Typescript: Generic enum types as index for another type

Question

I'm trying to create an indexed type whose index are the values of an enum BUT the enum comes from a generic argument.

export type EnumMap<TEnum extends string | undefined = undefined> = {
  [key: TEnum]: ValueClass;
}
export type EnumMap<TEnum extends string | undefined = undefined> = {
  [key: keyof TEnum]: ValueClass;
}

Both of those implementations shows the error:

An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.

When I implement it as a mapped object as suggested:

export type EnumMap<TEnum extends string | undefined = undefined> = {
  [TEnumKey in keyof TEnum]: ValueClass;
}

It doesn't show any errors at the definition but it only accepts TEnum instances as the value. It shows this error when I try to instantiate an EnumMap.

Type '{ "test": ValueClass; }' is not assignable to type 'TestEnum'.

I'm using this enum as a test:

export enum TestEnum {
  test = 'test'
}

Is there any way to use a generic enum as a type's index?

PS.: I understand I can implement it using string as the key, but I want to tie the indexes to the enum values;

export type EnumMap = {
  [key: string]: ValueClass;
}
const map: EnumMap = { [TestEnum.test]: {...} }; // works fine

Enums themselves are treated as "unions" when using them in a mapped type. After all, they are enumerations. — k.tten
– k.tten, Commented May 19, 2022 at 18:55

Tobias S. · Accepted Answer · 2022-05-19 18:52:34Z

3

Does this work for you?

enum Enum {
  "ABC" = "abc",
  "CDE" = "cde"
}

export type EnumMap<TEnum extends string = Enum> = {
  [key in TEnum]?: string;
}

const map: EnumMap = { [Enum.ABC]: "123" };

Since TEnum is a string union, you can use it as [key in TEnum] inside the index signature.

I am not sure why you used string | undefined for TEnum...

answered May 19, 2022 at 18:52

Tobias S.

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