<script type="text/javascript">
    function enter(obj) {
        var tr = obj.parentNode.parentNode;
        var cellIndex = obj.parentNode.cellIndex;
        var rowIndex = obj.parentNode.parentNode.rowIndex;
        if (event.keyCode == 37) //Left
        {
            if (cellIndex > 0)
                tr.parentNode.rows[rowIndex].cells[cellIndex - 1].getElementsByTagName('INPUT')[0].focus();
        }
        if (event.keyCode == 38) //Up
        {
            if (rowIndex > 1)
                tr.parentNode.rows[rowIndex - 1].cells[cellIndex].getElementsByTagName('INPUT')[0].focus();
        }
        if (event.keyCode == 39) //Right
        {
            if (cellIndex < tr.cells.length - 1)
                tr.parentNode.rows[rowIndex].cells[cellIndex + 1].getElementsByTagName('INPUT')[0].focus();
        }
        if (event.keyCode == 40) //Down
        {
            if (rowIndex < tr.parentNode.rows.length - 1)
                tr.parentNode.rows[rowIndex + 1].cells[cellIndex].getElementsByTagName('INPUT')[0].focus();
        }
    };

    var prm = Sys.WebForms.PageRequestManager.getInstance();
    if (prm != null) {
        prm.add_endRequest(function (sender, e) {
            if (sender._postBackSettings.panelsToUpdate != null) {
                var rowIndex = parseInt(document.getElementById("ContentPlaceHolder1_hfRowIndex").value);
                var row = document.getElementById("TeachersCommentsGrd").rows[rowIndex + 1];
                if (typeof (row) != "undefined") {
                    var celIndex = parseInt(document.getElementById("hfCellIndex").value);

                    var checkbox = row.cells[0].getElementsByTagName("INPUT");
                    if (!checkbox[0].checked) {
                        checkbox[0].checked = true;
                    }

                    var controls = row.cells[celIndex].getElementsByTagName("INPUT");
                    for (var i = 0; i < controls.length; i++) {
                        switch (controls[i].className) {
                            case "TeacherComments":
                                controls[i].focus();
                                return;
                            default:
                        }
                    }
                }
            }
        });
    }
</script>

 Error when i reach here

var row = document.getElementById("TeachersCommentsGrd").rows[rowIndex + 1];

and also another error when i reach here

var celIndex = parseInt(document.getElementById("hfCellIndex").value);