How to Obtain select the selected value?

<form action="#">
 <select id="select_test">
 <option value="1">VsCode</option>
  <option value="2">Atorm</option>
 </select>
 <input type="button" value="成功" οnclick="t1()">
 </form>
 <script>
 function t1(){
   var myselect=document.getElementById("select_test");
  var index=myselect.selectedIndex;
  alert(myselect.options[index].value)
 }

 </script>