Utilize a universal type for the property name in TypeScript

In my code, I am using an enum as a propName for an object-like type.

export enum WeaponClass {
    smallLaser = "smallLaser",
}

export type WeaponType = {
    [propName in WeaponClass]: number
};

const weapons: WeaponType = {
    [WeaponClass.smallLaser]: 1,
};

Check out the playground for this code snippet

I am curious if it is possible to use a generic for the propName and pass in an enum. However, when I tried this approach, it resulted in errors.

export enum WeaponClass {
    smallLaser = "smallLaser",
}

export type WeaponType<T> = {
    [propName in T]: number
};

const weapons: WeaponType<WeaponClass> = {
    [WeaponClass.smallLaser]: 1,
};

Find more details on this in the playground here

Answer №1

The main issue arises from the fact that a mapped type is only able to iterate over types that can be used as property keys, specifically string | number | symbol. If the type T is not narrowed down to one of these types, Typescript will not recognize it as a valid key. This necessitates narrowing down the type to a subset of types that are eligible to be used as property keys.

Luckily, there exists a type that precisely encapsulates this requirement - PropertyKey. Therefore, the simple solution is to specify:

T extends PropertyKey

Answer №2

The issue arises because T has no restrictions on its type, whereas keys in objects have specific type requirements. To address this problem, a condition can be added to limit the type T to one of those specifications:

export type ItemType<T extends (string | number | symbol)> = {
    [propertyName in T]: number
};

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