Type safety for enum property names

[schoel-bis]

Suggestion

Enums work in two directions, where an enum property value can be retrieved via AnEnum.aProperty and a property name can be retrieved via AnEnum[aValue]. In TypeScript 3.4.3, the type of the second expression is string. Instead, it should be equivalent to keyof typeof AnEnum | undefined.

Use Cases

While the nameof operator (suggested in #1579) does not exist yet, a construct such as AnEnum[AnEnum.aProperty] can currently be used to safely refer to an enum property name. Unfortunately, since the type of that expression is string it requires a cast when assigning it to a type safe target.

Examples

enum AnEnum { aProperty = 1, anotherProperty = 23, yetAnotherOne = 42 };
let target: keyof typeof AnEnum;

…

// Compilation error here: Type 'string' is not assignable to type '"aProperty" | "anotherProperty" | "yetAnotherOne"'.
target = AnEnum[AnEnum.aProperty];

Checklist

My suggestion meets these guidelines:

• [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [x] This wouldn't change the runtime behavior of existing JavaScript code
• [x] This could be implemented without emitting different JS based on the types of the expressions
• [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• [x] This feature would agree with the rest of TypeScript's Design Goals.

[Howard-Lam-UnitedVanning]

Would like this because for string enum, you can't really do the reverse keying like so Enum[Enum.key]. It needs to be nameof(Enum.key) / nameof Enum.key. Then, when there is refactoring, renaming will rename also the enum itself rather than just changing 1 string at a time.

Right now, my use case can be

export type RouteRecordCustom = RouteRecordRaw & {
    path: RoutePaths,
    name?: (keyof typeof RoutePaths)
};
routes: Array<RouteRecordCustom> =  = [
    {
        path: RoutePaths.Root,
        name: 'Root', //nameof RoutePaths.Root
        component: HomeView,
    },    {
        path: RoutePaths.HelloWorld,
        name: 'HelloWorld', //nameof RoutePaths.HelloWorld
        component: HelloWorld,
    },
]