javascript function for checking checkbox

function SelectAll(id) {
            //get reference of GridView control
            var grid = document.getElementById("<%= this.myGridView.ClientID %>");
            //variable to contain the cell of the grid
            var cell;

            if (grid.rows.length > 0) {
                //loop starts from 1. rows[0] points to the header.
                for (i = 1; i < grid.rows.length; i++) {
                    //get the reference of first column
                    cell = grid.rows[i].cells[0];

                    //loop according to the number of childNodes in the cell
                    for (j = 0; j < cell.childNodes.length; j++) {
                        //if childNode type is CheckBox
                        if (cell.childNodes[j].type == "checkbox") {
                            //assign the status of the Select All checkbox to the cell checkbox within the grid
                            //                            cell.childNodes[j].checked = document.getElementById(id).checked;
                            if (!cell.childNodes[j].checked) {
                                alert("Please check in serial order");
                            }
                        }
                    }
                }
            }
        }