Indexed access lookup on enum type could produce specific key types

Suggestion

Per the documentation, it is possible to reverse map an enum at runtime:

enum Enum {
  A,
}
 
let a = Enum.A;
let nameOfA = Enum[a]; // "A"

However, suppose we wanted to get the type of the enum name from the enum value at compile time. That is, get some type T = "A" from Enum.A. From the example above, we would expect something like the following to work:

type T = typeof Enum[Enum.a]; // evaluates to string rather than "A"

const A: T = "A"; // compiler has no issues
const B: T = "B"; // compiler has no issues

As written above, the compiler fails to get the correct type. However, I have found a workaround:

type T = keyof { [K in keyof typeof Enum as typeof Enum[K] extends Enum.A ? K : never]: any }; // evaluates to "A"

const A: T = "A"; // compiler has no issues
const B: T = "B"; // compiler now throws a fit as expected

It would be nice if this could be abstracted away from the developer.

[RyanCavanaugh]

Well, fair 😅

[fatcerberus]

@RyanCavanaugh I kind of get the impression this issue counts as that proposal (and you tagged it Declined 😉).

From the example above, we would expect something like the following to work:

type T = typeof Enum[Enum.a]; // evaluates to string rather than "A"

const A: T = "A"; // compiler has no issues
const B: T = "B"; // compiler has no issues