原生js获取下拉框选中值的下标

 //获取选中值的下标 function getselectscheckitemindex (idStr) {var o = document.getElementById(idStr).optionsfor (var i = 0; i < o.length; i++) {if (o[i].selected == true)return i}}//根据id和下标控制选中某个值function selectedByIndex (idStr,index) {var selectVal="";var o = document.getElementById(idStr).optionsfor (var i = 0; i < o.length; i++) {if (i == index){selectVal=o[i].value;o[i].selected == true;}}document.getElementById(idStr).value=selectVal;}