问题

我有一个对象。
/*
type A = {
  property1: any;
  property2: any;
}
*/

我知道对象中的值都是字符串，但我不想强制类型转换。

// 我不想这样做
const obj2 = obj1 as Record<keyof typeof obj1, string>

相反，我想以正确的方式推断它，使用typescript predicates。这是我的尝试。

function getIsCorrectType<T extends Record<string, any>>(
  obj: T
): obj is Record<keyof T, string>{
  return true; // 假设我手动检查了每个值是否为字符串
}

然而，我现在遇到了一个错误

A type predicate的type必须可分配给其parameter的type。
  Type 'Record<keyof T, string>' is not assignable to type 'T'.
    'Record<keyof T, string>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Record<string, any>'.
      Type 'string' is not assignable to type 'T[P]'.

对我来说这听起来很奇怪。我应该能够将string分配给T[P] = any，对吗？我做错了什么？是否有其他解决方案？

<details>
<summary>英文:</summary>

I have an object.

let obj1: A;
/*
type A = {
property1: any;
property2: any;
}
*/

I know that the values in the object are all strings, but I **don't** want to forcefully typecast.

// I don't want to do this
const obj2 = obj1 as Record<keyof typeof obj1, string>

Instead, I want to infer it in the right way, using **typescript predicates**. This is my attempt to do it.

function getIsCorrectType<T extends Record<string, any>>(
obj: T
): obj is Record<keyof T, string>{
return true; // assume that I manually checked each value to be a string
}

However I now get an error

A type predicate's type must be assignable to its parameter's type.
Type 'Record<keyof T, string>' is not assignable to type 'T'.
'Record<keyof T, string>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Record<string, any>'.
Type 'string' is not assignable to type 'T[P]'.

To me this sounds crazy. I should be able to assign `string` to `T[P] = any`, right? What am I doing wrong? Is there any alternative solution to this?

</details>


# 答案1
**得分**: 2

```typescript
为了使其工作，您只需要推断一组键，而不是整个对象：

const isString = (value: unknown): value is string => typeof value === 'string'

const getIsCorrectType = <K extends string>(
  obj: Record<K, unknown>
): obj is Record<K, string> =>
  Object.values(obj).every(isString)

const x = getIsCorrectType({ 1: 'sdf' })

K - 用于键的推断

还添加了isString自定义类型保护

const isString = (value: unknown): value is string => typeof value === 'string'

const getIsCorrectType = <K extends string>(
  obj: Record<K, unknown>
): obj is Record<K, string> =>
  Object.values(obj).every(isString)

const x = getIsCorrectType({ 1: 'sdf' })