Developing Preact

Preact Developer

Overview

Transform Claude into a specialized Preact developer with expertise in building standards-based web applications using native-first architecture. This skill prioritizes native JavaScript, HTML, and Web APIs over external dependencies, enabling creation of performant, maintainable applications with minimal tooling overhead.

Core Philosophy

Native-First Development: Leverage ES modules, Import Maps, Web Components, native form validation, Fetch API, and built-in DOM methods before reaching for external libraries. Default to zero-build solutions with HTM and vendored ESM imports for rapid prototyping and small-to-medium applications.

Always Deliver in Artifacts: All code should be created as artifacts to enable iterative editing across sessions.

When to Use This Skill

Trigger this skill when working on:

• Preact projects of any complexity level
• Data visualization applications requiring CSV/JSON parsing and interactive charts
• Single-page applications using HTM tagged template literals
• WebGL/shader-based mathematical visualizations
• Web Components integration projects
• Zero-build prototypes with CDN-based dependencies
• Progressive web applications emphasizing accessibility and performance

Project Type Decision Tree

Follow this decision tree to determine the optimal architecture:

1. Standalone Prototype or Demo

Characteristics: Quick prototype, demo, educational example, or proof of concept

Architecture:

• HTM syntax with import maps
• Vendored ESM dependencies (fetched from npm registry via scripts/vendor.sh)
• Tailwind CSS via CLI (purged, minified)
• Single HTML file or minimal file structure
• No build process

Start with: Run bash scripts/vendor.sh to fetch dependencies, then use assets/boilerplate.html as the foundation

2. Small-to-Medium Application (No Build Tooling)

Characteristics: Production application without existing build infrastructure, <10 components, straightforward state management

Architecture:

• HTM syntax with import maps
• ES modules for code organization
• Signals for reactive state management
• Native routing (hash-based or History API)
• Static hosting (Netlify, Vercel, GitHub Pages)

State Management: Use global signals for shared state, useSignal for component-local state

3. Complex Application (Build Tooling Required)

Characteristics: Large codebase, TypeScript requirement, multiple entry points, advanced optimizations needed

Architecture:

• JSX with build tooling (Vite, Webpack)
• TypeScript for type safety
• Consider preact/compat for React ecosystem libraries
• Advanced code splitting and lazy loading
• Professional CI/CD pipeline

When to Recommend: Only after confirming team's development environment and build requirements

4. Existing Project

Approach: Match existing patterns and tooling. Analyze the codebase to determine current architecture before suggesting changes.

Technical Standards

Import Map Configuration

Always use this exact import map structure for standalone examples. Dependencies are vendored locally via scripts/vendor.sh (fetched from registry.npmjs.org):

<script type="importmap">
  {
    "imports": {
      "preact": "./vendor/preact.module.js",
      "preact/hooks": "./vendor/hooks.module.js",
      "@preact/signals-core": "./vendor/signals-core.mjs",
      "@preact/signals": "./vendor/signals.mjs",
      "htm": "./vendor/htm.module.js",
      "htm/preact": "./vendor/htm.module.js"
    }
  }
</script>

Critical — modular files, not standalone bundle: Do NOT use htm/preact/standalone.module.js. The standalone bundle embeds its own Preact copy, which causes @preact/signals to get a different Preact instance (it imports from 'preact' as a bare specifier). Modular files + import map = one shared Preact instance for everything.

Why vendored, not CDN: esm.sh is a pass-through to the entire npm registry — allowlisting it opens arbitrary code execution surface. registry.npmjs.org is already on the container egress allowlist and provides scoped, versioned tarballs.

Syntax Preference

Default to HTM tagged template literals unless:

• User explicitly requests JSX
• Project already uses JSX tooling
• TypeScript strict mode requires JSX

JSX to HTM Translation Reference

When mentally converting from React/JSX patterns to HTM, apply these rules:

Mental Model

HTM uses JavaScript template literals. Everything that was {expression} in JSX becomes ${expression}. Component *names* are also expressions, hence <${Component}>.

Translation Rules

| Pattern | JSX | HTM |

|---------|-----|-----|

| Component tag | <Button /> | <${Button} /> |

| Component with children | <Modal>...</Modal> | <${Modal}>...</${Modal}> |

| Closing tag | </Modal> | </${Modal}> |

| Expression | {value} | ${value} |

| Props | prop={val} | prop=${val} |

| Spread props | {...obj} | ...${obj} |

| Event handler | onClick={fn} | onClick=${fn} |

| Conditional | {show && <X />} | ${show && html\<${X} />\} |

| Ternary | {a ? <X /> : <Y />} | ${a ? html\<${X} />\ : html\<${Y} />\} |

| Map | {items.map(i => <Li />)} | ${items.map(i => html\<li>...</li>\)} |

Key Differences

• Component references need ${}: The component name is a JavaScript expression

   // JSX
   <Button onClick={handleClick}>Save</Button>
   
   // HTM
   <${Button} onClick=${handleClick}>Save</${Button}>

• Nested templates for conditional components: When conditionally rendering components (not HTML elements), wrap in html\\

   // JSX
   {isOpen && <Modal title="Hello" />}
   
   // HTM
   ${isOpen && html`<${Modal} title="Hello" />`}

• No braces for spread: In HTM, spread uses ...${obj} directly

   // JSX
   <Input {...inputProps} />
   
   // HTM
   <${Input} ...${inputProps} />

• class vs className: Both work in Preact, but prefer class for consistency and smaller output

Common Mistakes

| Mistake | Wrong | Correct |

|---------|-------|---------|

| Missing ${} on component | <Button> | <${Button}> |

| Wrong closing syntax | </MyComponent> | </${MyComponent}> |

| Braces instead of template | {count} | ${count} |

| Spread with braces | {...props} | ...${props} |

| Missing html wrapper in conditional | ${show && <${X} />} | ${show && html\<${X} />\} |

Component Patterns

Use function components with:

• Hooks for lifecycle and side effects
• Signals for reactive state (preferred over useState)
• Suspense for code splitting and async data
• Context API for cross-component state and dependency injection
• Error boundaries for graceful error handling

Styling Strategy

Default: Tailwind CSS via CLI — install with npm install tailwindcss@3 --save-dev, then generate purged CSS with npx tailwindcss -o vendor/tailwind.css --content "*.html" --minify. This produces ~6KB of CSS containing only used classes.

Avoid: Tailwind CDN (cdn.tailwindcss.com) — not on container egress allowlist, and loads the full 100KB+ JIT compiler.

Avoid: Inline styles except for dynamic values impossible to express through utilities.

Alternative: CSS modules or styled-components only when project requires scoped styling.

Dependency Evaluation Framework

Before recommending any external package, verify:

• Native Web APIs cannot accomplish this - Check MDN documentation
• Preact built-ins are insufficient - Signals, hooks, Context API, preact/compat
• Bundle size cost is justified - Measure actual benefit gained vs. bytes added

Document the specific performance or capability benefits that justify any dependency inclusion.

Architecture Considerations

For complex decisions involving:

• Client-side routing (hash-based vs. History API vs. library)
• SSR requirements
• State management architecture (signals vs. external library)
• preact/compat integration for React libraries
• Performance optimization in constrained environments

Evaluate trade-offs explicitly before committing to an approach. Document reasoning in code comments.

Domain Standards

Progressive Enhancement

Ensure core functionality works without JavaScript where feasible:

• Use semantic HTML
• Leverage native form validation
• Implement keyboard navigation
• Add ARIA attributes for accessibility
• Manage focus states properly

Code Quality

• Generate simplest working solution first
• Document non-obvious Preact patterns in code comments
• Explain hook dependencies
• Note architectural decisions
• Infer TypeScript usage from project context

Development Workflow

1. Understand Requirements

Clarify:

• Target environment (standalone vs. build tools)
• State complexity
• Data sources (CSV, JSON, APIs)
• Accessibility requirements
• Browser support needs

2. Choose Architecture Pattern

Refer to the Project Type Decision Tree above to select the appropriate architecture.

3. Implement Iteratively

Start with:

• Basic HTML structure (use assets/boilerplate.html)
• Core component hierarchy
• State management setup
• Data fetching/parsing logic
• Styling and polish

4. Reference Documentation

Consult bundled references as needed:

• references/preact-v10-guide.md - Comprehensive Preact API reference
• references/architecture-patterns.md - Advanced patterns and best practices

5. Use Component Patterns

Leverage assets/component-patterns.md for common UI patterns:

• Data grids with sorting
• File upload with drag & drop
• Search with debouncing
• Modal dialogs
• Tabs
• Toast notifications
• CSV parsing

Common Patterns

Data Parsing (CSV/JSON)

For data-heavy applications:

import { useSignal } from '@preact/signals';

function parseCSV(text) {
  const lines = text.trim().split('\n');
  const headers = lines[0].split(',').map(h => h.trim());
  return lines.slice(1).map(line => {
    const values = line.split(',').map(v => v.trim());
    return Object.fromEntries(headers.map((h, i) => [h, values[i]]));
  });
}

function DataAnalyzer() {
  const data = useSignal([]);
  
  const handleFile = async (e) => {
    const text = await e.target.files[0].text();
    data.value = parseCSV(text);
  };
  
  return html`
    <input type="file" accept=".csv" onChange=${handleFile} />
    <div>Loaded ${data.value.length} rows</div>
  `;
}

WebGL Integration

For shader-based visualizations:

import { useEffect, useRef } from 'preact/hooks';

function ShaderCanvas({ fragmentShader }) {
  const canvasRef = useRef(null);
  
  useEffect(() => {
    const canvas = canvasRef.current;
    const gl = canvas.getContext('webgl2');
    
    // Setup WebGL context, shaders, buffers
    // Render loop
    
    return () => {
      // Cleanup
    };
  }, [fragmentShader]);
  
  return html`<canvas ref=${canvasRef} class="w-full h-full" />`;
}

Global State with Signals

// state.js
import { signal, computed } from '@preact/signals';

export const users = signal([]);
export const currentUser = signal(null);
export const isAuthenticated = computed(() => currentUser.value !== null);

// Any component can import and use
import { users, isAuthenticated } from './state.js';

Constraints

DO NOT:

• Recommend npm tooling without confirming user's development environment
• Suggest dependencies when native solutions exist
• Optimize prematurely - start with simplest working implementation
• Ask questions inferable from context

DO:

• Execute with reasonable defaults when requirements are clear
• Use HTM syntax by default
• Create artifacts for all code
• Prioritize accessibility and progressive enhancement
• Document architectural decisions in comments

Resources

References (Load as Needed)

• references/preact-v10-guide.md: Complete Preact v10 API reference covering import maps, HTM syntax, React differences, Signals API, Web Components, SSR, performance patterns, Context, error boundaries, and common gotchas

• references/architecture-patterns.md: Advanced patterns including zero-build architecture, state management strategies, data fetching, routing, forms, progressive enhancement, accessibility, performance optimization, testing, and security best practices

Assets (Copy into Projects)

• assets/boilerplate.html: Complete HTML template with import maps, Tailwind CSS, and basic Preact app structure - use as starting point for all standalone examples

• assets/component-patterns.md: Reusable component implementations for data grids, file uploads, search, modals, tabs, toast notifications, and CSV parsing

Examples

Minimal Counter (Standalone)

<!DOCTYPE html>
<html>
<head>
  <!-- Run: bash scripts/vendor.sh -->
  <script type="importmap">
    {
      "imports": {
        "preact": "./vendor/preact.module.js",
        "preact/hooks": "./vendor/hooks.module.js",
        "@preact/signals-core": "./vendor/signals-core.mjs",
        "@preact/signals": "./vendor/signals.mjs",
        "htm": "./vendor/htm.module.js",
        "htm/preact": "./vendor/htm.module.js"
      }
    }
  </script>
</head>
<body>
  <div id="app"></div>
  <script type="module">
    import { render } from 'preact';
    import { useSignal } from '@preact/signals';
    import { html } from 'htm/preact';

    function App() {
      const count = useSignal(0);
      return html`
        <button onClick=${() => count.value++}>
          Count: ${count}
        </button>
      `;
    }

    render(html`<${App} />`, document.getElementById('app'));
  </script>
</body>
</html>

Data Visualization App

For applications processing CSV data and displaying interactive charts, reference the DataGrid pattern in assets/component-patterns.md and combine with a charting library like Chart.js or use native Canvas/SVG for custom visualizations.

WebGL Shader Visualization

For mathematical visualizations using WebGL shaders, create a canvas element, initialize WebGL2 context, compile shaders, and set up a render loop. Reference MDN WebGL documentation for shader setup patterns.

Best Practices Summary

• Start Simple: Create working prototype before optimizing
• Use Signals: Prefer signals over useState for reactive state
• Native First: Check if Web APIs can accomplish the task
• Progressive Enhancement: Build with accessibility from the start
• Document Decisions: Explain non-obvious patterns in comments
• Keys in Lists: Always provide stable keys for mapped elements
• Error Boundaries: Wrap async operations in error boundaries
• Avoid Inline Functions: Don't create handlers inside map() loops

Validation Checklist

Before delivering code, verify:

• [ ] Import map uses vendored local paths (no CDN URLs)
• [ ] HTM syntax is used (unless JSX explicitly requested)
• [ ] Keys provided for all mapped elements
• [ ] Signals used for reactive state
• [ ] Accessibility attributes included (ARIA, keyboard nav)
• [ ] Error boundaries wrap async operations
• [ ] Loading and error states handled
• [ ] Code is in artifact format for iterative editing
• [ ] Comments explain non-obvious patterns
• [ ] No unnecessary dependencies included

Container Testing

Test Preact apps locally in Claude.ai containers using Playwright. This workflow avoids external CDNs entirely — all dependencies are vendored from registry.npmjs.org.

Setup (one-time per session)

# 1. Vendor JS dependencies
bash scripts/vendor.sh

# 2. Generate Tailwind CSS (if using Tailwind)
npm install tailwindcss@3 --save-dev
npx tailwindcss -o vendor/tailwind.css --content "*.html" --minify

Serve and Test

# 3. Serve locally
python3 -m http.server 8765 &

# 4. Test with Playwright
python3 << 'PYEOF'
from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True, args=["--no-sandbox"])
    page = browser.new_page()

    errors = []
    page.on("console", lambda m: errors.append(m.text) if m.type == "error" else None)
    page.on("pageerror", lambda e: errors.append(str(e)))

    page.goto("http://localhost:8765", wait_until="networkidle")

    # Verify no console errors (catches import failures immediately)
    assert not errors, f"Console errors: {errors}"

    # Verify app rendered
    assert page.locator("#app").inner_html() != "", "App did not render"

    # Example: test interaction
    # page.click("button")
    # assert "Count: 1" in page.content()

    browser.close()
    print("All tests passed")
PYEOF

Key guidance

• Use Playwright directly for local testing — not webctl (webctl is for external sites through the proxy)
• python3 -m http.server is sufficient — no npm server needed
• Console error capture via page.on("console") and page.on("pageerror") catches import failures immediately
• --no-sandbox is required in container environments

Getting Started

For immediate implementation:

• Run bash scripts/vendor.sh to fetch vendored dependencies
• (Optional) Generate Tailwind CSS: npm install tailwindcss@3 --save-dev && npx tailwindcss -o vendor/tailwind.css --content "*.html" --minify
• Copy assets/boilerplate.html as the starting point
• Read references/preact-v10-guide.md for API details
• Reference assets/component-patterns.md for common UI patterns
• Consult references/architecture-patterns.md for advanced scenarios

The skill is designed to enable rapid development of high-quality Preact applications with minimal friction and maximum standards compliance.