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>