32. Object Flattener

function flatten(obj, prefix = '') {
  let res = {};
  for (const k in obj) {
    const key = prefix ? `${prefix}.${k}` : k;
    if (typeof obj[k] === 'object') Object.assign(res, flatten(obj[k], key));
    else res[key] = obj[k];
  }
  return res;
}