Populating a Texbox with ListBox multiple selected items using Javascript

function ddlChange() {
var ddl = document.getElementById('<%=DDL.ClientID%>');
var textBox = document.getElementById('<%=txtSelected.ClientID%>');
len = ddl.length;
textBox.value = "";
for (var j = 0; j < len; j++) {
if (ddl.options[j].selected) {
textBox.value += ddl.options[j].text + "\r\n";
}
}
}


<div>Available Items:</div>
<div>
<asp:ListBox ID="DDL" runat="server" SelectionMode="Multiple" onChange="ddlChange()" Height="170px">
</asp:ListBox>
</div>


<div>Selected Items:</div>
<div>
<asp:TextBox ID="txtSelected" runat="server" TextMode="MultiLine"
ReadOnly="true" Height="170px" Width="365px"></asp:TextBox>
</div>