Drupal javascript behavior example

script.js

(function (Drupal, drupalSettings, once) {
  
  Drupal.behaviors.myBehavior = {
    attach: function (context, settings) {
      // Using once() to apply the myCustomBehaviour effect when you want to run just one function.
      once('myCustomBehavior', 'input.myCustomBehavior', context).forEach(function (element) {
        element.classList.add('processed');
      });
    }
  };
  
} (Drupal, drupalSettings, once));

THEME.libraries.yml

foobar:
  js:
    js/foobar.js: {}
  dependencies:
    - core/drupal
    - core/jquery
    - core/once