(function ($, Drupal) {
    Drupal.behaviors.promotionalOverlayBlock = {
        attach: function(context, settings) {
            console.log("hello world")
        }
    };
})(jQuery, Drupal);

mymodule/js/overlayblock.js

mymodule:
  js:
    js/overlayblock.js: {}
  dependencies:
    - core/jquery
    - core/jquery.once
    - core/drupalSettings