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;

            }

        }