Check gridview header checkbox if all item checkbox are checked

<script language="javascript" type="text/javascript">
<!--
        
        function ChangeCheckBoxState(id, checkState) {
            var cb = document.getElementById(id);
            if (cb != null)
                cb.checked = checkState;
        }

        function ChangeAllCheckBoxStates(checkState) {
            // Toggles through all of the checkboxes defined in the CheckBoxIDs array
            // and updates their value to the checkState input parameter
            if (CheckBoxIDs != null) {
                for (var i = 0; i < CheckBoxIDs.length; i++)
                    ChangeCheckBoxState(CheckBoxIDs[i], checkState);
            }
        }

        function ChangeHeaderAsNeeded() {
            // Whenever a checkbox in the GridView is toggled, we need to
            // check the Header checkbox if ALL of the GridView checkboxes are
            // checked, and uncheck it otherwise
            if (CheckBoxIDs != null) {
                // check to see if all other checkboxes are checked
                for (var i = 1; i < CheckBoxIDs.length; i++) {
                    var cb = document.getElementById(CheckBoxIDs[i]);
                    if (!cb.checked) {
                        // Whoops, there is an unchecked checkbox, make sure
                        // that the header checkbox is unchecked
                        ChangeCheckBoxState(CheckBoxIDs[0], false);
                        return;
                    }
                }

                // If we reach here, ALL GridView checkboxes are checked
                ChangeCheckBoxState(CheckBoxIDs[0], true);
            }
        }

-->
    </script> 


protected void grdStudents_DataBound(object sender, EventArgs e)
        {
            if (grdStudents.Rows.Count > 0)
            {
                CheckBox cbHeader = (CheckBox)grdStudents.HeaderRow.FindControl("HeaderLevelCheckBox");

                cbHeader.Attributes["onclick"] = "ChangeAllCheckBoxStates(this.checked);";

                List<String> ArrayValues = new List<string>();
                ArrayValues.Add(String.Concat("'", cbHeader.ClientID, "'"));

                foreach (GridViewRow gvr in grdStudents.Rows)
                {
                    CheckBox cb = (CheckBox)gvr.FindControl("chkSelect");
                    cb.Attributes["onclick"] = "ChangeHeaderAsNeeded();";
                    ArrayValues.Add(String.Concat("'", cb.ClientID, "'"));
                }

                CheckBoxIDsArray.Text = @"<script type=""text/javascript"">" + "\r\n" +
                            "<!--" + "\r\n" +
                            String.Concat("var CheckBoxIDs =  new Array(", String.Join(",", ArrayValues.ToArray()), ");") + "\r\n" +
                            "// -->" + "\r\n" +
                            "</script>";
            }
        }