我这个JavaScript程序主要是动态往表格"DataGrid"中添加单元格，可怎么不行呢？？

<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=gb2312">
    <title>DataGrid</title>
    <script language="JavaScript">
        var data = null;
        //var fields = null;
        var fields = new Array();
fields[0] = "编号";
fields[1] = "文件名";
fields[2] = "类型";
fields[3] = "时间";
fields[4] = "大小";
        var dg = null;
        function flush()
        {
var bdy = dg.firstChild
            if(bdy.hasChildNodes())
            {
                for(var i = 0; i < bdy.childNodes.length; i++)
                {
                    bdy.removeChild(bdy.childNodes[i]);
                }
            }
            if(fields != null)
            {
                var th = document.createElement("tr");
                for(var i = 0; i < fields.length; i++)
                {
                    var td = document.createElement("td");
                    var text = document.createTextNode(fields[i]);
                    td.appendChild(text);
                    th.appendChild(td);
                }
                bdy.appendChild(th);
            }
        }
        function init()
        {
            dg = document.getElementById("DataGrid");
            flush();
        }
    </script>
  </head>
  <body onload="init()">
    <table border="1" width="100%" id="DataGrid">
    </table>
  </body>
</html>