getQueryString.js

function getQueryString() {
  var url = arguments.length > 0 && arguments[0] !== undefined ?
    arguments[0] :
    window.location.search;

  var queryString = url.indexOf('?') !== -1 ?
    url.split('?')[1] :
    url;

  var queries = queryString.indexOf('&') !== -1 ?
    queryString.split('&') :
    [queryString];

  var params = {};

  for (let i = 0; i < queries.length; i += 1) {
    var element = queries[i].indexOf('=') !== -1 ?
      queries[i].split('=') :
      [queries[i], null];
    params[decodeURIComponent(element[0])] = decodeURIComponent(element[1] || '');
  }

  return params;
}

setQueryString.js

function setQueryString(params) {
  return '?' + Object
    .keys(params)
    .map(function (key) {
      return key + '=' + encodeURIComponent(params[key])
    })
    .join('&');
}