type NonNullObject = {} & object;

function getDeepKeys(obj: NonNullObject) {
  let keys: string[] = [];

  for (const key in obj) {
    if (typeof obj[key] === 'object') {
      const subKeys = getDeepKeys(obj[key]);
      keys = keys.concat(subKeys.map((subKey) => `${key}.${subKey}`));
    } else {
      keys.push(key);
    }
  }

  return keys;
}