js 随机选择下拉列表项

function selRandom(){
   var objSelectd;
   var countOption;
   var indexSel;
   objSelectd = document.getElementById("id");
   countOption = objSelectd.length;
   indexSel = Math.floor(Math.random()*countOption+1);
   objSelectd.value = objSelectd.options[indexSel].value;
}