function changenext()
     {
       var select1=document.getElementById("selectcompany");
       var select2=document.getElementById("selectdept");
       var cobalt1dep=new Array("前台","經理室","國內市場部","行政部","資訊組","倉庫","塑膠部","裝備一部","裝備二部","品質部","採購部","PMC","工程部(樣板組)","工程部","工模部","生產總部","財務部","移噴部");
       var cobalt2dep=new Array("行政","工程","工程IE組","BOM","品質部","引線部","工程部","裝備三部","倉庫","喇叭","海棉");
       var hkdep=new Array("administration","acounting","sale&marketing","shipping","R&D","conference","國內廠專線電話");
      switch (select1.options[select1.selectedIndex].value)
      {
      case "高寶一廠":
        for (i=0;i<cobalt1dep.Length ;i++ )
        {
          alert(i);
         var o=new option(cobalt1dep[0],cobaltdep[0]);
          select2.appendChild(o);
        }
         
      case "高寶二廠":
           for (i2=0;i2<cobalt2dep.Length ;i2++ )
        {
          var e1=document.createElement("option");
          alert(e1);
          e1.text=cobalt1dep[i2];
          e1.value=cobalt1dep[i2];
          select2.appendChild(e1);
        }
      case "香港":
         
      }
      }

cobalt1dep.Length
Length小写
cobalt1dep.length

select2.appendChild(o);
用
select2.options.push(o)