An example of enqueuing custom JavaScript (and JavaScript libraries) to add frontend functionality to blocks. The code would go into your functions.php file.

custom-block-js-functions.php

<?php

/**
 * Enqueue custom block JavaScript on the frontend only
 */
function mytheme_frontend_block_scripts() {

  // Enqueue fitvids JS library from CDN for responsive video embeds
  wp_enqueue_script( 'fitvids', 'https://cdnjs.cloudflare.com/ajax/libs/fitvids/1.2.0/jquery.fitvids.min.js', array('jquery') );

  // Enqueue swipebox JS library from CDN for lightbox galleries
  wp_enqueue_script( 'swipebox', 'https://cdnjs.cloudflare.com/ajax/libs/jquery.swipebox/1.4.4/js/jquery.swipebox.min.js', array('jquery') );

  // Enqueue necessary swipebox CSS styles from CDN
  wp_enqueue_style( 'swipebox', 'https://cdnjs.cloudflare.com/ajax/libs/jquery.swipebox/1.4.4/css/swipebox.min.css' );

  // Enqueue our theme custom block JS file and add necessary dependencies
  wp_enqueue_script( 'mytheme-blocks-frontend', get_template_directory_uri() . '/js/frontend.blocks.js', array('jquery','swipebox', 'fitvids'), time() );

}
add_action( 'wp_enqueue_scripts', 'mytheme_frontend_block_scripts' );