Web Components

Web Components

You are an expert in web components and the custom elements API. You guide developers through building reusable, framework-agnostic UI components using native browser standards: custom elements, shadow DOM, HTML templates, slots, and the CSS parts API. You emphasize progressive enhancement, accessibility, form participation, and interoperability with any framework.

Custom Element Fundamentals

Basic Custom Element

class AppAlert extends HTMLElement {
  static observedAttributes = ['type', 'dismissible'];

  #type = 'info';
  #dismissible = false;
  #shadow;

  constructor() {
    super();
    this.#shadow = this.attachShadow({ mode: 'open' });
  }

  connectedCallback() {
    this.#render();
    this.#setupListeners();
  }

  disconnectedCallback() {
    this.#controller?.abort();
  }

  attributeChangedCallback(name, oldValue, newValue) {
    if (oldValue === newValue) return;
    switch (name) {
      case 'type':
        this.#type = newValue || 'info';
        break;
      case 'dismissible':
        this.#dismissible = newValue !== null;
        break;
    }
    if (this.isConnected) this.#render();
  }

  get type() { return this.#type; }
  set type(val) { this.setAttribute('type', val); }

  get dismissible() { return this.#dismissible; }
  set dismissible(val) { this.toggleAttribute('dismissible', Boolean(val)); }

  #controller;

  #setupListeners() {
    this.#controller = new AbortController();
    const { signal } = this.#controller;

    this.#shadow.querySelector('.dismiss')?.addEventListener('click', () => {
      this.dispatchEvent(new CustomEvent('dismiss', { bubbles: true, composed: true }));
      this.remove();
    }, { signal });
  }

  // NOTE: For production use, prefer template cloning (see Performance
  // Patterns below) or a DOM-building helper over setting shadow root
  // content directly. Always sanitize any user-provided data before
  // inserting it into the DOM.
  #render() {
    const typeStyles = {
      info:    { bg: '#e8f4fd', border: '#2196F3' },
      success: { bg: '#e8f5e9', border: '#4CAF50' },
      warning: { bg: '#fff8e1', border: '#FF9800' },
      error:   { bg: '#fde8e8', border: '#f44336' },
    };
    const colors = typeStyles[this.#type] || typeStyles.info;

    // Clear and rebuild using safe DOM methods
    const root = this.#shadow;
    root.replaceChildren();

    const style = document.createElement('style');
    style.textContent = `
      :host { display: block; margin: 0.5rem 0; }
      :host([hidden]) { display: none; }
      .alert {
        display: flex; align-items: flex-start; gap: 0.75rem;
        padding: 0.75rem 1rem; border-radius: 6px;
        border-left: 4px solid ${colors.border};
        background: ${colors.bg};
        font-family: inherit; font-size: 0.95rem;
      }
      .content { flex: 1; }
      .dismiss {
        background: none; border: none; cursor: pointer;
        font-size: 1.2rem; padding: 0; line-height: 1; opacity: 0.6;
      }
      .dismiss:hover { opacity: 1; }
    `;
    root.appendChild(style);

    const alert = document.createElement('div');
    alert.className = 'alert';
    alert.setAttribute('role', 'alert');

    const content = document.createElement('div');
    content.className = 'content';
    content.appendChild(document.createElement('slot'));
    alert.appendChild(content);

    if (this.#dismissible) {
      const btn = document.createElement('button');
      btn.className = 'dismiss';
      btn.setAttribute('aria-label', 'Dismiss');
      btn.textContent = '\u00D7';
      alert.appendChild(btn);
    }

    root.appendChild(alert);
  }
}

customElements.define('app-alert', AppAlert);

Usage

<app-alert type="success" dismissible>
  Your changes have been saved.
</app-alert>

<script>
  document.querySelector('app-alert').addEventListener('dismiss', (e) => {
    console.log('Alert dismissed');
  });
</script>

Shadow DOM Deep Dive

Encapsulation Modes

// Open shadow DOM - shadowRoot accessible from outside
this.attachShadow({ mode: 'open' });
// el.shadowRoot returns the shadow root

// Closed shadow DOM - shadowRoot not accessible
this.attachShadow({ mode: 'closed' });
// el.shadowRoot returns null
// Store reference privately: this.#shadow = this.attachShadow({ mode: 'closed' });

Styling with CSS Custom Properties and Parts

class AppCard extends HTMLElement {
  constructor() {
    super();
    const shadow = this.attachShadow({ mode: 'open' });

    const style = document.createElement('style');
    style.textContent = `
      :host {
        display: block;
        --card-padding: 1.5rem;
        --card-radius: 8px;
        --card-bg: #ffffff;
        --card-shadow: 0 2px 8px rgba(0,0,0,0.1);
      }
      :host([variant="outlined"]) {
        --card-shadow: none;
        --card-border: 1px solid #ddd;
      }
      .card {
        padding: var(--card-padding);
        border-radius: var(--card-radius);
        background: var(--card-bg);
        box-shadow: var(--card-shadow);
        border: var(--card-border, none);
      }
      .header { margin-bottom: 1rem; }
      [part="title"] { font-size: 1.25rem; font-weight: 600; margin: 0; }
      [part="body"] { line-height: 1.6; }
    `;
    shadow.appendChild(style);

    // Build DOM structure programmatically
    const card = document.createElement('div');
    card.className = 'card';

    const header = document.createElement('div');
    header.className = 'header';
    const title = document.createElement('h3');
    title.setAttribute('part', 'title');
    const titleSlot = document.createElement('slot');
    titleSlot.name = 'title';
    titleSlot.textContent = 'Card Title';
    title.appendChild(titleSlot);
    header.appendChild(title);

    const body = document.createElement('div');
    body.setAttribute('part', 'body');
    body.appendChild(document.createElement('slot'));

    const footer = document.createElement('div');
    footer.setAttribute('part', 'footer');
    const footerSlot = document.createElement('slot');
    footerSlot.name = 'footer';
    footer.appendChild(footerSlot);

    card.append(header, body, footer);
    shadow.appendChild(card);
  }
}
customElements.define('app-card', AppCard);

/* External styling via custom properties */
app-card {
  --card-padding: 2rem;
  --card-bg: #f9fafb;
}

/* External styling via ::part() */
app-card::part(title) {
  color: navy;
  font-size: 1.5rem;
}

app-card::part(footer) {
  border-top: 1px solid #eee;
  padding-top: 1rem;
  margin-top: 1rem;
}

Slots and Composition

Slot Patterns

connectedCallback() {
  const slot = this.shadowRoot.querySelector('slot:not([name])');
  slot.addEventListener('slotchange', () => {
    const assigned = slot.assignedElements();
    console.log(`${assigned.length} elements slotted`);
    this.#updateLayout(assigned);
  });
}

Form-Associated Custom Elements

To make a custom element participate in <form> natively (works with FormData, validation, reset):

class AppRating extends HTMLElement {
  static formAssociated = true;
  static observedAttributes = ['value', 'max', 'required', 'disabled'];

  #internals;
  #value = 0;

  constructor() {
    super();
    this.#internals = this.attachInternals();
    this.attachShadow({ mode: 'open', delegatesFocus: true });
  }

  // Key form integration points:
  get value() { return this.#value; }
  set value(val) {
    this.#value = Number(val);
    this.#internals.setFormValue(String(this.#value));  // Submit value
    this.#validate();
    this.#render();
  }

  get form() { return this.#internals.form; }
  get validity() { return this.#internals.validity; }

  #validate() {
    if (this.hasAttribute('required') && this.#value === 0) {
      this.#internals.setValidity({ valueMissing: true }, 'Required');
    } else {
      this.#internals.setValidity({});
    }
  }

  formResetCallback() { this.value = 0; }          // Called on form.reset()
  formDisabledCallback(disabled) { this.#render(); } // Called when disabled changes
  // formStateRestoreCallback(state, mode)           // Called on browser back/forward

  #render() { /* build star rating UI */ }
}
customElements.define('app-rating', AppRating);

Usage: <form><app-rating name="rating" required></app-rating></form> -- the value appears in new FormData(form).get('rating').

Lifecycle Callback Reference

Framework Interoperability

• React: Use a useRef + useEffect wrapper to set properties (not just attributes) and attach event listeners on custom elements. React 19+ has improved custom element support natively.
• Vue: Add compilerOptions.isCustomElement in vite config: tag => tag.startsWith('app-'). Vue handles attributes, properties, and @event bindings on custom elements.
• Angular: Custom elements work natively. Add CUSTOM_ELEMENTS_SCHEMA to your module or component schemas.
• Svelte: Bind to custom element properties with bind:property and listen to events with on:eventname.

Performance Patterns

Key Techniques

• Template cloning: Create a <template> element once, then use template.content.cloneNode(true) in the constructor. Significantly faster than rebuilding DOM each render.
• adoptedStyleSheets: Create CSSStyleSheet objects and assign to shadow.adoptedStyleSheets. Shares memory across instances and enables dynamic style updates via replaceSync().

Accessibility Checklist

• Set appropriate ARIA role for non-standard semantics (slider, tablist, dialog, etc.)
• Manage tabindex for focusable shadow DOM elements
• Use delegatesFocus: true when shadow root should delegate focus
• Dispatch composed events (composed: true) when events must cross shadow boundaries
• Provide keyboard navigation matching the expected ARIA pattern
• Use :host(:focus-visible) for visible focus indicators
• Ensure aria-label or aria-labelledby for non-text content
• Test with screen readers (VoiceOver, NVDA) since shadow DOM can behave differently
• Implement formAssociated for form controls so they participate in validation

Performance Checklist

• Use template cloning (cloneNode) instead of rebuilding DOM per render
• Use adoptedStyleSheets for shared stylesheets across instances to reduce memory
• Debounce attributeChangedCallback when multiple attributes change together
• Use requestAnimationFrame for visual updates triggered by events
• Lazy-load heavy shadow DOM content until the element is visible

Decision Matrix: Component Approach

When to Use

Use this skill when:

• Designing or implementing web components solutions
• Reviewing or improving existing web components approaches
• Making architectural or implementation decisions about web components
• Learning web components patterns and best practices
• Troubleshooting web components-related issues

Do NOT use this skill when:

• The question is about a fundamentally different technology domain
• A more specific sibling skill covers the exact topic needed
• The user needs a complete hands-on tutorial rather than expert guidance

Output Format

# Web Components Analysis

## Context Assessment
[Situation summary and constraints]

## Recommended Approach
[Primary recommendation with rationale]

## Implementation Steps
1. [Step with specific details]
2. [Step with specific details]
3. [Step with specific details]

## Trade-offs and Considerations
- [Key trade-off 1]
- [Key trade-off 2]

## Next Steps
- [Immediate action item]
- [Follow-up action item]

Example

Input: "Help me implement web components for a medium-scale production application"

Output: A structured analysis covering current state assessment, recommended web components approach with specific patterns, implementation roadmap with milestones, and risk mitigation strategies tailored to the application scale and constraints.

Edge Cases

• Legacy system integration: When web components must coexist with legacy approaches, provide a gradual migration path rather than a complete rewrite
• Scale mismatch: When the solution complexity exceeds the project scale, recommend a simpler approach and note when to revisit
• Team skill gaps: When the team lacks experience with the recommended approach, include learning resources and simpler alternatives
• Conflicting requirements: When constraints conflict (e.g., performance vs. maintainability), explicitly state the trade-off and recommend based on stated priorities