Correct typing when using the index signature of an Enum

Search Terms

enum index signature

Suggestion

Imagine a non const enum (which can be accessed using properties and it's index signature).

enum example { ONE = 1, TWO = 2, THREE = 3 }

const a = example.ONE;
console.log(a); // Prints 1

const aName = example[a]; // Type of aName is string and should be "ONE" | "TWO" | "THREE". Or even "ONE" since a is const
console.log(aName); // Prints "ONE";

The implicit type of aName is string and should be "ONE" | "TWO" | "THREE" since those are the only possible values. Or even "ONE" since a is a constant.

Use Cases

More control over the values of a variable.

Examples

See above

Checklist

My suggestion meets these guidelines:

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

Start by reproducing the enum index-signature example from the issue and inspect the type-checking entry points that determine the type of example[a]. Compare the inferred result with the requested union of enum member names, including whether the constant value should narrow it to "ONE". Done means the intended type is inferred without changing emitted JavaScript and the relevant type-checking tests pass.