End-to-end guidance for AppRun apps in TypeScript using MVU: component patterns, event handling, state management (including async generators), routing/navigation with params and guards, and testing with vitest. Use when designing or reviewing AppRun components, wiring routes, managing state flows, or writing AppRun tests.
Scanned 5/31/2026
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Skills Directoryopenskills install diegosouzapw/awesome-omni-skill---
name: apprun-skills
description: "End-to-end guidance for AppRun apps in TypeScript using MVU: component patterns, event handling, state management (including async generators), routing/navigation with params and guards, and testing with vitest. Use when designing or reviewing AppRun components, wiring routes, managing state flows, or writing AppRun tests."
---
# AppRun Skills
## Overview
- Build AppRun apps with MVU (Model-View-Update) in TypeScript.
- Prefer pure update functions for testability.
- Use `mounted()` for components embedded in JSX.
- Use `state = async` only for top-level routed pages that must load async data.
## Project Setup
### Recommended Project Structure
```
web/ # Frontend application root
├── index.html # Entry HTML file
├── package.json # Dependencies and scripts
├── vite.config.js # Vite configuration
├── src/
│ ├── main.tsx # Application entry point (routes registration)
│ ├── api.ts # REST API client (optional)
│ ├── styles.css # Application styles
│ ├── tsconfig.json # TypeScript configuration
│ ├── components/ # Reusable UI components
│ │ ├── Layout.tsx # Root layout container
│ │ └── ... # Other reusable components
│ ├── domain/ # Business logic modules (optional)
│ │ └── ... # Pure functions and business logic
│ ├── pages/ # Top-level page components
│ │ ├── Home.tsx # Example: Home page
│ │ └── ... # Other route pages
│ ├── types/ # TypeScript type definitions
│ │ ├── index.ts # Shared types
│ │ └── jsx.d.ts # JSX type declarations
│ └── utils/ # Utility functions
└── public/ # Static assets (optional)
```
### Vite Configuration
```javascript
import { defineConfig } from 'vite'
export default defineConfig({
build: {
outDir: 'dist',
emptyOutDir: true,
},
server: {
port: 8080,
open: true,
historyApiFallback: true, // SPA mode
proxy: {
// Proxy API requests to backend
'/api': {
target: 'http://127.0.0.1:3000',
changeOrigin: true,
secure: false
}
}
}
})
```
### Package.json
```json
{
"name": "my-apprun-app",
"version": "1.0.0",
"type": "module",
"scripts": {
"dev": "vite",
"build": "vite build",
"preview": "vite preview",
"check": "tsc --noEmit"
},
"devDependencies": {
"apprun": "^3.38.0",
"typescript": "^5.0.0",
"vite": "^5.0.0"
}
}
```
### TypeScript Configuration
```json
{
"compilerOptions": {
"target": "ES2020",
"module": "ESNext",
"lib": ["ES2020", "DOM", "DOM.Iterable"],
"jsx": "react",
"jsxFactory": "app.createElement",
"jsxFragmentFactory": "app.Fragment",
"moduleResolution": "bundler",
"resolveJsonModule": true,
"isolatedModules": true,
"strict": true,
"esModuleInterop": true,
"skipLibCheck": true,
"forceConsistentCasingInFileNames": true
},
"include": ["src/**/*"],
"exclude": ["node_modules"]
}
```
**Critical Settings for AppRun:**
- `jsx: "react"` - Enables JSX syntax
- `jsxFactory: "app.createElement"` - Uses AppRun's JSX factory
- `jsxFragmentFactory: "app.Fragment"` - Uses AppRun's Fragment support
- `moduleResolution: "bundler"` - Optimized for Vite
### Entry Points
**HTML Entry (`index.html`):**
```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>My AppRun App</title>
</head>
<body>
<div id="root"></div>
<script type="module" src="src/main.tsx"></script>
</body>
</html>
```
**Application Entry (`src/main.tsx`):**
```tsx
import app from 'apprun';
import Layout from './components/Layout';
import Home from './pages/Home';
import About from './pages/About';
import './styles.css';
app.render('#root', <Layout />);
app.addComponents('#pages', {
'/': Home,
'/about': About,
});
```
**Layout Component (`src/components/Layout.tsx`):**
```tsx
import app from 'apprun';
export default () => (
<div id="app">
<div id="pages"></div>
</div>
);
```
### Styling Options
**Option 1: Vanilla CSS**
```css
/* src/styles.css */
:root {
--color-primary: #007bff;
--color-text: #333;
--spacing-unit: 8px;
}
body {
font-family: system-ui, -apple-system, sans-serif;
color: var(--color-text);
margin: 0;
padding: 0;
}
```
**Option 2: Tailwind CSS v4**
Install Tailwind v4:
```bash
npm install -D tailwindcss@next @tailwindcss/vite@next
```
Update `vite.config.js`:
```javascript
import { defineConfig } from 'vite'
import tailwindcss from '@tailwindcss/vite'
export default defineConfig({
plugins: [tailwindcss()],
// ... other config
})
```
Import in `src/styles.css`:
```css
@import "tailwindcss";
```
Use in components:
```tsx
<div className="flex items-center gap-4 p-4 bg-white rounded-lg shadow">
<h1 className="text-2xl font-bold">Hello World</h1>
</div>
```
**Option 3: CSS Modules**
```tsx
import styles from './MyComponent.module.css';
export default () => (
<div className={styles.container}>
<h1 className={styles.title}>Hello</h1>
</div>
);
```
### API Client Pattern
```typescript
// src/api.ts
const API_BASE_URL = '/api';
interface RequestOptions extends RequestInit {
params?: Record<string, string>;
}
async function request<T>(
endpoint: string,
options: RequestOptions = {}
): Promise<T> {
const { params, ...fetchOptions } = options;
let url = `${API_BASE_URL}${endpoint}`;
if (params) {
const query = new URLSearchParams(params).toString();
url += `?${query}`;
}
const response = await fetch(url, {
headers: {
'Content-Type': 'application/json',
...fetchOptions.headers,
},
...fetchOptions,
});
if (!response.ok) {
const error = await response.json().catch(() => ({}));
throw new Error(error.message || `HTTP ${response.status}`);
}
return response.json();
}
export const api = {
get: <T>(endpoint: string, params?: Record<string, string>) =>
request<T>(endpoint, { method: 'GET', params }),
post: <T>(endpoint: string, data?: unknown) =>
request<T>(endpoint, {
method: 'POST',
body: JSON.stringify(data),
}),
put: <T>(endpoint: string, data?: unknown) =>
request<T>(endpoint, {
method: 'PUT',
body: JSON.stringify(data),
}),
delete: <T>(endpoint: string) =>
request<T>(endpoint, { method: 'DELETE' }),
};
export default api;
```
### Quick Start
```bash
# 1. Create project
npm create vite@latest my-apprun-app -- --template vanilla-ts
cd my-apprun-app
# 2. Install AppRun
npm install
npm install -D apprun
# 3. Configure TypeScript (update tsconfig.json with settings above)
# 4. Rename entry file
mv src/main.ts src/main.tsx
# 5. Create basic app structure
# (Add Layout, pages, components as shown above)
# 6. Run development server
npm run dev
# 7. Build for production
npm run build
npm run preview
```
### Why Vite + AppRun?
**Why Vite:**
- Fast development with instant HMR
- Optimized builds with Rollup
- First-class TypeScript support
- Minimal configuration
**Why AppRun:**
- Lightweight (~7KB gzipped)
- Simple MVU pattern
- Direct DOM updates (no virtual DOM)
- Full TypeScript support
- Built-in routing
## Component Patterns - Decision Tree
1. **Manages state + user interactions?** → Stateful Class Component
2. **Popup/modal/overlay?** → Modal Component (use `mounted()`)
3. **Display-only from props?** → Functional Component
4. **10+ events needing type safety?** → Typed Events Pattern
## Stateful Class Component
**Structure Order:** Imports → Interfaces → Helpers → Actions → Component
```typescript
import { app, Component } from 'apprun';
interface Props { data?: any; }
export interface State {
loading: boolean;
error: string | null;
successMessage?: string;
// ... specific fields
}
const getStateFromProps = (props: Props): State => ({ /* ... */ });
export const saveData = async function* (state: State): AsyncGenerator<State> {
// Validation
if (!state.data.name.trim()) {
yield { ...state, error: 'Name required' };
return;
}
// Loading
yield { ...state, loading: true, error: null };
// API call
try {
await api.save(state.data);
yield { ...state, loading: false, successMessage: 'Saved!' };
app.run('data-saved');
} catch (error: any) {
yield { ...state, loading: false, error: error.message };
}
};
export default class MyComponent extends Component<State> {
declare props: Readonly<Props>;
mounted = (props: Props): State => getStateFromProps(props);
view = (state: State) => {
if (state.loading) return <div>Loading...</div>;
if (state.error) return <div className="error">{state.error}</div>;
return (
<form>
<input $bind="data.name" />
<button $onclick={[saveData]} disabled={state.loading}>Save</button>
</form>
);
};
}
```
**View Pattern:** Guard clauses → Early returns → Main content
## Modal Component
**CRITICAL:** Must use `mounted()` (embedded in JSX), not `state = async`
```typescript
export default class Modal extends Component<State> {
declare props: Readonly<Props>;
mounted = (props: Props): State => getStateFromProps(props);
view = (state: State) => (
<div className="modal-backdrop" onclick={closeModal}>
<div className="modal-content" onclick={(e) => e.stopPropagation()}>
<button onclick={closeModal}>×</button>
{/* content */}
</div>
</div>
);
}
```
**Requirements:** Close button + backdrop click + stopPropagation
## Functional Component
```typescript
export interface Props {
data: DataType[];
onItemClick?: (item: DataType) => void;
}
export default function DisplayComponent({ data, onItemClick }: Props) {
if (!data?.length) return <div>No items</div>;
return (
<ul>
{data.map(item => (
<li onclick={() => onItemClick?.(item)}>{item.name}</li>
))}
</ul>
);
}
```
**Pattern:** Destructure → Guard clauses → Main render
## Typed Events Pattern
**Payload Rules:**
1. Single value → `payload: string` | Call: `$onclick={['delete', id]}`
2. Multiple values → `payload: { id: string; name: string }` | Call: `$onclick={['edit', { id, name }]}`
3. No payload → `payload: void` | Call: `$onclick="save"`
4. Input events → `payload: { target: { value: string } }`
```typescript
// types/events.ts
export type MyEvents =
| { name: 'save'; payload: void }
| { name: 'delete'; payload: string }
| { name: 'edit'; payload: { id: string; name: string } };
export type MyEventName = MyEvents['name'];
// Component
class MyComponent extends Component<State, MyEventName> {
override update = myHandlers;
}
// Handlers (OBJECT format, not array)
export const myHandlers: Update<State, MyEventName> = {
save: (state): State => ({ ...state, saved: true }),
delete: (state, id: string): State => ({
...state,
items: state.items.filter(i => i.id !== id)
}),
edit: (state, { id, name }: { id: string; name: string }): State => ({
...state,
editing: { id, name }
})
};
```
**stopPropagation:** Add event as last parameter
```typescript
'click-item': (state, id: string, e?: Event): State => {
e?.stopPropagation();
return { ...state, selected: id };
}
```
## Event Directives
### AppRun Directives (Trigger Update Handlers)
| Directive | Use Case | Example |
|-----------|----------|---------|
| `$bind="field"` | Two-way binding (PREFERRED for forms) | `<input $bind="name" />` |
| `$bind="nested.field"` | Nested property | `<input $bind="user.profile.name" />` |
| `$onclick="action"` | String action | `<button $onclick="save" />` |
| `$onclick={['action', data]}` | Action with params | `<button $onclick={['delete', id]} />` |
| `$onclick={[func]}` | Direct function | `<button $onclick={[saveData]} />` |
| `$oninput="handler"` | Custom input handling | `<input $oninput="validate" />` |
**Other directives:** `$onchange`, `$onsubmit`, `$onfocus`, `$onblur`, `$onkeydown`
### Standard HTML Events (DOM Manipulation)
Use `onclick`, `oninput`, etc. for direct DOM manipulation only:
```tsx
<div onclick={(e) => e.stopPropagation()}>Content</div>
```
### When to Use What
- ✅ **`$bind`** - Simple form fields (no handler needed)
- ✅ **`$oninput`** - Validation, transformation, debouncing
- ✅ **`$onclick`** - Trigger update handlers
- ❌ **Never** - `$onclick={() => app.run('action')}`
**Validation Example:**
```typescript
$oninput="validate-email"
'validate-email': (state, e: Event) => {
const email = (e.target as HTMLInputElement).value;
const valid = /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
return { ...state, email, emailError: valid ? null : 'Invalid' };
}
```
## Update Handlers
**Sync:** Return new state
```typescript
'increment': (state) => ({ ...state, count: state.count + 1 })
```
**Async:** Use `async`
```typescript
'load': async (state) => {
this.setState({ ...state, loading: true });
const data = await api.fetch();
return { ...state, data, loading: false };
}
```
**Generator:** Multi-step with intermediate renders (PREFERRED for complex flows)
```typescript
'save': async function* (state) {
yield { ...state, loading: true };
await api.save(state.data);
yield { ...state, loading: false, success: true };
}
```
**Side Effects:** No return = no re-render
```typescript
'navigate': (state) => {
window.location.href = '/path';
// No return - no re-render
}
```
## Component Communication
| Pattern | Use Case | Implementation |
|---------|----------|----------------|
| Props | Parent → Child | Pass data via props |
| Callbacks | Child → Parent | Pass function via props |
| Global Events | Any → Any | `is_global_event = () => true` |
**Global Events:**
```typescript
// Modal component
class Modal extends Component {
is_global_event = () => true;
update = {
'open-modal': (state, data) => ({ ...state, visible: true, data }),
'close-modal': (state) => ({ ...state, visible: false })
};
}
// Any component can trigger
<button onclick={() => app.run('open-modal', data)}>Open</button>
```
## Critical Rules
### State Initialization
| Component Type | Use | Example |
|----------------|-----|---------|
| JSX Embedded | `mounted()` | `mounted = (props) => getStateFromProps(props)` |
| Top-Level Routed | `state = async` | `state = async () => { const data = await api.fetch(); return { data }; }` |
**❌ NEVER mix both** `mounted()` and `state = async`
### State Updates
**Returning state triggers re-render:**
- Immutable (recommended): `return { ...state, field: value }`
- Mutable (allowed): `state.field = value; return state`
- Side effects only: Don't return (no re-render)
### Required State Properties
```typescript
interface State {
loading: boolean; // For async operations
error: string | null; // For error messages
successMessage?: string; // For success feedback
// ... specific fields
}
```
### Deep Cloning
```typescript
// Nested object update
return {
...state,
user: {
...state.user,
profile: {
...state.user.profile,
name
}
}
};
```
## Anti-Patterns
**❌ DON'T:**
```typescript
// Don't use $onclick with arrow functions calling app.run
$onclick={() => app.run('action')}
// Don't forget error handling in async
async function save() { await api.save(); } // No try-catch!
// Don't use manual input when $bind available
$oninput={(e) => setState({ ...state, field: e.target.value })}
// Don't use state = async for JSX embedded components
class Modal extends Component {
state = async () => { /* WRONG */ };
}
// Don't forget defensive programming
messages.map() // messages might be undefined - use messages?.map()
// Don't use array format for update handlers
update = [['event', handler]] // WRONG - use object format
// Don't mutate state directly
state.count++; // WRONG
```
## Routing, Linking, and Component Registration
This section explains how AppRun applications handle routing, page navigation, and component registration.
### Overview
The app uses AppRun's built-in routing system without any external router libraries. Routes are defined declaratively, and navigation uses standard HTML anchor tags or programmatic methods.
### 1. Component Registration
Routes are registered centrally in `main.tsx`:
```tsx
import app from 'apprun';
import Layout from './components/Layout';
import Home from './pages/Home';
import World from './pages/World';
app.render('#root', <Layout />);
app.addComponents('#pages', {
'/': Home,
'/World': World,
// '/Agent': Agent, // commented out
// '/Settings': Settings, // commented out
});
```
**How It Works:**
- **`app.render('#root', <Layout />)`**: Renders the top-level Layout component into the `#root` DOM element
- **`app.addComponents('#pages', {...})`**: Registers route-to-component mappings
- Key: Route path (e.g., `'/'`, `'/World'`)
- Value: Component class (e.g., `Home`, `World`)
- Components are rendered into the `#pages` container defined in Layout
### 2. Layout Container
The Layout component provides the rendering container for routed pages:
```tsx
// web/src/components/Layout.tsx
export default () => <div id="main" className="w-full min-h-screen">
<div id="pages"></div>
</div>
```
- Minimal wrapper with full-width, full-height container
- The `#pages` div is where route components are dynamically rendered
- AppRun automatically swaps components based on the current route
### 3. Page Linking (Declarative Navigation)
The app uses **standard HTML anchor tags** for navigation:
**Example from Home Component:**
```tsx
// Navigate to a specific world
<a href={'/World/' + worldName}>
<button className="btn btn-primary">
Enter {worldName}
</button>
</a>
```
**Example from World Component:**
```tsx
// Navigate back to home
<a href="/">
<button className="back-button" title="Back to Worlds">
<span className="world-back-icon">←</span>
</button>
</a>
```
**How It Works:**
- Standard `<a href="">` links trigger AppRun's routing
- AppRun intercepts link clicks and updates the route without full page reload
- Route parameters (like world name) are included in the URL path
- No special Link component required—just plain HTML
### 4. Programmatic Navigation
Components can navigate programmatically using `window.location.href`:
**Example from Home Component Update Handler:**
```tsx
update = {
'enter-world': (state: HomeState, world: World): void => {
// Navigate to the world page
window.location.href = '/World/' + world.name;
}
}
```
**When to Use:**
- Inside event handlers that need to navigate after logic
- When navigation is a side effect (return `void` instead of new state)
- For conditional navigation based on user actions
### 5. Route Parameters
Routes can include dynamic parameters in the path:
**URL Pattern:**
```
/World/:worldName
```
**Parsing Parameters:**
Components can access route parameters from the URL:
```tsx
// Example: /World/MyWorld
const worldName = window.location.pathname.split('/')[2]; // "MyWorld"
```
**Route Handler Pattern:**
```tsx
update = {
'/World': async (state, worldName: string) => {
// worldName is parsed from the URL
return {
...state,
worldName,
// ... load world data
};
}
}
```
### 6. Component Architecture (MVU Pattern)
Page components follow AppRun's **Model-View-Update** pattern:
```tsx
export default class PageComponent extends Component<StateType> {
// 1. STATE: Initial data and loading states
state = {
loading: true,
data: null,
// ...
};
// 2. VIEW: Render function that returns JSX
view = (state: StateType) => {
return <div>
{/* JSX markup */}
</div>;
};
// 3. UPDATE: Event handlers
update = {
'event-name': (state, payload) => {
// Return new state to trigger re-render
return { ...state, newData: payload };
},
'navigation-event': (state) => {
// Return void for side effects (no re-render)
window.location.href = '/path';
}
};
}
```
**Key Principles:**
- **State**: Plain object with component data
- **View**: Pure function that converts state to JSX
- **Update**: Event handlers that return new state or void
- **Immutability**: Always return new state objects, never mutate
### 7. Event System
**Local vs Global Events:**
Components can be configured to listen to global events:
```tsx
export default class WorldComponent extends Component {
// Make all events global (visible across components)
override is_global_event = () => true;
}
```
**Event Propagation:**
- **Local events**: Only visible within the component
- **Global events**: Can be triggered from child components or other parts of the app
- Use `app.run('event-name', payload)` to trigger events programmatically
**Event Handler Types:**
```tsx
update = {
// Returns new state → triggers re-render
'update-data': (state, newData) => ({
...state,
data: newData
}),
// Returns void → no re-render (side effect only)
'navigate': (state) => {
window.location.href = '/path';
}
}
```
### 8. Best Practices
**Navigation:**
- ✅ Use `<a href="">` for simple links
- ✅ Use `window.location.href` for programmatic navigation
- ✅ Include route parameters in the path: `/World/${name}`
- ❌ Don't use client-side routing for external URLs
**Component Registration:**
- ✅ Register all routes in a single place (`main.tsx`)
- ✅ Use clear, semantic route paths
- ✅ Keep the route structure flat and simple
- ❌ Don't nest routes deeply
**Event Handling:**
- ✅ Return new state to trigger re-render
- ✅ Return void for navigation or side effects
- ✅ Use descriptive event names: `'load-world'`, `'delete-chat'`
- ❌ Don't mutate state directly
**URL Structure:**
```
/ → Home page (world selection)
/World/:name → World page (chat interface)
/Agent/:id → Agent page (currently disabled)
/Settings → Settings page (currently disabled)
```
### 9. Example Flow: Entering a World
**Step 1: User clicks "Enter World" button on Home page**
```tsx
// Home.tsx
<a href={'/World/' + world.name}>
<button className="btn btn-primary">
Enter {world.name}
</button>
</a>
```
**Step 2: AppRun intercepts the link and updates route**
- URL changes to `/World/MyWorld`
- AppRun's router detects the route change
- Router looks up the registered component for `/World`
**Step 3: World component is mounted and initialized**
```tsx
// World.tsx
update = {
'/World': async (state, worldName: string) => {
// Load world data from API
const world = await api.getWorld(worldName);
const messages = await api.getMessages(worldName);
return {
...state,
worldName,
world,
messages,
loading: false
};
}
}
```
**Step 4: World component renders with loaded data**
- View function receives the updated state
- Chat interface displays with agents and messages
- Component is now interactive and listening for events
### 10. Debugging Tips
**Check Current Route:**
```tsx
console.log(window.location.pathname); // "/World/MyWorld"
```
**Monitor Route Changes:**
```tsx
app.on('//', (route) => {
console.log('Route changed to:', route);
});
```
**Verify Component Registration:**
```tsx
// Check if component is registered for a route
// Look for the component rendering in #pages container
console.log(document.querySelector('#pages').innerHTML);
```
### Summary
- **Registration**: `app.addComponents('#pages', { path: Component })`
- **Navigation**: Use `<a href="">` or `window.location.href`
- **Route Params**: Parsed from URL path in route handlers
- **Component Pattern**: MVU (Model-View-Update)
- **Events**: Local by default, can be made global with `is_global_event()`
- **No Router Library**: AppRun's built-in routing handles everything
## Testing (Vitest)
- Unit test pure update functions.
- Iterate async generators to capture each yield.
- Mock APIs with `vi.mock`.
```typescript
import { describe, it, expect, vi } from 'vitest';
import { save } from './Form';
import api from '../api';
vi.mock('../api');
describe('save', () => {
it('yields validation then stops', async () => {
const state = { loading: false, error: null, form: { name: '' } } as State;
const gen = save(state);
const first = await gen.next();
expect(first.value?.error).toBe('Name is required');
});
});
```
## Development Checklist
### Component Structure
- [ ] Imports at top
- [ ] Props interface with `?` for optional
- [ ] State interface (exported)
- [ ] Helper functions
- [ ] Action functions (exported for `$onclick` and testing)
- [ ] Component class with `mounted` or `state = async`
### TypeScript Types
- [ ] Props interface
- [ ] State interface exported
- [ ] Event types for 10+ events (discriminated union)
- [ ] Generic types: `Component<State, EventName>`
- [ ] Async generators: `AsyncGenerator<State>`
### View Method
- [ ] Guard clauses first (loading, error, success)
- [ ] Early returns for special states
- [ ] Main content last
- [ ] Defensive programming (`data?.map()`, defaults)
### State Management
- [ ] Include `loading`, `error`, `successMessage?`
- [ ] Return new state to re-render
- [ ] Use `mounted()` for JSX embedded
- [ ] Use `state = async` only for routed pages
- [ ] Never mix both
### Event Handling
- [ ] Use `$bind` for simple forms
- [ ] Use `$onclick` (not `onclick={() => app.run()}`)
- [ ] Export action functions for reusability
- [ ] Use async generators for multi-step
- [ ] Add try-catch in async functions
### Error Handling
- [ ] Try-catch in async operations
- [ ] Error state in interface
- [ ] Error display in view
- [ ] Loading states during async
- [ ] Success messages
### Best Practices
- [ ] Keep update logic pure when possible
- [ ] Use global events for cross-component
- [ ] Add catch-all route for 404
- [ ] Test update logic and error paths
- [ ] Use descriptive event names
## Quick Reference
**Component Selection:**
- State + interactions → Stateful Class
- Modal/popup → Modal Component (`mounted()`)
- Display only → Functional
- 10+ events → Typed Events
**State Init:**
- JSX embedded → `mounted()`
- Routed page → `state = async`
**Events:**
- `$bind` for forms (preferred)
- `$onclick` for actions
- Typed for large components
**Updates:**
- Return state → re-render
- No return → side effect
- Generators → multi-step
**Communication:**
- Props: parent → child
- Callbacks: child → parent
- Global: any → any
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