Javacript: Set or Update a URL/QueryString Parameter, and update URL using HTML history.replaceState()

gistfile1.js

// Explicitly save/update a url parameter using HTML5's replaceState().
var updateQueryStringParam = function (key, value) {
    var baseUrl = [location.protocol, '//', location.host, location.pathname].join(''),
        urlQueryString = document.location.search,
        newParam = key + '=' + value,
        params = '?' + newParam;

    // If the "search" string exists, then build params from it
    if (urlQueryString) {
        keyRegex = new RegExp('([\?&])' + key + '[^&]*');
        // If param exists already, update it. If value is undefined - unset it
        var match = urlQueryString.match(keyRegex);
        if (match !== null) {
            params = urlQueryString.replace(keyRegex, value == undefined ? (match[1] == '?' ? '?' : ''): "$1" + newParam);
        } else { // Otherwise, add it to end of query string
            params = urlQueryString + (value != undefined ? '&' + newParam : '');
        }
    }
    params = params.replace('?&', '?');
    window.history.replaceState({}, "", baseUrl + params);
};