I have written this code to add querystring to url

exports.addQueryString = function(url, queryString) {
  var isQuestionMarkPresent = url && url.indexOf('?') !== -1,
    separator = '';

  if (queryString) {
    separator = isQuestionMarkPresent ? '&' : '?';
    url += separator + queryString;
  }

  return url;
};

Is there any way I can write this better  ?

Usage: addQueryString('http://example.com', 'q=1&val=3&user=me');

I have written this code which adds a querystring to a URL:

exports.addQueryString = function(url, queryString) {
  var isQuestionMarkPresent = url && url.indexOf('?') !== -1,
    separator = '';

  if (queryString) {
    separator = isQuestionMarkPresent ? '&' : '?';
    url += separator + queryString;
  }

  return url;
};

Is there any way I can write this better?

Usage:

addQueryString('http://example.com', 'q=1&val=3&user=me');