怎么用javascript取listbox的多选的值！

<%@ Page language="c#" Codebehind="w3.aspx.cs" AutoEventWireup="false" Inherits="stuinfo.w3" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<HTML>
<HEAD>
<title>w3</title>
<meta name="GENERATOR" Content="Microsoft Visual Studio .NET 7.1">
<meta name="CODE_LANGUAGE" Content="C#">
<meta name="vs_defaultClientScript" content="JavaScript">
<meta name="vs_targetSchema" content="http://schemas.microsoft.com/intellisense/ie5">
<script>
 function f1()
 {
   var obj=document.all.ListBox1
   for(var i=0;i<obj.length;i++)
    {
      if (obj[i].selected)
       {
         alert(obj[i].text);
       }
    }
 }
</script>
</HEAD>
<body MS_POSITIONING="GridLayout">
<form id="Form1" method="post" runat="server">
<asp:ListBox id="ListBox1" style="Z-INDEX: 101; LEFT: 352px; POSITION: absolute; TOP: 160px"
runat="server" Width="104px" SelectionMode="Multiple">
<asp:ListItem Value="aaaa">aaaa</asp:ListItem>
<asp:ListItem Value="bbbb">bbbb</asp:ListItem>
<asp:ListItem Value="cccc">cccc</asp:ListItem>
<asp:ListItem Value="dddd">dddd</asp:ListItem>
</asp:ListBox><INPUT style="Z-INDEX: 102; LEFT: 520px; WIDTH: 112px; POSITION: absolute; TOP: 184px; HEIGHT: 24px"
type="button" value="Button" onclick="f1()">
</form>
</body>
</HTML>