JS code doesn't work

Hi! I have a problem with JC code. So, I'm including JavaScript Code with Load JS in Events, but this code work only when I click:"Test Form", but doesn't work in page in joomla. When I looked in source pages I saw this same JS code in test page and in site. This code:

var checkbox_id, textbox_id, checkbox, textbox, div;
/* edit the next two lines to match the ids of the elements in your form */
checkbox_id = 'checkbox';
textbox_id = 'wlasna_modlitwa';

window.addEvent('domready', function() {
  checkbox = $(checkbox_id);
  textbox = $(textbox_id);
  div = $(textbox_id+'_container_div');
  div.dissolve();
  showHide();
  checkbox.addEvent('click', showHide);
});

function showHide() {
  if ( checkbox.checked ) {
    div.reveal();
    textbox.disabled = false;
  } else {
    div.dissolve();
    textbox.value = '';
    textbox.disabled = true;
  }
}		

Hi wiciu2.0,

It sounds as if there is a conflict with some other script on your page. Please use your browser web develop tools to check the JavaScript Console for error messages.

Bob