selectedIndex 属性可设置或返回下拉列表中被选选项的索引号。
注释：若允许多重选择，则仅会返回第一个被选选项的索引号。
selectObject.selectedIndex=number
下面的例子可提示出被选选项的索引号：
<html>
<head>
<script type="text/javascript">
function getIndex()
{
var x=document.getElementById("mySelect")
alert(x.selectedIndex)
}
</script>
</head>
<body>
<form>
Select your favorite fruit:
<select id="mySelect">
<option>Apple</option>
<option>Orange</option>
<option>Pineapple</option>
<option>Banana</option>
</select>
<br /><br />
<input type="button" onclick="getIndex()"
value="Alert index of selected option">
</form>
</body>
</html>
<html>
<head>
<script type="text/javascript">
function insertBeforeSelected()
{
var x=document.getElementById("mySelect");
if (x.selectedIndex>=0)
{
var y=document.createElement(‘option‘);
y.text=‘Kiwi‘;
var sel=x.options[x.selectedIndex];
try
{
x.add(y,sel); // standards compliant
}
catch(ex)
{
x.add(y,x.selectedIndex); // IE only
}
}
}
</script>
</head>
<body>
<form>
<select id="mySelect">
<option>Apple</option>
<option>Pear</option>
<option>Banana</option>
<option>Orange</option>
</select>
<input type="button" onclick="insertBeforeSelected()" value="Insert option before selected" />
</form>
</body>
</html>
原文：http://www.cnblogs.com/jianxingjianyuan/p/6740683.html