Use enum as restricted key type

Can an enum be used as a key type instead of only number or string? Currently it seems like the only possible declaration is { [key: number]: any }, where key can be of type number or string. Is it possible to make something like in this example:

enum MyEnum
{
    First,
    Second
}

var layer: { [key: MyEnum]: any };

Solution

Since 2018, there is an easier way in Typescript, without using keyof typeof:

let obj: { [key in MyEnum]: any} =
 { [MyEnum.First]: 1, [MyEnum.Second]: 2 };

To not have to include all keys:

let obj: { [key in MyEnum]?: any} =
 { [MyEnum.First]: 1 };

To know the difference between in and keyof typeof, continue reading.

`in Enum` vs `keyof typeof Enum`

in Enum compiles to enum values and keyof typeof to enum keys.

Other differences

With keyof typeof, you cannot change the enum properties:

let obj: { [key in keyof typeof MyEnum]?: any} = { First: 1 };
obj.First = 1;
// Cannot assign to 'First' because it is a read-only property.

... unless you use -readonly:

let obj: { -readonly [key in keyof typeof MyEnum]?: any} = { First: 1 };
obj.First = 1; // works

But you can use any integer key?!:

let obj: { [key in keyof typeof MyEnum]?: any} = { First: 1 };
obj[2] = 1;

keyof typeof will compile to:

{
    [x: number]: any;
    readonly First?: any;
    readonly Second?: any;
}

Note both the [x: number] and the readonly properties. This [x: number] property doesn't exist with a string enum.

But with in Enum, you can change the object:

enum MyEnum {
    First,  // default value of this is 0
    Second, // default value of this is 1
}

let obj: { [key in  MyEnum]?: any} = { [MyEnum.First]: 1 };
obj[MyEnum.First] = 1; // can use the enum...
obj[0] = 1;            // but can also use the enum value, 
                       // as it is a numeric enum by default

It's a numeric enum. But we can't use any number:

obj[42] = 1;
// Element implicitly has an 'any' type because 
// expression of type '42' can't be used to index type '{ 0?: any; 1?: any; }'.
// Property '42' does not exist on type '{ 0?: any; 1?: any; }'.

The declaration compiles to:

{
    0?: any;
    1?: any;
}

We allow only 0 and 1, the values of the enum.

This is in line with how you would expect an enum to work, there are no surprises unlike keyof typeof.

It works with string and heterogenous enums:

enum MyEnum
{
    First = 1,
    Second = "YES"
}

let obj: { [key in  MyEnum]?: any} = { [MyEnum.First]: 1, [MyEnum.Second]: 2 };
obj[1] = 0;
obj["YES"] = 0;

Here the type is:

{
    1?: any;
    YES?: any;
}

Get immutability with readonly:

let obj: { readonly [key in MyEnum]?: any} = { 
    [MyEnum.First]: 1,
};
obj[MyEnum.First] = 2;
// Cannot assign to '1' because it is a read-only property.

... which makes these keys readonly:

{
    readonly 1?: any;
    readonly 2?: any;
}

Summary

`in Enum`	`keyof typeof Enum`
Compiles to enum values	Compiles to enum keys
Does not allow values outside the enum	Can allow numeric values outside the enum if you use a numeric enum
Can change the object, immutability opt-in with `readonly`	Can't change enum props without `-readonly`. Other numeric values outside the enum can be

Use in Enum if possible.

Use enum as restricted key type

in Enum vs keyof typeof Enum

Other differences

Summary

`in Enum` vs `keyof typeof Enum`