JavaScript function to update/replace the value in a URL query string parameter; or insert the key/value if it doesn't exist.

/**
 * Update URL Query String
 *
 * Updates the URL query string with the specified parameter value.
 * If the key doesn't exist in the url, a new one will be created.
 *
 * @param {String} key  The query string key to search for in the specified url parameter.
 * @param {String|Number} value  The value to replace in the existing query string.
 * @param {String} url A url containing the query string key to update, or
 *     insert if one doesn't currently exist. If the url parameter is not
 *     provided, the browser url (window.location.href) will be used.
 *
 * @return {String}  The updated url and query string.
 */
function updateUrlQueryString(key, value, url) {
    value = encodeURIComponent(value);

    var _url = url || window.location.href;
    var re = new RegExp("([?&])" + key + "=.*?(&|#|$)(.*)", "gi"),
        hash;

    if (re.test(_url)) {
        if (typeof value !== "undefined" && value !== null)
            return _url.replace(re, "$1" + key + "=" + value + "$2$3");
        else {
            hash = _url.split("#");
            _url = hash[0].replace(re, "$1$3").replace(/(&|\?)$/, "");
            if (typeof hash[1] !== "undefined" && hash[1] !== null) {
                _url += "#" + hash[1];
            }
            return _url;
        }
    } else {
        if (typeof value !== "undefined" && value !== null) {
            var separator = _url.indexOf("?") !== -1 ? "&" : "?";
            hash = _url.split("#");
            _url = hash[0] + separator + key + "=" + value;
            if (typeof hash[1] !== "undefined" && hash[1] !== null) {
                _url += "#" + hash[1];
            }
            return _url;
        } else return _url;
    }
}

/**
 * Gets URL Query String Parameter Value by Name
 *
 * Gets the value for the specified URL query string parameter name.
 *
 * @param {String} parameterName  The name of the url query string parameter to search for.
 * @param {String} url  The url to search. If the url parameter is not provided,
 *     the browser url (window.location.href) will be used.
 *
 * @return {String} The value found in the specified url for the matched
 *     parameterName, or null if not found.
 */
function getUrlQueryStringParameter(parameterName, url) {
    var _url = url || window.location.href;
    parameterName = parameterName.replace(/[\[\]]/g, "\\$&");
    var regex = new RegExp("[?&]" + parameterName + "(=([^&#]*)|&|#|$)", "i"),
        results = regex.exec(_url);
    if (!results) return null;
    if (!results[2]) return "";
    return decodeURIComponent(results[2].replace(/\+/g, " "));
}