map through enum

TypeScript Version: 3.1.6

Search Terms: enum

Code

enum ABC {
  A = 'A',
  B = 'B',
  C = 'C',
}
// No Error
for (const key in ABC) {
  console.log(ABC[key]);
}
// Error: Element implicitly has an 'any' type because index expression is not of type 'number'
Object.keys(ABC).map(key => console.log(ABC[key]));

Expected behavior: Object.keys, values, entries on enum ABC infer type of ABC’s key and value. Actual behavior: Cannot get inferred type.

Shouldn’t type of key from enum be ‘A’ | ‘B’ | ‘C’?

[Nathan-Fenner]

This is related to, but distinct from, issues like #13254 or #12870.

Usually, Object.keys(foo) can’t just return Array<keyof typeof foo>, because typeof foo might not completely specify the fields that foo actually has.

But enum objects are created by the TS compiler and normal code isn’t allowed to add more fields to them. So the exact fields will be known by the compiler.

[RyanCavanaugh]

We’ve rejected like 30 PRs trying to change Object.keys(ABC) to return keyof typeof ABC. It’s not clear how this would work without some mechanism to traffic through the ABC-ness of the keys return value?