extensions/theme-extension/
├── blocks/                    # App blocks (merchant can position)
│   ├── feature-block.liquid   # Main feature block
│   └── badge.liquid           # Small badge/snippet
├── snippets/                  # Reusable Liquid snippets
│   └── component.liquid       # Shared components
├── assets/                    # JS/CSS files
│   ├── app-embed.js           # Minimal JS (forms, interactions)
│   └── styles.css             # Scoped styles
└── locales/
    └── en.default.json        # Translations

{% schema %}
{
  "name": "Feature Name",
  "target": "section",
  "enabled_by_default": false,
  "settings": [
    {
      "type": "range",
      "id": "items_per_page",
      "label": "Items per page",
      "min": 3,
      "max": 20,
      "default": 10
    },
    {
      "type": "color",
      "id": "primary_color",
      "label": "Primary color",
      "default": "#000000"
    }
  ]
}
{% endschema %}

{% comment %} Block content {% endcomment %}
<div class="app-feature" data-resource-id="{{ product.id }}">
  {% render 'app-component', resource: product, settings: block.settings %}
</div>

{% stylesheet %}
  .app-feature { /* scoped styles */ }
{% endstylesheet %}

{% javascript %}
  // Minimal JS only when needed
{% endjavascript %}

{% schema %}
{
  "name": "App Embed",
  "target": "body",
  "enabled_by_default": true,
  "settings": []
}
{% endschema %}

{% comment %} Load styles globally {% endcomment %}
{{ 'styles.css' | asset_url | stylesheet_tag }}

{% comment %} Minimal JS - forms, interactions only {% endcomment %}
<script src="{{ 'app-embed.js' | asset_url }}" defer></script>

{% comment %} Pass data to JS {% endcomment %}
<script>
  window.APP_CONFIG = {
    shopDomain: '{{ shop.permanent_domain }}',
    customerId: '{{ customer.id | default: "" }}',
    locale: '{{ request.locale.iso_code }}'
  };
</script>

[app_proxy]
url = "https://<firebase-url>/clientApi"
subpath = "app-name"
subpath_prefix = "apps"

{% comment %}
  App proxy URL: /apps/app-name/endpoint
  Shopify adds shop param automatically
{% endcomment %}

{% assign api_url = shop.url | append: '/apps/app-name/data/' | append: resource.id %}

// clientApi controller - receives 'shop' query param from app proxy
export async function getData(ctx) {
  const shopDomain = ctx.query.shop;
  const {id} = ctx.params;

  const shop = await shopRepository.getShopByShopifyDomain(shopDomain);
  if (!shop) {
    ctx.body = {success: false, error: 'Shop not found'};
    return;
  }

  const data = await service.getData(shop.id, id);
  ctx.body = data;
}

{% comment %}
  Generic async widget pattern
  - Unique IDs using resource ID (product, collection, page, etc.)
  - Data attributes for JS configuration
  - Loading/content/error states
{% endcomment %}

{% assign widget_id = product.id | default: collection.id | default: 'global' %}

<div class="app-widget"
     data-resource-id="{{ widget_id }}"
     data-resource-type="{{ resource_type | default: 'product' }}"
     data-shop="{{ shop.permanent_domain }}"
     id="app-widget-{{ widget_id }}">

  <div class="app-widget__loading" id="app-loading-{{ widget_id }}">
    <span class="app-widget__spinner"></span>
    {{ 'app.loading' | t | default: 'Loading...' }}
  </div>

  <div class="app-widget__content" id="app-content-{{ widget_id }}" style="display: none;">
    {%- comment -%} Content populated by JS {%- endcomment -%}
  </div>

  <div class="app-widget__error" id="app-error-{{ widget_id }}" style="display: none;">
    {{ 'app.error' | t | default: 'Unable to load content' }}
  </div>
</div>

(function() {
  // Configuration
  var PROXY_PATH = '/apps/proxy';  // Your app proxy path
  var ENDPOINT = '/data';          // Your endpoint

  // Get elements
  var containers = document.querySelectorAll('.app-widget');

  containers.forEach(function(container) {
    var resourceId = container.dataset.resourceId;
    var resourceType = container.dataset.resourceType;
    var shop = container.dataset.shop;

    var loadingEl = document.getElementById('app-loading-' + resourceId);
    var contentEl = document.getElementById('app-content-' + resourceId);
    var errorEl = document.getElementById('app-error-' + resourceId);

    async function fetchData() {
      try {
        var params = new URLSearchParams({
          resource_id: resourceId,
          resource_type: resourceType,
          shop: shop
        });

        var response = await fetch(PROXY_PATH + ENDPOINT + '?' + params);
        var result = await response.json();

        if (!result.success) {
          throw new Error(result.error || 'Request failed');
        }

        if (!result.data) {
          container.dataset.hidden = 'true';
          return;
        }

        renderContent(result.data, result.settings);
        loadingEl.style.display = 'none';
        contentEl.style.display = 'block';
      } catch (error) {
        console.error('Widget error:', error);
        loadingEl.style.display = 'none';
        errorEl.style.display = 'block';
      }
    }

    function renderContent(data, settings) {
      // Build your HTML from data
      contentEl.innerHTML = buildHTML(data, settings);
    }

    // Initialize on DOM ready
    if (document.readyState === 'loading') {
      document.addEventListener('DOMContentLoaded', fetchData);
    } else {
      fetchData();
    }
  });
})();

/* Loading spinner */
.app-widget__spinner {
  width: 16px;
  height: 16px;
  border: 2px solid #e5e5e5;
  border-top-color: var(--app-accent-color, #008060);
  border-radius: 50%;
  animation: app-spin 0.8s linear infinite;
}

@keyframes app-spin {
  to { transform: rotate(360deg); }
}

/* Hide widget when no data */
.app-widget[data-hidden="true"] {
  display: none;
}

/* Loading state */
.app-widget__loading {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 16px;
  color: #666;
}

/* Error state */
.app-widget__error {
  padding: 16px;
  color: #c00;
  text-align: center;
}

{% comment %} Data from metafields or app proxy {% endcomment %}
{% assign items = product.metafields.app_namespace.items.value %}

{% if items.size > 0 %}
  <div class="items-list">
    {% for item in items %}
      {% render 'item-card', item: item %}
    {% endfor %}
  </div>
{% else %}
  <p class="no-items">{{ 'app.no_items' | t }}</p>
{% endif %}

{% comment %} snippets/rating-display.liquid {% endcomment %}
{% comment %}
  @param value {number} - Value to display
  @param max {number} - Maximum value
  @param size {string} - 'small' | 'medium' | 'large'
{% endcomment %}

<div class="rating-display rating-display--{{ size | default: 'medium' }}">
  {% for i in (1..max) %}
    {% if i <= value %}
      <span class="rating-unit rating-unit--filled"></span>
    {% else %}
      <span class="rating-unit rating-unit--empty"></span>
    {% endif %}
  {% endfor %}
</div>

// assets/app-embed.js

(function() {
  'use strict';

  const config = window.APP_CONFIG || {};

  // Form handling
  function initForms() {
    document.querySelectorAll('[data-app-form]').forEach(form => {
      form.addEventListener('submit', handleSubmit);
    });
  }

  async function handleSubmit(e) {
    e.preventDefault();
    const form = e.target;
    const data = new FormData(form);

    try {
      const response = await fetch(form.action, {
        method: 'POST',
        body: JSON.stringify(Object.fromEntries(data)),
        headers: {'Content-Type': 'application/json'}
      });

      const result = await response.json();
      if (result.success) {
        showMessage(form, 'success', result.data.message);
      } else {
        showMessage(form, 'error', result.error);
      }
    } catch (err) {
      showMessage(form, 'error', 'Something went wrong');
    }
  }

  // Initialize when DOM ready
  if (document.readyState === 'loading') {
    document.addEventListener('DOMContentLoaded', initForms);
  } else {
    initForms();
  }
})();

{
  "app": {
    "title": "Feature Title",
    "submit": "Submit",
    "success": "Submitted successfully",
    "error": "Something went wrong",
    "no_items": "No items yet"
  }
}

{% comment %} In Liquid {% endcomment %}
{{ 'app.title' | t }}

{% comment %} With variables {% endcomment %}
{{ 'app.count' | t: count: items.size }}

- Schema defines all settings
- Uses snippets for reusable parts
- Translations for all text
- Scoped CSS (no global pollution)
- Minimal/no inline JS

- shopify.app.toml configured
- Backend validates shop parameter
- Returns JSON for JS consumption
- HTML rendering for Liquid consumption

- JS deferred or async
- CSS in stylesheet (not inline)
- Images lazy loaded
- No external dependencies
- Bundle < 20KB total