js验证checkboxlist是否有选中的项

function Check() {
            var cbl = document.getElementById("<%= ddlSurveyCompanyName.ClientID %>");
            var cbs = cbl.document.getElementsByTagName("input");
            var b = false;
            for (var i = ; i < cbs.length; i++) {
                if (cbs[i].type == "checkbox" && cbs[i].checked) {
                    b = true;
                    return b;
                }
            }
            if (!b) {
                alert("请至少选择一项！");
                return false;

            }
        }