Object.keys return string[] instead of keyof type

[chriskrycho]

TypeScript Version: 2.6.2 (but tsc isn't the problem here; the typings are).

Code

function getValue<T, K extends keyof T>(obj: T, a: K): T[K] {
  return obj[a];
}

const foo = { a: true };
const keys = Object.keys(foo);
getValue(foo, keys[0]);

Expected behavior:

This should type-check!

Actual behavior:

Argument of type 'string' is not assignable to parameter of type '"a"'.

The obvious solution here is to make the type definition for Object.keys return keyof property, and I'm happy to submit that small PR to the appropriate lib files:

// within the Object type definition
keys<T, K extends keyof T>(o: T): K[];

This automatically works everywhere that Object.keys works today because (keyof T)[] is always a proper subtype of string[].

This resolves it nicely. However, I'm unsure what the relationship between the versions of the typings in lib here and TS versions is – do they need to be backwards compatible to previous versions of TS? Otherwise, as noted, I'll happily open that PR.

[jwbay]

See #20503 for the most recent request for the same.

[chriskrycho]

My search-fu is the worst.