Record mapping as a map function (e.g. `[P in keyof T]: Transform<T[P]>`)

objMap.ts

export function objMap<T extends Record<string, any>, U>(
  template: T,
  eachKey: <P extends Extract<keyof T, string>>(name: P, value: T[P]) => U,
): {
  [P in Extract<keyof T, string>]: U;
} {
  // @ts-ignore
  return Object.fromEntries(
    Object.entries(template)
      .filter(([name]) => typeof name === "string")
      .map(([name, value]) => {
        // @ts-ignore
        return [name, eachKey(name, value)];
      }),
  );
}

objMapWithProxy.ts

export function objMapWithProxy<T extends Record<string, any>, U>(
  template: T,
  eachKey: <P extends Extract<keyof T, string>>(name: P, value: T[P]) => U,
): {
  [P in keyof T]: U;
} {
  return new Proxy(template, {
    get(target, prop, receiver) {
      type P = Extract<keyof T, string>;
      if (typeof prop === "string") {
        return eachKey(prop as P, Reflect.get(target, prop, receiver) as T[P]);
      }
      return undefined;
    },
  });
}