How do I add my own custom Javascript in Drupal 8?

 Save your custom script in themes/THEMENAME/js/script.js

 Declare your script in themes/THEMENAME/THEMENAME.libraries.yml:

 ts-corescripts:
   version: VERSION
   js:
     js/script.js: {}

 In THEMENAME.theme

 <?php
      /**
         * Add custom scripts
         */
        function THEMENAME_page_alter(&$page) {
          $page['#attached']['library'][] = 'thirstysix/ts-corescripts';
          //$page['#attached']['library'][] = 'core/jquery';
        }
    ?>