用户控件问题？？？？ 谢谢各位大哥大姐了！

[zhuwenli]

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="UserReg.ascx.cs" Inherits="Web.asa.UserReg" %>
<script language="javascript" type="text/javascript">
    function CheckUsername(var uname)
    {
        alert(HH.CheckUsername(uname).value);
    }
</script>
<asp:Panel ID="Panel1" runat="server">
<table width="100%" border="0" cellpadding="10" cellspacing="0">
  <tr>
    <td width="25%" align="right">用户名</td>
    <td width="50%">
    <asp:TextBox ID="SM_USER" runat="server" Width="44%"></asp:TextBox>
        <input id="Button1" type="button" value="检查用户是否存在" onclick="CheckUsername(?????);" /></td>
    <td style="width: 25px"> </td>
  </tr>
</table>

<input id="Button1" type="button" value="检查用户是否存在" onclick="CheckUsername(?????);" />
我想获取<asp:TextBox ID="SM_USER" runat="server" Width="44%"></asp:TextBox>
里的参数直接传递到javascript的function:CheckUsername(var uname);中应该怎么写 谢谢各位大哥大姐了

[coodd]

document.getElementById("SM_USER").value