---
name: web-components-architecture
description: Build web components using Custom Elements v1 API with Declarative Shadow DOM, attribute-driven state, handleEvent pattern, and zero DOM selection. Use when creating custom elements, extending built-in HTML elements, or implementing component-based architecture. NO querySelector, NO innerHTML, NO external libraries - pure web platform APIs only.
allowed-tools: Read, Write, Edit, Grep, Glob, Bash
---

# Web Components Architecture

## Core Principles

This skill enforces a strict architectural pattern for web components:

1. **Zero DOM Selection**: NEVER use `querySelector`, `querySelectorAll`, or `getElementById`
2. **Attribute-Driven State**: All state flows through HTML attributes
3. **Event Delegation on `this`**: Use `this.addEventListener` and the `handleEvent` pattern
4. **No External Dependencies**: Use only standard Web Platform APIs
5. **Declarative Shadow DOM**: Use `<template shadowrootmode="open">` for instant rendering
6. **Progressive Enhancement**: Components work (degraded) even if JavaScript fails
7. **Customized Built-ins Over Autonomous**: Extend native elements when possible to preserve accessibility

## Relationship with JavaScript Best Practices

This skill defines the **architectural pattern** for building web components. When implementing components, combine this skill with `javascript-pragmatic-rules` for production-quality code:

**This skill provides the WHAT (component architecture):**
- Component structure (extends HTMLElement)
- Lifecycle callbacks (connectedCallback, disconnectedCallback)
- State management (attribute-driven)
- Event patterns (handleEvent, CustomEvent)
- Shadow DOM and encapsulation

**`javascript-pragmatic-rules` provides the HOW (implementation quality):**
- Async operation handling (timeouts, cancellation)
- Resource cleanup patterns
- Error handling strategies
- Memory leak prevention
- Performance optimization

**Example:** Building an `<async-button>` component:

```javascript
// Architecture from web-components-architecture skill
class AsyncButton extends HTMLButtonElement {
  #controller = null; // Private field for cleanup

  connectedCallback() {
    this.addEventListener('click', this);
  }

  // Using handleEvent pattern from web-components-architecture
  async handleEvent(e) {
    if (e.type === 'click') {
      // Rule 2 from javascript-pragmatic-rules: Timeout async operations
      this.#controller = new AbortController();
      const timeoutId = setTimeout(() => this.#controller.abort(), 5_000);

      try {
        const response = await fetch(this.getAttribute('data-url'), {
          signal: this.#controller.signal
        });
        clearTimeout(timeoutId);
        // Handle response...
      } catch (error) {
        // Rule 1 from javascript-pragmatic-rules: Handle rejections
        if (error.name === 'AbortError') {
          console.warn('Request timed out');
        } else {
          throw new Error('Request failed', { cause: error });
        }
      }
    }
  }

  // Rule 4 from javascript-pragmatic-rules: Clean up resources
  disconnectedCallback() {
    this.removeEventListener('click', this);
    if (this.#controller) this.#controller.abort();
  }
}
```

**Key Integration Points:**
- Use this skill's `connectedCallback` with `javascript-pragmatic-rules` Rule 4 (cleanup)
- Use this skill's `handleEvent` with `javascript-pragmatic-rules` Rules 1-2 (async safety)
- Use this skill's attribute patterns with `javascript-pragmatic-rules` Rule 5 (immutability)

See `javascript-pragmatic-rules` skill for comprehensive JavaScript best practices.

---

## Component Types

### 1. Customized Built-in Elements (PREFERRED)

Extend native HTML elements to preserve built-in accessibility and behavior.

```javascript
class AsyncAction extends HTMLButtonElement {
  connectedCallback() {
    // Principle: Event Delegation on Self
    this.addEventListener('click', this);
  }

  // Principle: HandleEvent pattern avoids binding 'this'
  async handleEvent(e) {
    if (e.type === 'click') {
      // Principle: Attribute-Driven State (Input)
      this.setAttribute('aria-busy', 'true');
      this.disabled = true;

      // Simulate async work
      await new Promise((resolve) => setTimeout(resolve, 1_500));

      this.removeAttribute('aria-busy');
      this.disabled = false;

      // Principle: Events are the ONLY output
      this.dispatchEvent(new CustomEvent('action-complete', {
        bubbles: true,
        detail: { originalEvent: e }
      }));
    }
  }

  disconnectedCallback() {
    this.removeEventListener('click', this);
  }
}

// Register with { extends: 'button' }
customElements.define('async-action', AsyncAction, { extends: 'button' });
```

**Usage in HTML:**
```html
<button is="async-action" aria-label="Add to cart">
  Add to Cart
</button>
```

### 2. Autonomous Custom Elements

Use for layout containers and composite components.

```javascript
class ProductCard extends HTMLElement {
  // Purely declarative - no logic needed if using DSD
}

customElements.define('product-card', ProductCard);
```

## Declarative Shadow DOM

Use `<template shadowrootmode="open">` for instant rendering without JavaScript.

```html
<product-card>
  <template shadowrootmode="open">
    <style>
      :host {
        display: block;
        max-width: 300px;
      }

      /* Attribute-Driven Styling */
      button[aria-busy="true"] {
        opacity: 0.7;
        cursor: wait;
      }

      .card {
        background: var(--card-bg, white);
        border: 1px solid #e5e7eb;
        border-radius: 8px;
        padding: 1rem;
        display: flex;
        flex-direction: column;
        gap: 0.5rem;
      }

      .footer {
        margin-top: 1rem;
        display: flex;
        justify-content: space-between;
        align-items: center;
      }
    </style>

    <article class="card" part="container">
      <slot name="header"></slot>
      <slot></slot>

      <div class="footer">
        <slot name="price">Free</slot>

        <button is="async-action" part="action-btn">
          Add to Cart
        </button>
      </div>
    </article>
  </template>

  <h3 slot="header">Product Title</h3>
  <p>Product description goes here.</p>
  <strong slot="price">$450.00</strong>
</product-card>
```

## Attribute-Driven State Pattern

State flows IN via attributes and OUT via events.

### Input: Reading Attributes

```javascript
class DataDisplay extends HTMLElement {
  connectedCallback() {
    // Read initial state from attributes
    const apiUrl = this.getAttribute('data-url');
    const refreshInterval = this.getAttribute('refresh-interval') || 5_000;

    this.setAttribute('status', 'loading');
    this.loadData(apiUrl);
  }

  async loadData(url) {
    try {
      const response = await fetch(url);
      const data = await response.json();

      // State change via attribute
      this.setAttribute('status', 'loaded');
      this.setAttribute('data-count', data.items.length);

      // Output via event
      this.dispatchEvent(new CustomEvent('data-loaded', {
        bubbles: true,
        detail: { data }
      }));
    } catch (error) {
      this.setAttribute('status', 'error');
      this.dispatchEvent(new CustomEvent('data-error', {
        bubbles: true,
        detail: { error: error.message }
      }));
    }
  }

  // Observe attribute changes
  static get observedAttributes() {
    return ['data-url', 'refresh-interval'];
  }

  attributeChangedCallback(name, oldValue, newValue) {
    if (oldValue === newValue) return;

    if (name === 'data-url' && newValue) {
      this.loadData(newValue);
    }
  }
}
```

### Output: Events Only

```javascript
// CORRECT: Dispatch events for output
this.dispatchEvent(new CustomEvent('value-changed', {
  bubbles: true,
  composed: true, // Cross shadow DOM boundaries
  detail: { value: newValue }
}));

// WRONG: Never modify external DOM
// document.querySelector('.result').textContent = value; // ❌ FORBIDDEN
```

## HandleEvent Pattern

Use the `handleEvent` interface to avoid `.bind(this)` and maintain clean memory management.

```javascript
class MultiEventHandler extends HTMLElement {
  connectedCallback() {
    // Single listener for multiple event types
    this.addEventListener('click', this);
    this.addEventListener('keydown', this);
    this.addEventListener('focus', this);
  }

  handleEvent(e) {
    // Route by event type
    switch(e.type) {
      case 'click':
        this.handleClick(e);
        break;
      case 'keydown':
        this.handleKeydown(e);
        break;
      case 'focus':
        this.handleFocus(e);
        break;
    }
  }

  handleClick(e) {
    this.setAttribute('last-interaction', 'click');
  }

  handleKeydown(e) {
    if (e.key === 'Enter' || e.key === ' ') {
      this.setAttribute('last-interaction', 'keyboard');
    }
  }

  handleFocus(e) {
    this.setAttribute('focused', 'true');
  }

  disconnectedCallback() {
    this.removeEventListener('click', this);
    this.removeEventListener('keydown', this);
    this.removeEventListener('focus', this);
  }
}
```

## Styling API

### CSS Custom Properties (Theming)

```css
/* Consumer defines theme */
:root {
  --primary-color: #6200ea;
  --bg-color: #ffffff;
  --spacing: 1rem;
}

product-card {
  --card-bg: var(--bg-color);
  --card-padding: var(--spacing);
}
```

### ::part() Pseudo-element

```html
<template shadowrootmode="open">
  <style>
    .internal-element {
      /* internal styles */
    }
  </style>

  <!-- Expose parts for external styling -->
  <div class="internal-element" part="container">
    <button part="action-btn">Click me</button>
  </div>
</template>
```

```css
/* External styling via ::part() */
product-card::part(container) {
  box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1);
}

product-card::part(action-btn) {
  background-color: var(--primary-color);
  color: white;
}
```

### :host and :host-context

```css
<style>
  /* Style the host element */
  :host {
    display: block;
    font-family: system-ui, sans-serif;
  }

  /* State-based styling */
  :host([disabled]) {
    opacity: 0.5;
    pointer-events: none;
  }

  /* Context-based styling */
  :host-context(.dark-mode) {
    background: #1a1a1a;
    color: #ffffff;
  }
</style>
```

## Lifecycle Callbacks

```javascript
class LifecycleExample extends HTMLElement {
  constructor() {
    super();
    // ONLY: Initialize instance variables
    // DO NOT: Access attributes, children, or DOM
  }

  connectedCallback() {
    // Element added to DOM
    // DO: Set up event listeners, start timers, fetch data
    this.addEventListener('click', this);

    const initialValue = this.getAttribute('value');
    if (initialValue) {
      this.initialize(initialValue);
    }
  }

  disconnectedCallback() {
    // Element removed from DOM
    // DO: Clean up event listeners, timers, connections
    this.removeEventListener('click', this);

    if (this.intervalId) {
      clearInterval(this.intervalId);
    }
  }

  attributeChangedCallback(name, oldValue, newValue) {
    // Observed attribute changed
    if (oldValue === newValue) return;

    switch(name) {
      case 'value':
        this.updateValue(newValue);
        break;
      case 'disabled':
        this.updateDisabled(newValue !== null);
        break;
    }
  }

  adoptedCallback() {
    // Element moved to new document
    // Rarely needed - usually for iframes
  }

  static get observedAttributes() {
    return ['value', 'disabled', 'theme'];
  }
}
```

## Form Integration with ElementInternals

Use ElementInternals API for form-associated custom elements.

```javascript
class CustomInput extends HTMLElement {
  static formAssociated = true;

  constructor() {
    super();
    this.internals = this.attachInternals();
  }

  connectedCallback() {
    this.addEventListener('input', this);
    this.updateValue(this.getAttribute('value') || '');
  }

  handleEvent(e) {
    if (e.type === 'input') {
      const value = e.target.value;
      this.updateValue(value);
    }
  }

  updateValue(value) {
    // Update form value
    this.internals.setFormValue(value);

    // Validate
    if (!value) {
      this.internals.setValidity(
        { valueMissing: true },
        'This field is required'
      );
    } else {
      this.internals.setValidity({});
    }

    // Emit event
    this.dispatchEvent(new CustomEvent('value-changed', {
      bubbles: true,
      detail: { value }
    }));
  }

  // Form lifecycle callbacks
  formResetCallback() {
    this.updateValue('');
  }

  formDisabledCallback(disabled) {
    this.setAttribute('aria-disabled', disabled);
  }
}

customElements.define('custom-input', CustomInput);
```

## Accessibility Patterns

### ARIA Attributes

```javascript
class AccessibleToggle extends HTMLButtonElement {
  connectedCallback() {
    this.addEventListener('click', this);

    // Set initial ARIA state
    if (!this.hasAttribute('aria-pressed')) {
      this.setAttribute('aria-pressed', 'false');
    }
  }

  handleEvent(e) {
    if (e.type === 'click') {
      const isPressed = this.getAttribute('aria-pressed') === 'true';
      this.setAttribute('aria-pressed', String(!isPressed));

      this.dispatchEvent(new CustomEvent('toggle', {
        bubbles: true,
        detail: { pressed: !isPressed }
      }));
    }
  }
}

customElements.define('accessible-toggle', AccessibleToggle, { extends: 'button' });
```

### Keyboard Navigation

```javascript
class KeyboardNav extends HTMLElement {
  connectedCallback() {
    if (!this.hasAttribute('tabindex')) {
      this.setAttribute('tabindex', '0');
    }

    this.addEventListener('keydown', this);
  }

  handleEvent(e) {
    if (e.type === 'keydown') {
      switch(e.key) {
        case 'Enter':
        case ' ':
          e.preventDefault();
          this.activate();
          break;
        case 'Escape':
          this.deactivate();
          break;
      }
    }
  }

  activate() {
    this.setAttribute('active', 'true');
    this.dispatchEvent(new CustomEvent('activated', { bubbles: true }));
  }

  deactivate() {
    this.removeAttribute('active');
    this.dispatchEvent(new CustomEvent('deactivated', { bubbles: true }));
  }
}
```

## Complete Example: Async Action Button

```javascript
/* async-action.js */
class AsyncAction extends HTMLButtonElement {
  connectedCallback() {
    this.addEventListener('click', this);
  }

  async handleEvent(e) {
    if (e.type === 'click') {
      // Prevent double-clicks
      if (this.getAttribute('aria-busy') === 'true') {
        return;
      }

      // Set loading state
      this.setAttribute('aria-busy', 'true');
      this.disabled = true;

      const originalText = this.textContent;
      const loadingText = this.getAttribute('loading-text') || 'Loading...';
      this.textContent = loadingText;

      try {
        // Dispatch event for external handler
        const actionEvent = new CustomEvent('async-action', {
          bubbles: true,
          cancelable: true,
          detail: {
            button: this,
            originalEvent: e
          }
        });

        const shouldContinue = this.dispatchEvent(actionEvent);

        if (shouldContinue) {
          // Simulate async work (in real usage, parent handles this)
          await new Promise((resolve) => setTimeout(resolve, 1_500));

          // Success state
          this.setAttribute('aria-busy', 'false');
          this.removeAttribute('aria-busy');

          this.dispatchEvent(new CustomEvent('action-complete', {
            bubbles: true,
            detail: { success: true }
          }));
        }
      } catch (error) {
        // Error state
        this.setAttribute('aria-busy', 'false');
        this.removeAttribute('aria-busy');

        this.dispatchEvent(new CustomEvent('action-error', {
          bubbles: true,
          detail: { error: error.message }
        }));
      } finally {
        // Reset state
        this.textContent = originalText;
        this.disabled = false;
      }
    }
  }

  disconnectedCallback() {
    this.removeEventListener('click', this);
  }
}

customElements.define('async-action', AsyncAction, { extends: 'button' });
```

**HTML Usage:**
```html
<button
  is="async-action"
  loading-text="Saving..."
  aria-label="Save changes">
  Save
</button>

<script>
  document.addEventListener('async-action', async (e) => {
    // Handle the action
    const response = await fetch('/api/save', {
      method: 'POST',
      body: JSON.stringify({ data: 'example' })
    });

    if (!response.ok) {
      throw new Error('Save failed');
    }
  });
</script>
```

## Anti-Patterns (NEVER DO THIS)

### ❌ DOM Selection
```javascript
// WRONG
connectedCallback() {
  const button = this.querySelector('button'); // ❌
  button.addEventListener('click', ...);
}
```

### ❌ Direct DOM Manipulation of External Elements
```javascript
// WRONG
handleClick() {
  document.getElementById('result').textContent = 'Done'; // ❌
}
```

### ❌ innerHTML for Dynamic Content
```javascript
// WRONG
updateContent(data) {
  this.innerHTML = `<div>${data}</div>`; // ❌
}
```

### ❌ Imperative Shadow DOM Creation
```javascript
// WRONG (use Declarative Shadow DOM instead)
constructor() {
  super();
  this.attachShadow({ mode: 'open' }); // ❌
  this.shadowRoot.innerHTML = '<div>...</div>'; // ❌
}
```

### ❌ Global State
```javascript
// WRONG
let globalState = {}; // ❌

class MyComponent extends HTMLElement {
  connectedCallback() {
    globalState.value = 'bad'; // ❌
  }
}
```

## Testing Pattern

```javascript
// test.html
<!DOCTYPE html>
<html>
<head>
  <script type="module" src="./async-action.js"></script>
</head>
<body>
  <button is="async-action" id="test-btn">Test</button>

  <script type="module">
    const btn = document.getElementById('test-btn');

    // Test attribute-driven state
    btn.addEventListener('action-complete', (e) => {
      console.log('✓ Action completed', e.detail);
    });

    // Programmatically trigger
    btn.click();

    // Verify state
    console.assert(
      btn.getAttribute('aria-busy') === 'true',
      'Should be busy during action'
    );
  </script>
</body>
</html>
```

## Progressive Enhancement

Components should degrade gracefully without JavaScript:

```html
<!-- Works as regular button if JS fails -->
<button is="async-action" formaction="/submit" formmethod="post">
  Submit Form
</button>

<!-- Works as regular link if JS fails -->
<a is="spa-link" href="/page">
  Navigate
</a>
```

## When to Use This Pattern

- Building reusable UI components
- Creating design systems
- Server-side rendering (SSR) with hydration
- Progressive web apps (PWAs)
- Micro-frontends
- Accessibility-first applications

## When NOT to Use

- Simple static websites (use plain HTML)
- Heavy data visualization (consider Canvas/WebGL)
- Applications requiring IE11 support (no polyfills allowed)

## Instructions for Implementation

When implementing web components:

1. **Choose Element Type**: Prefer customized built-ins over autonomous elements
2. **Define State Contract**: Document all attributes and events
3. **Use HandleEvent**: Implement `handleEvent()` for event delegation
4. **Avoid DOM Selection**: NEVER use `querySelector` or similar methods
5. **Use DSD**: Implement Declarative Shadow DOM for instant rendering
6. **Expose Styling API**: Use CSS custom properties and `::part()`
7. **Implement Accessibility**: Add ARIA attributes and keyboard support
8. **Test Progressive Enhancement**: Verify behavior with JavaScript disabled
9. **Document Usage**: Provide clear HTML examples

## File Organization

```
components/
├── async-action/
│   ├── async-action.js
│   ├── async-action.test.html
│   └── README.md
├── product-card/
│   ├── product-card.js
│   ├── product-card.test.html
│   └── README.md
└── index.js (exports all components)
```

## References

- [Custom Elements Spec](https://html.spec.whatwg.org/multipage/custom-elements.html)
- [Declarative Shadow DOM](https://web.dev/declarative-shadow-dom/)
- [ElementInternals API](https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals)
- [ARIA Authoring Practices](https://www.w3.org/WAI/ARIA/apg/)