Adding script conditionally
To attach a script in Drupal 8 you need to define a library first in your modules *.libraries.yml (replace * with your modules machine name).
The file should look as follows:

libraryname:
  version: 1.x
  js:
    js/script.js: {}
  dependencies:
    - core/jquery


After that you have the possibilities to attach a script through a preprocess function or directly in the template with a twig function.
I believe you are trying to achieve the first. In that case you need to modify the render array, like in this example:

&lt;?php
function modulename_element_info_alter(array &amp;$types) {
  if (isset($types['table'])) {
    $types['table']['#attached']['library'][] = 'modulename/libraryname';
  }
}
?&gt;


If you want to append a script to every page depending if a custom setting is activated in your module, you should do as follows:

function modulename_page_attachments_alter(array &amp;$page) {
  if ($settingIsTrue) {
    $page['#attached']['library'][] = 'modulename/libraryname';
  }
}


You can read more about it in the manual.