Is it possible to map object value types in TypeScript?

Question

For example, I have a function of the following form:

function wrap<T>(item: T): W<T> {
}
interface W<T> {
  item: T;
}

Is it possible to write function (mapValues), that will transform given object values by applying this wrap function and return properly typed object? I understand how to write the actual function (lodash's _.mapValues will work fine), but I don't know if it is possible to apply proper types.

Is it possible to write mapValues function so the following will compile?

interface Before {
  a: number;
  b: string;
}

interface After {
  a: W<number>;
  b: W<string>;
}

let obj1: Before = { a: 1, b: "2" };
let obj2: After = mapValues(obj1);

function typecheck_number(arg: number) {}
function typecheck_string(arg: string) {}
typecheck_number(obj2.a.item);
typecheck_string(obj2.b.item);

Answer 1

Here's one possible solution:

interface W<T> {
  item: T;
}
function mapValues<T>(obj: T): {[K in keyof T]: W<T[K]>} {
  // implementation
}

Unfortunately, similar recursive approach does not seem to be possible, but there is hope that it will be possible with type constraints from this proposal.

This link was very helpful in arriving to this solution: https://blog.mariusschulz.com/2016/05/31/type-queries-and-typeof-in-typescript