In <code>self.previous_page</code> function we don't need <code>parseInt</code>, but in <code>self.next_page</code> we need, because <code>self.current_page</code> is a string, and when we subtract string and int we got int but when we add string and int we got string. eq:
'1' - 1 = 0 but '1' + 1 = 11