Thinking about single file Drupal frontend components... Edit: Now live at https://drupal.org/project/sfc !

DrupalComponents.php

<?php

class SayHello implements ComponentInterface {

  const name = 'sayhello';

  const template = <<<TWIG
  {% embed "@components/bigtext" %}
    {% block content Hello {{ name }}! %}
  {% endembed %}
TWIG;

  function preProcess($variables) {
    if (!isset($variables['name'])) {
      $variables['name'] = \Drupal::currentUser()->getDisplayName();
    }
  }

}

class BigText implements ComponentInterface {

  const name = 'bigtext';

  const template = <<<TWIG
<p class="component-big-text">{% block content %}{% endblock %}</p>
TWIG;

  // Use inline library definitions to make development easier.
  // Paths are relative to file.
  const library = [
    'css' => [
      'theme' => [
        'big-text.css' => [],
      ],
    ],
    'dependencies' => [
      'drupal/core',
      'drupal/jquery',
    ],
  ];

  // Or maybe do everything inline?
  const dependencies = [
    'drupal/core',
    'drupal/jquery',
  ];

  const css = <<<CSS
.component-big-text { font-size: 100px; }
CSS;

  const js = <<<JS
(function($, Drupal) {
  Drupal.behaviors.copyFieldValue = {
    attach: function attach(context) {
      $('.component-big-text', context).once(function () {
        $(this).on('click', function () {});
      });
    }
  };
})(jQuery, Drupal);
JS;

}