Js检测单选框是否选中

/**
* 修改监管频度
*/
function modSF() {
        var radios = document.getElementsByName("sfId");
        var id = "";
        var flag = 0;
        for (var k = 0; k < radios.length; k++) {
                if (radios[k].checked) {
                        id = radios[k].value;
                        flag = 1;
                        var retValue = window.showModalDialog(rootPath + "/addSFOrUpdateUI.do?sfId=" + id, "", 'resizable:no;status:no;scroll:no;dialogWidth:' + 600 + 'px;DialogHeight=' + 250 + 'px');
                        if (retValue != "undefined") {
                                doSearch();
                        }
                }
        }
        if (!flag) { //不在for循环内
                alert("请选中要修改的内容");
                return false;
        }
}