Given we have a generic indexable type how can I retrieve only the types of its values to be able to narrow down to only some of the keys?
// imagine check is a function and its second argument only allows the property `a` as it's string and raise an error if 'b' is passed
const test2 = check({ a: 'string', b: 22 }, 'b'); // error only 'a' is allowed
const test2 = check({ a: 'str', b: 'str2' }, 'b'); // ok returns 'str2'
Sure, you can do this by using conditional and mapped types to extract just the keys of an object type T whose values match a value type V:
type KeysMatching<T, V> = { [K in keyof T]: T[K] extends V ? K : never }[keyof T];
declare function check<T>(obj: T, k: KeysMatching<T, string>): string;
const test1 = check({ a: 'string', b: 22 }, 'b'); // error
const test2 = check({ a: 'str', b: 'str2' }, 'b'); // ok
Hope that helps. Good luck!
