var url = window.location.href;
if (url.indexOf('?') > -1){
   url += '&param=1'
}else{
   url += '?param=1'
}
window.location.href = url;

window.location.search += '&param=42';

location.href = location.href + "&parameter=" + value;

https://stackoverflow.com/?&param=1&param=1&param=1&param=1&param=1&param=1&param=1&param=1&param=1

function URL_add_parameter(url, param, value){
    var hash       = {};
    var parser     = document.createElement('a');

    parser.href    = url;

    var parameters = parser.search.split(/?|&/);

    for(var i=0; i < parameters.length; i++) {
        if(!parameters[i])
            continue;

        var ary      = parameters[i].split('=');
        hash[ary[0]] = ary[1];
    }

    hash[param] = value;

    var list = [];
    Object.keys(hash).forEach(function (key) {
        list.push(key + '=' + hash[key]);
    });

    parser.search = '?' + list.join('&');
    return parser.href;
}

location.href = URL_add_parameter(location.href, 'param', 'value');

function gotoItem( item ){
    var url = window.location.href;
    var separator = (url.indexOf('?') > -1) ? "&" : "?";
    var qs = "item=" + encodeURIComponent(item);
    window.location.href = url + separator + qs;
}

function gotoItem( item ){
    var url = window.location.href;
    url += (url.indexOf('?') > -1)?"&":"?" + "item=" + encodeURIComponent(item);
    window.location.href = url;
}

location.href = location.href + "&parameter=" + value;