Use getElementById in Javascript to iterate through table

var SelectedRowsCount = new Array();
var AllRowsCount = new Array();
var HeaderCheckBox = new Array();

function CountCheckedRowsByCol(detailTableName, colNum)
{
    AllRowsCount[colNum] = 0;
    SelectedRowsCount[colNum] = 0;
   
    var detailTable = document.getElementById(detailTableName);
   
    if (detailTable == null)
        return;

    var Rows = detailTable.getElementsByTagName("tr");

    if (colNum == null)
        colNum = 0;

    for (i = 0; i < Rows.length; i++)
    {
        var cols = Rows[i].getElementsByTagName("td");
       
        if (cols.length <= colNum)
            continue;
       
        var chks = cols[colNum].getElementsByTagName("input");
       
        if (chks.length == 0)
            continue;
        if (chks[0].type != "checkbox")
            continue;
       
        if (AllRowsCount[colNum] == 0)
        {
            if (HeaderCheckBox[colNum] == null)
                HeaderCheckBox[colNum] = chks[0];
        }
        else
        {
            if (chks[0].checked)
                SelectedRowsCount[colNum]++;
        }
       
        AllRowsCount[colNum]++;
    }
   
    AllRowsCount[colNum]--;
   
//    alert(AllRowsCount[colNum]);
   
    //CheckHeaderIfAllColChkBoxSelected(colNum);
}