computer()
...
  // no good move found - make id_loser move & lose
  do_x(id_loser);
  check();
  show_result();
  g_message_el.textContent='you win';
}
square_clicked()
function square_clicked(el) {
  if (g_player && g_squares[el.id]==0) {
    g_player=false;
    do_x(el.id);
    if (check()) {
      show_result();
      g_message_el.textContent='computer wins';
    } else {
      setTimeout(computer,2000);
    }
  }
}