;

type N<T extends unknown[] = []> = {
  "0": [...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T];
  "1": [...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, unknown];
  "2": [
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    unknown,
    unknown
  ];
  "3": [
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    unknown,
    unknown,
    unknown
  ];
  "4": [
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    unknown,
    unknown,
    unknown,
    unknown
  ];
  "5": [
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown
  ];
  "6": [
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown
  ];
  "7": [
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown
  ];
  "8": [
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown
  ];
  "9": [
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    ...T,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown,
    unknown
  ];
};

Construct Tuple ​

Construct Tuple