Typescript类型体操 - Fill

题目

中文

Fill是 javascript 中常用的方法, 现在让我实现类型版本的 Fill
Fill<T, N, Start?, End?>, 正如你看到的那样, Fill接受四个泛型参数, 其中 T 和 N 是必填参数, Start 和 End 是可选参数
这些参数的要求如下: T 必须是一个元组(tuple), N 可以是任何值类型, Start 和 End 必须是大于或等于 0 的整数

为了模拟真实的函数, 测试用例包含了一些边界条件, 我希望你喜欢它 😃

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

为了

English

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]

In order to simulate the real function, the test may contain some boundary conditions, I hope you can enjoy it 😃

答案

type Fill<
    T extends unknown[],
    N,
    Start extends number = 0,
    End extends number = T['length'],
    Pending extends any[] = [],
    Filled extends any[] = []
> = T extends [infer L, ...infer R]
    ? [
          Pending['length'] extends Start
              ? Filled['length'] extends End
                  ? L
                  : N
              : L,
          ...Fill<
              R,
              N,
              Start,
              End,
              Pending['length'] extends Start ? Pending : [...Pending, 0],
              Filled['length'] extends End ? Filled : [0, ...Filled]
          >
      ]
    : [];

在线演示