Brand all Enums

[LukeAbby]

I've added a new Brand type to helperTypes. One of the things it's most useful for are enums.

The pattern to implement is as follows:

namespace ExampleClass {
    type EXAMPLE_ENUM = Brand<number, "ExampleClass.EXAMPLE_ENUM">;
}

declare class ExampleClass {
    static EXAMPLE_ENUM: Record<"a" | "b" | "c", ExampleClass.EXAMPLE_ENUM>;

    usesEnum(value: ExampleClass.EXAMPLE_ENUM);
}

const exampleClass = new ExampleClass();
exampleClass.usesEnum(ExampleClass.EXAMPLE_ENUM.a); // Works
exampleClass.usesEnum(1); // Fails

This helps people use enums appropriately and avoid mistakes.

---

Brand is implemented as follows:

declare class Branded<in out BrandName extends string> {
  #brand: BrandName;
}

export type Brand<BaseType, BrandName extends string> = BaseType & Branded<BrandName>;

The principle is that the truly private property #brand must exist for it to be assignable. This property does not really exist at runtime but its existence is expected at compile time and so has these useful properties.

This can't be replicated outside of going through the Brand type because the Branded class does not exist anywhere else and a truly private property from one class is not assignable to another. A similar effect could be achieved through a unique symbol but this would show up in keyof Brand<...> hence why a truly private property is used. Plus it would make it more difficult to make BrandName explicitly invariant outside of a class.