如何利用javascript获取表单中select下拉列表中所选中项的值value

1.html代码如下:
<html>
<head>
</head>
<body>
<form name="form1" >
Select your favorite fruit:
<select >
<option value="11">Apple</option>
<option value="22" >Orange</option>
<option value="33">Pineapple</option>
<option value="44">Banana</option>
</select>
<br /><br />
<input type="button" onclick="getIndex()"
value="Alert index of selected option">
</form>
</body>
</html>

 2.javascript代码如下:

<script type="text/javascript">
function getIndex()
{
  //从document对象中,获取select标签
  var a=document.getElementById("mySelect");
  //select标签获取的值其实是一个数组--a.options[]; 然后,选定项的下标是--a.selectedIndex
   var b=a.options[a.selectedIndex].value;   
   alert(b); 
} 
</script>