TypeScript函数返回类型适配问题：动态数组返回空元组而非boolean[]

要实现「编译时已知数组长度返回对应长度boolean元组，未知则返回boolean数组」的需求，需要通过条件类型区分具体数字字面量和宽泛number类型，修复原类型在动态数组场景下返回空元组的问题。

修改后的代码 #

type Tuple<
  T,
  Length extends number,
  Accumulator extends T[] = [],
> = Accumulator["length"] extends Length
  ? Accumulator
  : Tuple<T, Length, [...Accumulator, T]>;

// 核心：判断长度类型是具体字面量还是宽泛number，返回对应类型
type Result<Length extends number> = 
  number extends Length ? boolean[] : Tuple<boolean, Length>;

function useHasPermissions<
  const T extends {
    schemaName: string;
    action: string;
  }[],
>(args: { permissions: T }): Result<T["length"]> {
  return [] as any;
}

原代码问题解析 #

原Tuple类型在处理宽泛number类型（动态数组的length类型就是number）时，初始累加器Accumulator是空数组，其length为0。由于0 extends number始终为true，类型递归直接终止，返回空元组[]，导致访问数组索引时报错。

测试验证 #

编译时已知长度的场景

// const tup: [boolean, boolean, boolean]
const tup = useHasPermissions({
  permissions: [
    { schemaName: "User", action: "read" },
    { schemaName: "User", action: "create" },
    { schemaName: "User", action: "update" },
  ],
});

动态数组场景

// 模拟运行时获取的数组
function getAtRuntime() {
  return [{ schemaName: "User", action: "read" }];
}

const arr = useHasPermissions({ permissions: getAtRuntime() });
// 此时 arr 类型为 boolean[]，访问索引不再报错
const a = arr[0];

内容的提问来源于stack exchange，提问作者Ruudy