updatequerystringparam.js

 var UpdateQueryStringParameter = 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;
    }
  };

https://gist.github.com/sgreenfield/2802070 <-- another good one you can use to make improvements to yours