Fill

题目

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 😃

解答

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;

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这题需要引入两个变量，一个用来计数，一个用来作为是否替换的标志
Count extends any[] = []
Flag extends boolean = Count['length'] extends Start ? true : false 几个关键点
在 Count 等于 End 的时候需要结束替换，也就是结束条件
当 Count 等于 Start 的时候是开始替换的条件，递归处理数组替换即可，注意需要把 T 换成新的
在开始替换后，需要把 Flag 继续传下去，不然 Flag 会被置为 false 不再替换
ts
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;

Fill ​

Fill