Generics 上的“keyof typeof”

Question

Hello I have some struggle with my generic typing.

interface ReturnValue {
    keyA: string,
    keyB: string
}


interface FilterA {
    keyA: boolean;
}

interface FilterB {
    keyB: boolean;
}


const func = function<T>(args: T) : Pick<ReturnValue, keyof typeof args> {
    const res :Pick<ReturnValue, keyof typeof T> = {};
    Object.keys(args).map(key => {
        res[key] = key
    }) 

    return res;
}

console.log(func({keyB: true}).keyB); // should work
console.log(func({keyA: true}).keyA); // should work

console.log(func({keyA: true}).keyB); // should not work
console.log(func({keyC: true}).keyC); // should not work

But I have this error:

Type 'keyof T' does not satisfy the constraint 'keyof ReturnValue'.
  Type 'string | number | symbol' is not assignable to type 'keyof ReturnValue'.
    Type 'string' is not assignable to type 'keyof ReturnValue'.
      Type 'keyof T' is not assignable to type '"keyB"'.
        Type 'string | number | symbol' is not assignable to type '"keyB"'.
          Type 'string' is not assignable to type '"keyB"'.(2344)

Any ideas?

When I'm using the function call it's working fine, I can access only to what I gave as arguments but TS still display this error.

The context is GraphQl. My input is an object where value of each keys is true or false . From this object I'll build a GQL string query, fetch to api, and the return will have the same structure but value wont be boolean but string for example.

TS Playground

Answer 1

It wort using Array.prototype.reduce and mapped types here:

interface FilterA {
    keyA: boolean;
}

interface FilterB {
    keyB: boolean;
}

type Filters = FilterA | FilterB

type Reduce<Obj> = {
    [Prop in keyof Obj]: Prop
}

const func = <Obj extends Filters>(args: Obj) =>
    Object.keys(args).reduce((acc, key) => ({
        ...acc,
        [key]: key
    }), {} as Reduce<Obj>)

func({ keyB: true }).keyB; // ok
func({ keyA: true }).keyA; // ok

func({ keyA: true }).keyB; // error
func({ keyC: true }).keyC // error

Playground

I have used immutable version of this approach not because it is better. I just prefer immutable data structures.

UPDATE

Code example of alternative version of Reduce from comments:

type Reduce<Obj> = {
    [Prop in keyof Obj]: Prop extends keyof ReturnValue ? ReturnValue[Prop] : never
}


interface ReturnValue {
    keyA: string,
    keyB: string
}