<!-- BEGIN COPY / PASTE -->
    // Enqueue block editor assets
    function my_custom_block_enqueue() {
      wp_enqueue_script(
        'my-custom-block',
        get_template_directory_uri() . '/js/my-custom-block.js',
        array('wp-blocks', 'wp-element', 'wp-editor'),
        filemtime(get_template_directory() . '/js/my-custom-block.js')
      );
    }
    add_action('enqueue_block_editor_assets', 'my_custom_block_enqueue');

    // JavaScript file: my-custom-block.js
    wp.blocks.registerBlockType('my-plugin/my-custom-block', {
      title: 'My Custom Block',
      icon: 'smiley',
      category: 'common',
      edit: function() {
        return wp.element.createElement('p', null, 'Hello from the editor!');
      },
      save: function() {
        return wp.element.createElement('p', null, 'Hello from the saved content!');
      },
    });
    <!-- END COPY / PASTE -->