下拉菜单返回空错误

Question

当我运行Firefox错误控制台时，我在下拉菜单中收到此错误：ddmenuitem为null。除此之外，页面似乎运行良好。

我收到的错误就是说：ddmenuitem.style.visibility =＆＃39; visible＆＃39;;

   var timeout  = 500;
    var closetimer  = 0;
    var ddmenuitem  = 0;

    // open hidden layer
    function mopen(id)
    {   
        // cancel close timer
        mcancelclosetime();

        // close old layer
        if(ddmenuitem) ddmenuitem.style.visibility = 'hidden';

        // get new layer and show it
        ddmenuitem = document.getElementById(id);
        ddmenuitem.style.visibility = 'visible';

    }
    // close showed layer
    function mclose()
    {
        if(ddmenuitem) ddmenuitem.style.visibility = 'hidden';
    }

    // go close timer
    function mclosetime()
    {
        closetimer = window.setTimeout(mclose, timeout);
    }

    // cancel close timer
    function mcancelclosetime()
    {
        if(closetimer)
        {
            window.clearTimeout(closetimer);
            closetimer = null;
        }
    }

    // close layer when click-out
    document.onclick = mclose; 

Answer 1

没有id=m2的元素：<a href="photo_gallery_pg1.html" onmouseout="mclosetime()" onmouseover="mopen('m2')">