Push state function - изменение урла при ajax запросе

pushstate.js

function pushState(){

//1 рабочий вариант
//    var form = $('#products_filter');
//    var params = form.serialize();
//    var url = document.products_filter.action+'?'+params;
//    if(url != window.location){
//        window.history.pushState(null, null, url);
//    }
//2 рабочий вариант
    var params = serializeFilter();
    var url = document.products_filter.action+'?'+params;
    if(url != window.location){
        window.history.pushState(null, null, url);
    }

}

function serializeFilter (){
    var fields = document.products_filter.elements;
    var field, name, value, type;
    var res = '';
    for(var z = 0; z < fields.length; z++){
        field = fields[z];
        name = field.name;
        value = field.value;
        type = field.type;
        if(typeof name == "undefined" || name == ""){continue;}
        if(type == 'checkbox' || type == 'radio'){
            if(field.checked){
                res += name +"="+ encodeURIComponent(value) +"&";
            }
            continue;
        }
        if(type == "select-multiple"){
            for(var so = 0; so < field.length; so++){
                if(field[so].selected){
                    res += name +"="+ encodeURIComponent(field[so].value) +"&";
                }
            }
            continue;
        }
        if(value){
            res += name +"="+ encodeURIComponent(value) +"&";
        }
    }
    return res.substring(0, res.length - 1);
};