serialize form

function serialize(form) {
    if (!(form instanceof HTMLFormElement)) {
        return;
    }
    var q = [];
    function push(k, v) {
        q.push(encodeURIComponent(k) + '=' + encodeURIComponent(v));
    }
    for (var i = 0, el, j; i < form.elements.length; ++i) {
        el = form.elements[i];
        if (!el.name || (el.tagName == 'INPUT' && (el.type == 'file' || ((el.type == 'radio' || el.type == 'checkbox') && !el.checked)))) {
            continue;
        }
        if (el.tagName == 'SELECT') {
            if (el.multiple) {
               for (j = 0; j < el.options.length; ++j) {
                    if (el.options[j].selected) {
                        push(el.name, el.options[j].value);
                    }
                }
            }
            else {
                push(el.name, el.options[el.selectedIndex].value);
            }
        }
        else {
            push(el.name, el.value);
        }
    }
    return q.join('&');
}