获得页面的复选框的值

var box = document.getElementsByName("checkBoxs");
var packPolicyRelationId = '';
var isChecked = false;
for (var i=0;i<box.length;i++ )
{
    if(box[i].checked)
    {
    isChecked = true;
packPolicyRelationId=packPolicyRelationId + box[i].value + ',';
}
}
if (!isChecked)
{
alert("请至少选择一项进行修改");
return;
}