javascript的求和问题

<script language="JavaScript">
<!--
function doSum(){
var tb=document.getElementById("tbSum");
var sumrow=0;
for (var i=0;i<6;i++){
for(var j=0;j<6;j++){
var tmpVal=parseInt(tb.rows[i].cells[j].children[0].value);
sumrow += isNaN(tmpVal)?0:tmpVal;
}
tb.rows[i].cells[6].children[0].value = sumrow;
sumrow=0;
}
var sumcell=0;
for (var i=0;i<7;i++){
for(var j=0;j<6;j++){
var tmpVal=parseInt(tb.rows[j].cells[i].children[0].value);
sumcell += isNaN(tmpVal)?0:tmpVal;
}
tb.rows[6].cells[i].children[0].value = sumcell;
sumcell=0;
}
}
document.write('<table width="360" cellspacing="0" cellpadding="0" id="tbSum">');
for (var i=0;i<7 ;i++){
document.write('<tr>');
for(var j=0;j<7;j++){
if(i!=6 && j!=6)
document.write('<td width="60"><input type="text" style="width:100%;text-align:right" value="'+i+j+'"></td>');
else
document.write('<td width="60"><input type="text" style="width:100%;text-align:right;background-color:#DDDDDD" readonly></td>');
}
document.write('</tr>');
}

document.write('</table><br><input type="button" value="求和" onclick="doSum()">');
//-->
</script>