map-props.ts

type MapFn<T> = (x: T) => unknown;
type MapSchema<T> = { [K in keyof T]?: MapFn<T[K]> | MapSchema<T[K]> };
type MapReturnType<T, TMap extends MapSchema<T>> = {
  [K in keyof T]: TMap[K] extends Function
    ? ReturnType<Extract<TMap[K], MapFn<T[K]>>>
    : MapReturnType<T[K], TMap[K]>;
};

function isMapFn<T = any>(x: unknown): x is MapFn<T> {
  return typeof x === "function";
}

function getKeys<T, K extends keyof T>(o: T): K[] {
  return Object.keys(o) as any;
}

function mapProps<T, TMap extends MapSchema<T>>(
  object: T,
  mapSchema: TMap
): MapReturnType<T, TMap> {
  const result: any = {};
  const keys = getKeys(object);

  for (const key of keys) {
    const value = object[key];
    let mappedValue: unknown;

    if (mapSchema[key]) {
      const fnOrSchema = mapSchema[key];

      if (isMapFn(fnOrSchema)) {
        mappedValue = fnOrSchema(value);
      } else {
        mappedValue = mapProps(value, fnOrSchema);
      }
    } else {
      mappedValue = value;
    }

    if (mappedValue !== undefined) {
      result[key] = mappedValue;
    }
  }

  return result;
}