TypeScript: Infer enum value type from enum type

Question

I wrote a function returning all values of a given enum as array. The implementation works, but I have a problem with the type of the return value.

enum Foo {
    FOO_1 = "FOO_1",
    FOO_2 = "FOO_2",
}

function getEnumValues<T>(e:T): T[] {
    let keys: string[] = Object.keys(e);
    keys = keys.filter(key => e[key] !== undefined);
    return keys.map(key => e[key]);
}

const fooValues:Foo[] = getEnumValues(Foo);

I get this error:

Error:(46, 7) TS2322: Type '(typeof Foo)[]' is not assignable to type 'Foo[]'. Type 'typeof Foo' is not assignable to type 'Foo'.

How can I change the signature of getEnumValues() in order to return the type Foo[] here?

Titian Cernicova-Dragomir Titian Cernicova-Dragomir · Accepted Answer · 2018-05-08T06:56:38

You need to change the definition a bit to infer the type of the enum member, right now T will be the enum object itself (aka typeof T)

enum Foo {
    FOO_1 = "FOO_1",
    FOO_2 = "FOO_2",
}

function getEnumValues<TEnum, TKeys extends string>(e: { [key in TKeys]: TEnum }): TEnum[] {
    let keys = Object.keys(e) as Array<TKeys>;
    keys = keys.filter(key => e[key] !== undefined);
    return keys.map(key => e[key]);
}

const fooValues: Foo[] = getEnumValues(Foo);

Note that while this works for enums, it will work for any object it is not restricted to enums

TypeScript: Infer enum value type from enum type

1 Answers