点击按钮，表格增加/删除一行

<table width="100%" border="1" id="tb">
<tr>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</table>
<p>
<script>
/*
操作表格,为表格添加行,删除行操作
*/
function delrow(){
var i=tb.rows.length;
tb.deleteRow(i-1);
}
function addrow(){
    var tr = document.createElement('tr');
    var cellsNum = tb.rows[0].cells.length;
for(var j = 0 ; j < cellsNum ; j++){
        var td = document.createElement('td');
        td.innerHTML='test';
        tr.appendChild(td);
    }
    tb.tBodies[0].appendChild(tr);
}
</script>
<input type="submit" name="Submit" value="删除" onClick="delrow();">
<input type="submit" name="Submit" value="添加" onClick="addrow();">