关于下面的JS的问题


function order_UpItem() {
        document.forms[1].changed.value=1;
        UpItem(document.forms[1].idValues);
    } 
function UpItem(selectBox)
{
var j;
var sObj;
    var len=selectBox.length-1;
for (j=0;j<=len;j++)
      {        if (selectBox.options[j].selected)
             {
             if (j <= 0)
return;
                 sObj=document.createElement("option");
     sObj.text = selectBox.options[j].text;// selectBox.options是什么意思
     sObj.value = selectBox.options[j].value;//
       if (sObj.value!="")
                     {
                         selectBox.options[j]=null;
              selectBox.add(sObj,j-1);// selectBox.add什么意思
              selectBox.options[j-1].selected=1;
                          j-=1;
             }
     }
       }
}