function parseQuery(queryString) {
  var query = {};
  var pairs = (queryString[0] === '?' ? queryString.substr(1) : queryString).split('&');
  for (var i = 0; i < pairs.length; i++) {
    var pair = pairs[i].split('=');
    query[decodeURIComponent(pair[0])] = decodeURIComponent(pair[1] || '');
  }
  return query;
}

function put(obj, keyParts, value) {
  const key = keyParts.shift();
  if (keyParts.length === 0) {
    obj[key] = value;
  } else {
    obj[key] = obj[key] || {};
    put(obj[key], keyParts, value);
  }
}

function putAll(obj, query) {
  Object.keys(query).forEach((key) => {
    if (query[key]) {
      put(obj, key.split('.'), query[key])
    }
  });
  return obj;
}

const input = "db.host=&db.user=root&db.pass=1234";

const query = parseQuery(input);
// query = { 'db.host': '', 'db.user': 'root', 'db.pass': '1234' }

const result = putAll({}, query);
// result = { db: { user: 'root', pass: '1234' } }

console.log(result);