Master programming syntax, solve interactive challenges & run live code in Syntaxis Explore Arena →

menu_book Course Curriculum (Web Components) expand_more

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 / Styling & APIs

Custom states concept

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

12 min read • Advanced

web-components-037.js

javascript

class AsyncPanel extends HTMLElement{#i=this.attachInternals();set loading(v){v?this.#i.states.add("loading"):this.#i.states.delete("loading")}}customElements.define("async-panel",AsyncPanel);
// CSS: async-panel:state(loading){opacity:.6}

What to watch

Content source & attribution

Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

Custom states concept Mastery Check →

ARIA via internals

Declarative shadow DOM