Lit Web Components Best Practices
Best practices for building Lit web components, optimized for AI-assisted code generation and review.
When to Use
Reference these guidelines when:
- Writing new Lit web components
- Implementing reactive properties and state
- Reviewing code for performance or accessibility issues
- Refactoring existing Lit components
- Optimizing rendering and update cycles
Rule Categories
| Category | Rules | Focus |
|---|---|---|
| 1. Component Structure | 4 rules | Properties, state, TypeScript |
| 2. Rendering | 5 rules | Templates, directives, derived state |
| 3. Styling | 4 rules | Static styles, theming, CSS parts |
| 4. Events | 3 rules | Custom events, naming, cleanup |
| 5. Lifecycle | 4 rules | Callbacks, timing, async |
| 6. Accessibility | 3 rules | ARIA, focus, forms |
| 7. Performance | 4 rules | Updates, caching, lazy loading |
Priority Levels
| Priority | Description | Action |
|---|---|---|
| CRITICAL | Major correctness or accessibility issues | Fix immediately |
| HIGH | Significant maintainability/performance impact | Address in current PR |
| MEDIUM | Best practice violations | Address when touching related code |
| LOW | Style preferences, micro-optimizations | Consider during refactoring |
Rules Index
1. Component Structure
rules/1-1-use-decorators.md- Use TypeScript Decorators (HIGH)rules/1-2-separate-state.md- Separate Public Properties from Internal State (HIGH)rules/1-3-reflect-sparingly.md- Reflect Properties Sparingly (MEDIUM)rules/1-4-default-values.md- Always Provide Default Values (HIGH)
2. Rendering
rules/2-1-pure-render.md- Keep render() Pure (CRITICAL)rules/2-2-use-nothing.md- Use nothing for Empty Content (MEDIUM)rules/2-3-use-repeat.md- Use repeat() for Keyed Lists (HIGH)rules/2-4-use-cache.md- Use cache() for Conditional Subtrees (MEDIUM)rules/2-5-derived-state.md- Compute Derived State in willUpdate() (HIGH)
3. Styling
rules/3-1-static-styles.md- Always Use Static Styles (CRITICAL)rules/3-2-host-styling.md- Style the Host Element Properly (HIGH)rules/3-3-css-custom-properties.md- CSS Custom Properties for Theming (MEDIUM)rules/3-4-css-parts.md- CSS Parts for Deep Styling (MEDIUM)
4. Events
rules/4-1-composed-events.md- Dispatch Composed Events (CRITICAL)rules/4-2-event-naming.md- Event Naming Conventions (MEDIUM)rules/4-3-cleanup-listeners.md- Clean Up Event Listeners (HIGH)
5. Lifecycle
rules/5-1-super-call-order.md- Correct super() Call Order (CRITICAL)rules/5-2-first-updated.md- Use firstUpdated for DOM Operations (HIGH)rules/5-3-will-update.md- Use willUpdate for Derived State (HIGH)rules/5-4-update-complete.md- Async Operations with updateComplete (MEDIUM)
6. Accessibility
rules/6-1-delegates-focus.md- delegatesFocus for Interactive Components (HIGH)rules/6-2-aria-attributes.md- ARIA for Custom Interactive Components (CRITICAL)rules/6-3-form-associated.md- Form-Associated Custom Elements (HIGH)
7. Performance
rules/7-1-has-changed.md- Custom hasChanged for Complex Types (HIGH)rules/7-2-batch-updates.md- Batch Property Updates (MEDIUM)rules/7-3-lazy-loading.md- Lazy Load Heavy Dependencies (HIGH)rules/7-4-memoization.md- Memoize Expensive Computations (MEDIUM)
Quick Reference
Essential Imports
// Core
import { LitElement, html, css, nothing } from 'lit';
import { customElement, property, state, query } from 'lit/decorators.js';
// Common Directives
import { repeat } from 'lit/directives/repeat.js';
import { cache } from 'lit/directives/cache.js';
import { classMap } from 'lit/directives/class-map.js';
import { until } from 'lit/directives/until.js';Component Skeleton
@customElement('my-component')
export class MyComponent extends LitElement {
static styles = css`
:host { display: block; }
:host([hidden]) { display: none; }
`;
@property({ type: String }) value = '';
@property({ type: Boolean, reflect: true }) disabled = false;
@state() private _internal = '';
render() {
return html`<slot></slot>`;
}
}