Adding a parameter to the URL with JavaScript

gistfile1.js

function addParam(url, param, value) {
      var a = document.createElement('a'), regex = /[?&]([^=]+)=([^&]*)/g;
      var params = {}, match, str = []; a.href = url; value=value||"";
      while (match = regex.exec(a.search))
            if (param != match[1]) str.push(match[1] + "=" + match[2]);
      str.push(encodeURIComponent(param) + "=" + encodeURIComponent(value));
      a.search = (a.search.substring(0,1) == "?" ? "" : "?") + str.join("&");
      return a.href;
}