checkbox 复选框不能为空

<script type="text/javascript">
function check_check(checkname){
var flag;
var checks = document.getElementsByName(checkname);
for(var i=0;i<checks.length;i++){
if(checks[i].checked==true){
flag = true;
return true;
break;
}
}
if(!flag){
//alert("请至少选择一项");
return false;
}
}
</script>