Failing at getting parametised runtime picking in typescript

broken_pick.ts

// this doesn't work... I want to extract types from the process.env, and ensure they're required...!
// not an easy task it seems

function safeLoadEnv<T, K extends keyof T>(env: T, ...keys: Array<K>): Required<Pick<T, K>> {
  // extension of https://stackoverflow.com/a/47232883
  function pick(obj: T, ...keys: Array<K>): Pick<T, K> {
    const ret: Partial<Pick<T, K>> = {};
    const missing: any = [];
    keys.forEach(key => {
      if (!obj[key]) {
        missing.push(key);
      } else {
        ret[key] = obj[key]!;
      }
    });
    if (missing.length) {
      throw new Error(`missing keys ${JSON.stringify(missing)}`);
    }
    // cast, as we've guaranteed there's no optionals in the partial.
    return ret as Required<T>;
  }

  return pick(env, ...keys) as Required<Pick<T, K>>;
};