SOURCE

function alertIndex()
  {
  var x=document.getElementById("mySelect").selectedIndex;
  var y=document.getElementsByTagName("option");
  alert(y[x].text + " has the index of: " + y[x].index);
  }

<form>
   请选择您喜欢的水果： 
  <select id="mySelect"> <option>苹果</option> <option>桃子</option> <option>香蕉</option> <option>桔子</option> </select> 
  <br> 
  <br> 
  <input type="button" onclick="alertIndex()" value="显示被选水果的 index"> 
 </form>