In TypeScript, how to get the keys of an object type whose values are of a given type?

This can be done with conditional types and lookup types, like this:

type KeysMatching<T, V> = {[K in keyof T]-?: T[K] extends V ? K : never}[keyof T];

and then you pull out the keys whose properties match string like this:

const key: KeysMatching<Thing, string> = 'other'; // ERROR!
// '"other"' is not assignable to type '"id"'

In detail:

KeysMatching<Thing, string> ➡

{[K in keyof Thing]-?: Thing[K] extends string ? K : never}[keyof Thing] ➡

{ 
  id: string extends string ? 'id' : never; 
  price: number extends string ? 'number' : never;
  other: { stuff: boolean } extends string ? 'other' : never;
}['id'|'price'|'other'] ➡

{ id: 'id', price: never, other: never }['id' | 'price' | 'other'] ➡

'id' | never | never ➡

'id'

---

Note that what you were doing:

type SetNonStringToNever<T> = { [P in keyof T]: T[P] extends string ? T[P] : never };

was really just turning non-string property values into never property values. It wasn't touching the keys. Your Thing would become {id: string, price: never, other: never}. And the keys of that are the same as the keys of Thing. The main difference with that and KeysMatching is that you should be selecting keys, not values (so P and not T[P]).

Hope that helps. Good luck!