判断checkbox是否被选中

用javascript实现判断checkbox是否被选中
HTML代码如下：
       <input type=checkbox name='nID' value='1'>
       <input type=checkbox name='nID' value='2'>
       <input type=checkbox name='nID' value='3'>

javascript如下：      
        var len = document.OrdersForm.nID.length;
        var checked = false;

        for (i = 0; i < len; i++)
        {
            if (document.OrdersForm.nID[i].checked == true)
            {
                checked = true;
                break;
            }
        }
        if (!checked)
        {
            alert("请至少选择一个订单！");
            return;
        }