大家帮忙调试，怎么解决？

<html>
<head><title>test</title></head>
<body>
<script language="javascript">
            function OnMouseDown(obj)
            {
//           你传过来的是一个对象，
               var oobj=eval("document.all."+obj.id);
// 在这里我们需要的是它的ID号。               alert(oobj.rows[0].cells[0].innerText);
            }
</script>
<table id="tab1" onmousedown="OnMouseDown(this)"  style="Z-INDEX: 103; LEFT: 176px; WIDTH: 384px; POSITION: absolute; TOP: 112px; HEIGHT: 348px">
<TR>
<TD>test1</TD>
<TD>test2</TD>
<TD>test3</TD>
<TD>test4</TD>
</TR>
<TR>
<TD>test1</TD>
<TD>test2</TD>
<TD>test3</TD>
<TD>test4</TD>
</TR>
<TR>
<TD>test1</TD>
<TD>test2</TD>
<TD>test3</TD>
<TD>test4</TD>
</TR>
<TR>
<TD colSpan="4">
<TABLE id="tmp1" onmousedown="OnMouseDown(this)" style="WIDTH: 376px; HEIGHT: 147px">
<TR>
<TD>test11</TD>
<TD>test21</TD>
<TD>test31</TD>
<TD>test41</TD>
</TR>
<TR>
<TD>test12</TD>
<TD>test22</TD>
<TD>test32</TD>
<TD>test42</TD>
</TR>
<TR>
<TD>test13</TD>
<TD>test23</TD>
<TD>test33</TD>
<TD>test43</TD>
</TR>
</TABLE>
</TD>
</TR>
<TR>
<TD>test1</TD>
<TD>test2</TD>
<TD>test3</TD>
<TD>test4</TD>
</TR>
</table>
</body>
</HTML>