Gitako/packages/pjax/lib/util/update-query-string.js

module.exports = function(uri, key, value) {
  var re = new RegExp("([?&])" + key + "=.*?(&|$)", "i")
  var separator = uri.indexOf("?") !== -1 ? "&" : "?"
  if (uri.match(re)) {
    return uri.replace(re, "$1" + key + "=" + value + "$2")
  }
  else {
    return uri + separator + key + "=" + value
  }
}