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).options
        for (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).options
        for (var i = 0; i < o.length; i++) {
            if (i == index){
                selectVal=o[i].value;
                o[i].selected == true;
            }
        }
        document.getElementById(idStr).value=selectVal;
    }