AEM + Vite Registry Architecture (Island Pattern)

In modern AEM implementations using Vite, we avoid injecting fragmented <script type="module"> tags via server-side HTL (<sly data-sly-use... />). Instead, we use a single entry point (app.ts) that relies on a MutationObserver to watch the DOM for specific data attributes (data-ui-module).

When an element is found (either on page load or when an AEM author drags a component onto the page), the global app dynamically imports the required script via registry.ts and initializes it.

Benefits:

1. Perfect Vite Chunking: One entry point allows Vite to resolve the full dependency graph and generate optimized vendor chunks.
2. Authoring DX: Components initialize instantly upon AJAX injection in the AEM Edit dialog without page reloads.
3. No Side Effects: Scripts are pure and only execute when explicitly invoked with a target DOM element.
4. Memory Safety: Teardown is handled automatically when AEM removes a component from the DOM.

Architecture File Structure

ui.frontend/src/
├── app.ts                  ← The main entry point (MutationObserver + WeakMap)
├── registry.ts             ← Maps DOM attribute strings to dynamic Vite imports
├── types/
│   └── UIModule.ts         ← TypeScript interface for module contract
└── components/
    └── carousel/
        ├── carousel.ts     ← The component JS (exports initModule)
        └── carousel.scss   ← The component styling

The Module Contract (UIModule.ts)

Every UI module must adhere to this signature. It prevents top-level execution and enforces cleanup.

import type { AppContext } from "@ui/types/AppContext";

export interface UIModule {
  /** Returns a teardown function, or void if no teardown is required. */
  destroy?: () => void;
}

export interface UIModuleDefinition {
  initModule: (el: HTMLElement, context: AppContext) => Promise<UIModule | void> | UIModule | void;
}

Agent Workflow: Creating a New Frontend Module

Step 1: Add the trigger to the HTL

Tag the root element of your AEM component with data-ui-module. Do not use <sly> to include the JS.

<div class="cmp-my-component" data-ui-module="components/my-component">
    <button class="cmp-my-component__btn">Click Me</button>
</div>

Step 2: Create the component script

Write the TypeScript module. Do not execute any DOM queries at the top level. Use an AbortController to manage event listeners so they can be easily destroyed.

// components/my-component/my-component.ts
import { createConsolePrefix } from "@ui/utils/common-utils";
import type { AppContext } from "@ui/types/AppContext";

const cp = createConsolePrefix("my-component");

export async function initModule(el: HTMLElement, ctx: AppContext) {
  const btn = el.querySelector<HTMLButtonElement>(".cmp-my-component__btn");
  if (!btn) return;

  // 1. Create state for cleanup
  const abortController = new AbortController();
  const { signal } = abortController;

  // 2. Bind events using the signal
  btn.addEventListener("click", () => {
    console.log(...cp, "Button clicked!");
  }, { signal });

  // 3. Return the teardown function
  return {
    destroy: () => {
      console.debug(...cp, "Cleaning up my-component");
      abortController.abort(); // Instantly removes all event listeners
    }
  };
}

Step 3: Register the module

Map the string used in the HTL to the dynamic import in the registry.

// registry.ts
import type { UIModuleDefinition } from "./types/UIModule";

export const registry: Record<string, () => Promise<UIModuleDefinition>> = {
  "components/carousel": () => import("./components/carousel/carousel"),
  "components/my-component": () => import("./components/my-component/my-component"),
  // Add new modules here...
};

The Engine: App Initialization (app.ts)

The core application relies on a WeakMap to track element state and a MutationObserver to handle AEM authoring lifecycle events. (Reference this when debugging initialization flows).

// app.ts
import { registry } from "./registry";

const attribute = "data-ui-module";
const moduleInstances = new WeakMap<HTMLElement, "pending" | { destroy?: () => void }>();

export async function initModule(el: HTMLElement): Promise<void> {
  if (moduleInstances.has(el)) return; // Prevent double initialization

  const moduleName = el.getAttribute(attribute);
  if (!moduleName) return;

  const loader = registry[moduleName];
  if (!loader) {
    console.warn(`No module registered for: ${moduleName}`);
    return;
  }

  moduleInstances.set(el, "pending"); // Lock

  try {
    const uiModule = await loader();
    const instanceAPI = await uiModule.initModule(el, { /* AppContext */ });
    
    // Store the returned destroy function (if any)
    moduleInstances.set(el, instanceAPI || { destroy: undefined });
  } catch (error) {
    console.error(`Failed to load module: ${moduleName}`, error);
    moduleInstances.delete(el); // Unlock on failure so it can be retried
  }
}

// Observe DOM for AEM Component Additions/Removals
new MutationObserver((mutations) => {
  for (const mutation of mutations) {
    // Additions
    mutation.addedNodes.forEach((node) => {
      if (!(node instanceof HTMLElement)) return;
      if (node.hasAttribute(attribute)) void initModule(node);
      node.querySelectorAll<HTMLElement>(`[${attribute}]`).forEach(initModule);
    });

    // Removals (Teardown)
    mutation.removedNodes.forEach((node) => {
      if (!(node instanceof HTMLElement)) return;
      const elements = node.hasAttribute(attribute) 
        ? [node, ...node.querySelectorAll<HTMLElement>(`[${attribute}]`)]
        : Array.from(node.querySelectorAll<HTMLElement>(`[${attribute}]`));

      for (const el of elements) {
        const instance = moduleInstances.get(el);
        if (instance && instance !== "pending" && typeof instance.destroy === "function") {
          instance.destroy();
          moduleInstances.delete(el);
        }
      }
    });
  }
}).observe(document.body, { childList: true, subtree: true });

Critical Rules

1. NO top-level side effects: Never run document.querySelector or addEventListener at the root level of a module. Code must only execute inside initModule.
2. Always scope selectors to the element: Use el.querySelector, not document.querySelector, to ensure you only manipulate the specific component instance.
3. Always use AbortController: Pass { signal: abortController.signal } to all addEventListener calls, especially global ones (like window.addEventListener('scroll')).
4. Return a destroy method: If your component creates global listeners, intervals, or third-party library instances (like Swiper), you MUST return a { destroy: () => void } object from initModule.
5. NEVER use HTL script injection for UI modules: Do not use <sly data-sly-call="${... @ modulePath='...'}"> for standard component JavaScript. Rely strictly on data-ui-module. Web Components (Lit) are the only exception as they self-register.
6. Use WeakMap for state: Do not store component instances in global arrays or objects, as this causes memory leaks. Rely on the app.ts WeakMap.