基于参数的 Typescript 泛型返回类型

Question

如何根据参数进行动态打字稿返回类型未定义或未定义。请帮忙。

还添加了Typescript Playground Snippet供参考

我想出了这个，但没有按预期工作。

V extends keyof T ? ToObject<T> : ToObject<T[keyof T]>

代码：

export interface ToObject<T> {
  [k: string]: T;
}

export const toObject = <T, V = (keyof T)>(
  list: T[],
  key: keyof T,
  valueKey?: V
): V extends keyof T ? ToObject<T> : ToObject<T[keyof T]> => valueKey === undefined
    ? list.reduce((all, cur) => ({
      ...all,
      [cur[key] as unknown as string]: cur
    }), {} as ToObject<T>)
    : list.reduce((all, cur) => ({
      ...all,
      [cur[key] as unknown as string]: cur[valueKey as unknown as keyof T]
    }), {} as ToObject<T[keyof T]>);

用法：

interface Data{
  a: string;
  b: string;
  c: string;
}
const list: Data[] = [
  {a:'a', b: 'b', c: 'c'},
  {a:'1', b: '2', c: '3'}
];

// use 1
toObject<Data>(list, 'a');
// response
{
  {'a': {a:'a', b: 'b', c:'c'}},
  {'1': {a:'1', b:'2', c: '2'}},
}

// Use 2
toObject<Data>(list, 'a', 'b');
//response
{ {'a': 'b'}, {'1': '2'} }

问题：

// use 1 ERROR -> Cant do this
const dataMap: ToObject<Data> = toObject<Data>(list, 'a');
// Type 'ToObject<Data> | ToObject<string>' is not assignable to type 'ToObject<Data>'.
// Type 'ToObject<string>' is not assignable to type 'ToObject<Data>'.
// Type 'string' is not assignable to type 'Data'.(2322)

// Use 2 ERROR -> Cant do this
const dataKeyMap: ToObject<string> = toObject<Data>(list, 'a', 'b');
// Type 'ToObject<Data> | ToObject<string>' is not assignable to type 'ToObject<string>'.
// Type 'ToObject<Data>' is not assignable to type 'ToObject<string>'.
// Type 'Data' is not assignable to type 'string'.(2322)

Answer 1

正如所写，TS 没有足够的信息来区分toObject 的两种返回类型。

如果我们添加function overloads，我们可以将返回类型绑定到函数签名。如果通过valueKey， toObject 返回ToObject<T[keyof T]>，如果不是，则函数返回ToObject<T>。

Revised Playground

export function toObject <T, V = (keyof T)>(list: T[], key: keyof T): ToObject<T>
export function toObject <T, V = (keyof T)>(list: T[], key: keyof T, valueKey: V): ToObject<T[keyof T]>
export function toObject <T, V = (keyof T)>(list: T[], key: keyof T, valueKey?: V): ToObject<T> | ToObject<T[keyof T]> {
  return valueKey === undefined
    ? list.reduce((all, cur) => ({
      ...all,
      [cur[key] as unknown as string]: cur
    }), {} as ToObject<T>)
    : list.reduce((all, cur) => ({
      ...all,
      [cur[key] as unknown as string]: cur[valueKey as unknown as keyof T]
    }), {} as ToObject<T[keyof T]>);
}  

除了让 TS 编译器满意之外，重载还告诉人类用户应该如何调用函数。