Typescript:如何提取const对象的值类型，并将它们用作新类型中的键

Question

我想取一个常量对象，比如{key1: 'value1', key2: value2'} as const，将其值转换为键，并在类型为{value1: number; value2: number}的新对象中使用它。我想在函数中执行此操作，并且我希望函数类型正确。

下面是我正在努力工作的代码。当我尝试将类型为Partial<Blah>的变量强制转换为Blah时，会发生错误。有人能解释一下为什么会发生这种事吗？如果有办法做我想做的事吗？谢谢

type BaseObject = Record<string, string>;

type ValuesAsKeys<T extends BaseObject> = {
[K in keyof T as T[K]]: number;
};
function useValuesAsKeys<T extends BaseObject>(arg: T): ValuesAsKeys<T> {
const x: Partial<ValuesAsKeys<T>> = {};
for (const value of Object.values(arg) as Array<T[keyof T]>) {
x[value] = 1;
}
// This line doesn't work
return x as ValuesAsKeys<T>;
}
// Example use case
const obj = {key1: 'value1', key2: 'value2'} as const;
const result = useValuesAsKeys(obj); // Type of result is {value1: number; value2: number}

// Everything below here works
// Only including this because I'm contrasting this code with the code above.
type KeysAsKeys<T extends BaseObject> = {
[K in keyof T]: number;
};
function useKeysAsKeys<T extends BaseObject>(arg: T): KeysAsKeys<T> {
const x: Partial<KeysAsKeys<T>> = {};
for (const value of Object.values(arg) as Array<T[keyof T]>) {
x[value] = 1;
}
return x as KeysAsKeys<T>;
}

Answer 1

带有reduce的显式返回类型几乎总是不起作用，因为initial的值每次迭代都会更改。

为了使其工作，最好超载您的功能：

type BaseObject = Record<string, string>;

type ValuesAsKeys<T extends BaseObject> = {
[K in keyof T as T[K]]: number;
};
type Values<T> = T[keyof T]
function useValuesAsKeys<T extends BaseObject>(arg: T): ValuesAsKeys<T>
function useValuesAsKeys<T extends BaseObject>(arg: T) {
return (Object.values(arg) as Array<Values<T>>).reduce((acc, elem) => ({
...acc,
[elem]: 1
}), {})
}
// Example use case
const obj = { key1: 'value1', key2: 'value2' } as const;
const result = useValuesAsKeys(obj); // Type of result is {value1: number; value2: number}
result.value1 //ok

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Answer 2

我仍然不确定为什么将Partial<Blah>转换为Blah在这种情况下不起作用，但这段代码有效，并且对我来说足够安全：

function useValuesAsKeys<T extends BaseObject>(arg: T): ValuesAsKeys<T> {
const x = {} as ValuesAsKeys<T>;
for (const value of Object.values(arg) as Array<T[keyof T]>) {
x[value] = 1;
}
return x;
}