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Custom Elements

Define custom element Autonomous custom element Custom element constructor connectedCallback disconnectedCallback attributeChangedCallback observedAttributes Custom element upgrade

Shadow DOM

attachShadow Open shadow root Closed shadow root Shadow tree styles Style encapsulation CSS custom properties across shadow ::part exportparts

Templates & Slots

template element DocumentFragment cloning slot element Named slots Default slot slotchange assignedElements Fallback slot content

Attributes & Properties

Reflect property to attribute Boolean attributes Property-only complex values Attribute parsing Custom events Event composed option Event bubbling across shadow Form-associated concept

Styling & APIs

adoptedStyleSheets Constructable stylesheet ElementInternals ARIA via internals Custom states concept Declarative shadow DOM Scoped registry concept Lifecycle-safe listeners

Production Patterns

Lazy definition define idempotently Versioning tag names SSR hydration strategy Testing shadow DOM Accessibility naming Performance template reuse Progressive enhancement

Web Components / Custom Elements

connectedCallback

Learn connectedCallback in Web Components with a dedicated explanation, a concept-specific example, a production caution, and a mastery check.

12 min read • Beginner

web-components-004.js

javascript

class LiveClock extends HTMLElement { connectedCallback(){ this.textContent=new Date().toLocaleTimeString(); } }
customElements.define("live-clock",LiveClock);

What to watch

Content source & attribution

Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

connectedCallback Mastery Check →

Custom element constructor

disconnectedCallback