笨笨的问题：listbox双击鼠标事件报错？ 何解？

Question

我从一个listbox里双击某一项添加到另外一个listbox里，同时这边除去那一项。 在选到项时还好，就是如果没选中项直接双击的话，就会报页面错误说是“发生意外”我的js是这样的：
//雙擊鼠標添加email
        function addDbclick()
        {   
            var addOption=document.createElement("option");
            var index;
            if(document.all("emailaddr").length==0)
            {
                alert("無傚選項");
                return(false);
            }else
            {
                index=document.all("emailaddr").selectedIndex;
            }
            addOption.text=document.all("emailaddr").options(index).text;
            addOption.value=document.all("emailaddr").options(index).value;
            
            document.all("outselect").add(new Option(addOption.text,addOption.value));
            if(!(index<0))
            {
                document.all("emailaddr").options.remove(index);
            }
        }请问怎么解决这个问题？ 我对js不熟

Answer 1

Maybe you should check whether the element be null before use it.
BTW, I'm not familiar with js either.

Answer 2

要先判断listbox中是否有项目或是否选中了项目，否则退出程序

Answer 3

function addDbclick()
        {   
            if(document.all("emailaddr").value==null||document.all("emailaddr").value=="")
         {
             return false;
         }
            var addOption=document.createElement("option");
            var index;
            if(document.all("emailaddr").length==0)
            {
                alert("無傚選項");
                return(false);
            }else
            {
                index=document.all("emailaddr").selectedIndex;
            }
            addOption.text=document.all("emailaddr").options(index).text;
            addOption.value=document.all("emailaddr").options(index).value;
            
            document.all("outselect").add(new Option(addOption.text,addOption.value));
            if(!(index<0))
            {
                document.all("emailaddr").options.remove(index);
            }
        }