Chrome Extension Development

This skill provides expert-level guidance for Chrome extension development, covering JavaScript/TypeScript, browser extension APIs, and modern web development practices.

Workflow: Building a Chrome Extension from Scratch

1. Initialize the project — Create the directory structure with manifest.json, background service worker, content scripts, and popup files.
2. Configure the manifest — Define permissions, content script matches, service worker registration, and action settings in Manifest V3 format.
3. Implement the background service worker — Set up event listeners for extension lifecycle, messaging, and alarms using the chrome.* API.
4. Build content scripts — Write scripts that interact with web page DOM, communicate with the background worker via chrome.runtime.sendMessage, and respect CSP.
5. Create the popup UI — Design the popup HTML/CSS and wire up interactivity with the background and content scripts.
6. Add storage and state management — Use chrome.storage.local or chrome.storage.sync to persist user settings and extension state.
7. Test and debug — Load the extension unpacked via chrome://extensions, use Chrome DevTools to inspect the service worker and content scripts, and run unit tests.
8. Package and publish — Prepare store assets (icons, screenshots, description), create a privacy policy, and submit to the Chrome Web Store.

Example: Minimal Manifest V3 Configuration

{
  "manifest_version": 3,
  "name": "My Extension",
  "version": "1.0.0",
  "description": "A sample Chrome extension using Manifest V3",
  "permissions": ["storage", "activeTab"],
  "action": {
    "default_popup": "popup/popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "background": {
    "service_worker": "background/service-worker.js",
    "type": "module"
  },
  "content_scripts": [
    {
      "matches": ["https://*.example.com/*"],
      "js": ["content/content-script.js"],
      "css": ["content/styles.css"]
    }
  ]
}

Example: Background Service Worker with Messaging

// background/service-worker.ts

// Listen for extension install or update
chrome.runtime.onInstalled.addListener((details) => {
  if (details.reason === 'install') {
    chrome.storage.local.set({ initialized: true, count: 0 });
    console.log('Extension installed');
  }
});

// Handle messages from content scripts or popup
chrome.runtime.onMessage.addListener(
  (message: { type: string; payload?: unknown }, sender, sendResponse) => {
    if (message.type === 'GET_COUNT') {
      chrome.storage.local.get('count', (result) => {
        sendResponse({ count: result.count ?? 0 });
      });
      return true; // keep message channel open for async response
    }

    if (message.type === 'INCREMENT') {
      chrome.storage.local.get('count', (result) => {
        const newCount = (result.count ?? 0) + 1;
        chrome.storage.local.set({ count: newCount }, () => {
          sendResponse({ count: newCount });
        });
      });
      return true;
    }
  }
);

// Schedule periodic tasks with chrome.alarms
chrome.alarms.create('sync-data', { periodInMinutes: 30 });
chrome.alarms.onAlarm.addListener((alarm) => {
  if (alarm.name === 'sync-data') {
    console.log('Running scheduled sync');
  }
});

Code Style and Structure

• Write clear, modular TypeScript code with proper type definitions
• Follow functional programming patterns; avoid classes
• Use descriptive variable names (e.g., isLoading, hasPermission)
• Structure files logically: popup, background, content scripts, utils
• Implement proper error handling and logging
• Document code with JSDoc comments

Architecture and Best Practices

• Strictly follow Manifest V3 specifications
• Divide responsibilities between background, content scripts and popup
• Configure permissions following the principle of least privilege
• Use modern build tools (webpack/vite) for development
• Implement proper version control and change management

Chrome API Usage

• Use chrome.* APIs correctly (storage, tabs, runtime, etc.)
• Handle asynchronous operations with Promises
• Use Service Worker for background scripts (MV3 requirement)
• Implement chrome.alarms for scheduled tasks
• Use chrome.action API for browser actions
• Handle offline functionality gracefully

Security and Privacy

• Implement Content Security Policy (CSP)
• Handle user data securely
• Prevent XSS and injection attacks
• Use secure messaging between components
• Handle cross-origin requests safely
• Implement secure data encryption
• Follow web_accessible_resources best practices

Performance and Optimization

• Minimize resource usage and avoid memory leaks
• Optimize background script performance
• Implement proper caching mechanisms
• Handle asynchronous operations efficiently
• Monitor and optimize CPU/memory usage

UI and User Experience

• Follow Material Design guidelines
• Implement responsive popup windows
• Provide clear user feedback
• Support keyboard navigation
• Ensure proper loading states
• Add appropriate animations

Internationalization

• Use chrome.i18n API for translations
• Follow _locales structure
• Support RTL languages
• Handle regional formats

Accessibility

• Implement ARIA labels
• Ensure sufficient color contrast
• Support screen readers
• Add keyboard shortcuts

Testing and Debugging

• Use Chrome DevTools effectively
• Write unit and integration tests
• Test cross-browser compatibility
• Monitor performance metrics
• Handle error scenarios

Publishing and Maintenance

• Prepare store listings and screenshots
• Write clear privacy policies
• Implement update mechanisms
• Handle user feedback
• Maintain documentation

Follow Official Documentation

• Refer to Chrome Extension documentation
• Stay updated with Manifest V3 changes
• Follow Chrome Web Store guidelines
• Monitor Chrome platform updates

Output Expectations

• Provide clear, working code examples
• Include necessary error handling
• Follow security best practices
• Ensure cross-browser compatibility
• Write maintainable and scalable code