Feature Request: Allow typeof with index types

[SLaks]

Revised feature request: Please allow typeof in mapped types:

Given a type like this:

type Enum<TEnum> = {
    [TName in keyof typeof TEnum]: TEnum[TName]
}

I want to write

type Schema<TObject> = {
    [TName in keyof TObject]: Enum<typeof (TObject[TName])>
}

This way, given the property declared as foo: SomeEnum, the mapped type would produce foo: Enum<typeof SomeEnum>, which would then match actual property foo: SomeEnum.

TObject[TName] is a value, so typeof should apply. However, this currently gives a syntax error.
Removing the parentheses complains (correctly) that TObject itself is a type, not a value.

---

TypeScript Version: 2.7.0

Search Terms: typeof keyof generic

Playground Link:
Minimal: http://www.typescriptlang.org/play/index.html#src=enum%20Color%20%7B%0D%0A%20%20%20%20Red%2C%20Green%2C%20Blue%0D%0A%7D%0D%0A%0D%0Atype%20Enum%3CTEnum%3E%20%3D%20%7B%0D%0A%20%20%20%20%5BTName%20in%20keyof%20TEnum%5D%3A%20TEnum%5BTName%5D%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20MyClass%20%7B%0D%0A%20%20%20%20color%3A%20Color%3B%0D%0A%7D%0D%0A%0D%0Aconst%20test%3A%20Enum%3Ctypeof%20(MyClass%5B'name'%5D)%20%3E%20%3D%20Color%3B%0D%0A

With motivating real-world example:
http://www.typescriptlang.org/play/index.html#src=enum%20Color%20%7B%0D%0A%20%20%20%20Red%2C%20Green%2C%20Blue%0D%0A%7D%0D%0A%0D%0Aenum%20Shape%20%7B%0D%0A%20%20%20%20Square%2C%20Circle%2C%20Triangle%0D%0A%7D%0D%0A%0D%0Atype%20Enum%3CTEnum%3E%20%3D%20%7B%0D%0A%20%20%20%20%5BTName%20in%20keyof%20TEnum%5D%3A%20TEnum%5BTName%5D%3B%0D%0A%7D%3B%0D%0A%0D%0A%2F%2F%20Simple%20sample%3A%0D%0Aconst%20test%3A%20Enum%3Ctypeof%20Shape%3E%20%3D%20Shape%3B%0D%0A%0D%0A%2F%2F%20Real-world%20sample%20usage%3A%0D%0Ainterface%20MyClass%20%7B%0D%0A%20%20%20%20name%3A%20Date%3B%0D%0A%20%20%20%20color%3A%20Color%3B%0D%0A%20%20%20%20shape%3A%20Shape%3B%0D%0A%7D%0D%0Atype%20Schema%3CT%3E%20%3D%20%7B%0D%0A%20%20%20%20%5BTName%20in%20keyof%20T%5D%3A%20(%0D%0A%20%20%20%20%20%20%20%20%2F%2F%20For%20classes%3A%0D%0A%20%20%20%20%20%20%20%20%7B%20new()%3A%20T%5BTName%5D%20%7D%0D%0A%20%20%20%20%20%20%20%20%2F%2F%20For%20enums%3A%0D%0A%20%20%20%20%20%20%20%20%7CEnum%3CT%5BTName%5D%3E%0D%0A%20%20%20%20)%0D%0A%7D%0D%0A%0D%0Aconst%20myClassSchema%3A%20Schema%3CMyClass%3E%20%3D%20%7B%0D%0A%20%20%20%20name%3A%20Date%2C%0D%0A%20%20%20%20color%3A%20Color%2C%0D%0A%20%20%20%20shape%3A%20Shape%2C%0D%0A%7D%3B

[s-ve]

I don't think that supporting typeof TEnum here would make much sense since TEnum can only refer to something that is already a type. typeof always expects a value as operand.

You can refactor your code like this :

type Enum<TEnum> = {
    [TName in keyof TEnum]: TEnum[TName]
};

const test: Enum<typeof Color> = Color;

But then it would be equivalent to simply writing :

const test = Color;

Could you maybe clarify the purpose of your Enum<TEnum> type ? 🙂

Got this working for string enums:

enum Color {
    Red = "Red", Green = "Green", Blue = "Blue",
}

enum Shape {
    Square = "Square", Circle = "Circle", Triangle = "Triangle",
}
type Enum<TEnum extends string> = {
    [TName in TEnum]: string;
};
const test: Enum<Shape> = Shape;

// Real-world sample usage:
interface MyClass {
    name: Date;
    color: Color;
    shape: Shape;
}
type SchemaForType<T> = T extends string ? Enum<T> : { new(): T }
type Schema<T> = {
    [TName in keyof T]: SchemaForType<T[TName]>
}
const myClassSchema: Schema<MyClass> = {
    name: Date,
    color: Color, // There is correctly a compile error if you write `color: Shape`!
    shape: Shape,
};

Unfortunately this does not work for normal number enums right now, because we can't put those in mapped types:

type Enum<TEnum extends number> = {
    [TName in TEnum]: string; // Error: TEnum is not a string type...
};
const test: Enum<Shape> = Shape;

Closest issue I can find for that is #22105

[SLaks]

@s-ve See the end of my playground link.

Basically, I want to use Enum<T> together with mapped types, to create a type that maps an object to its property types.
Something like

type Schema<TObject> = {
    [TName in keyof TObject]: Enum<TObject[TName]>
}

interface MyClass {
    color: Color;
}

const schema: Schema<MyClass> = {
    color: Color,
};

My feature request should actually be to allow [TName in keyof TObject]: Enum<typeof (TObject[TName])>, which is currently a syntax error.

@s-ve is correct that you can't get the typeof something that's already a type. This might be confusing since there is a type Color and a value Color that share the same name, but they're two different things. If we have x: Color there's no way to get access to the value Color. You can't use typeof typeof x since typeof x is a type, not a value. typeof Color works because we interpret Color as referring to the value and not the type.

[SLaks]

But TObject[TName] is a value, so typeof that should work.

No, TObject[TName] is the type of the property.

[SLaks]

@Andy-MS Your solution only works when the enum values match their names (since you're using TName in TEnum instead of keyof).

[SLaks]

I see; I was misunderstanding how typeof works for enums.

Is there any way to access the static members of a type?

Can you add such a feature?

Just realized my example wouldn't work because T extends string ? Enum<T> : { new(): T } maps over the union T, so it just has to be Enum<"Red"> | Enum<"Green"> | Enum<"Blue">, while I wanted Enum<"Red" | "Green" | "Blue">. @sandersn Was there a workaround for that?

Is there any way to access the static members of a type?

We don't have the notion of a static member of a type -- a type just describes instances. If you write class C { static a() {} b() {} }, the type C just describes { b(): void } and has no reference to the static side of the class, which is described by a separate type typeof C (where C is the value, not the type).

[mhegazy]

[T] extends [string] ? ... will not distribute

[s-ve]

@mhegazy Unfortunately Typescript will not infer that T extends string in the conditional true branch.

I don't know if this is expected or not.

Related : #21870

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.