What could be the issue causing this indexed signature to fail?

I'm attempting to extract an object's values and use them as keys for the interface. Here is my approach:

const obj = {
    a: 'foo',
    b: 'bar',
} as const;

type A<T extends object, K extends keyof T = keyof T> = {
    [x: string]: T[K];
}[number];

// I encounter an error with this one:
// Type 'T[string]' is not assignable to type 'symbol'
type B<T extends object> = {
    [K in A<T>]: any;
};

// Even when trying variations like this, it still produces the same error.
type C<T extends typeof obj = typeof obj> = {
    [K in A<T>]: any;
};

// However, this alternative works without issues.
type D = {
    [K in A<typeof obj>]: any;
};

// Successfully defining this one.
const a: D = {
    foo: 1,
    bar: 1,
};

This problem has me stumped, can someone help explain why this isn't working? (or if there are better alternatives)?

Playground link

Answer №1

An issue was flagged indicating that it cannot be assured that the values of the specified type are suitable to serve as keys in a new type (

Type 'T[string]' is not assignable to type 'string | number | symbol'
). This can be resolved by implementing the right generic constraint (for example,
T extends Record<PropertyKey, PropertyKey>
instead of T extends object)

type ValuesAsKeys<T extends Record<PropertyKey, PropertyKey>> =
    Record<T[keyof T], any>;

const a: ValuesAsKeys<typeof obj> = {
    foo: 1,
    bar: 1,
};

Access the Playground

Answer №2

To access the values stored within your object, you can utilize the following custom type

type ValuesFromObject<T extends object> = T[keyof T]

Link to Playground for Testing

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