平成30/ 2018-12-17 10:54
html select multiple selectindex 複数
<script>
function alCoChg(){
    var opt = document.getElementById('al_co_id').options;
    for(var i=0; i<opt.length; i++){
        if(
        opt[i].selected
        ){
              alert(i);
        }
    }
}
</script>

(Thanks.)
http://phpjavascriptroom.com/?t=js&p=selectoption#a_select-multiple