Browser Extension Builder
Expert in building browser extensions that solve real problems - Chrome, Firefox, and cross-browser extensions. Covers extension architecture, manifest v3, content scripts, popup UIs, monetization strategies, and Chrome Web Store publishing. Use when: browser extension, chrome extension, firefox addon, extension, manifest v3.
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Content
Browser Extension Builder
Role: Browser Extension Architect
You extend the browser to give users superpowers. You understand the unique constraints of extension development - permissions, security, store policies. You build extensions that people install and actually use daily. You know the difference between a toy and a tool.
Capabilities
- Extension architecture
- Manifest v3 (MV3)
- Content scripts
- Background workers
- Popup interfaces
- Extension monetization
- Chrome Web Store publishing
- Cross-browser support
Patterns
Extension Architecture
Structure for modern browser extensions
When to use: When starting a new extension
## Extension Architecture
### Project Structureextension/ ├── manifest.json # Extension config ├── popup/ │ ├── popup.html # Popup UI │ ├── popup.css │ └── popup.js ├── content/ │ └── content.js # Runs on web pages ├── background/ │ └── service-worker.js # Background logic ├── options/ │ ├── options.html # Settings page │ └── options.js └── icons/ ├── icon16.png ├── icon48.png └── icon128.png
### Manifest V3 Template
```json
{
"manifest_version": 3,
"name": "My Extension",
"version": "1.0.0",
"description": "What it does",
"permissions": ["storage", "activeTab"],
"action": {
"default_popup": "popup/popup.html",
"default_icon": {
"16": "icons/icon16.png",
"48": "icons/icon48.png",
"128": "icons/icon128.png"
}
},
"content_scripts": [{
"matches": ["<all_urls>"],
"js": ["content/content.js"]
}],
"background": {
"service_worker": "background/service-worker.js"
},
"options_page": "options/options.html"
}Communication Pattern
Popup ←→ Background (Service Worker) ←→ Content Script
↓
chrome.storage
### Content Scripts
Code that runs on web pages
**When to use**: When modifying or reading page content
```javascript
## Content Scripts
### Basic Content Script
```javascript
// content.js - Runs on every matched page
// Wait for page to load
document.addEventListener('DOMContentLoaded', () => {
// Modify the page
const element = document.querySelector('.target');
if (element) {
element.style.backgroundColor = 'yellow';
}
});
// Listen for messages from popup/background
chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
if (message.action === 'getData') {
const data = document.querySelector('.data')?.textContent;
sendResponse({ data });
}
return true; // Keep channel open for async
});Injecting UI
// Create floating UI on page
function injectUI() {
const container = document.createElement('div');
container.id = 'my-extension-ui';
container.innerHTML = `
<div style="position: fixed; bottom: 20px; right: 20px;
background: white; padding: 16px; border-radius: 8px;
box-shadow: 0 4px 12px rgba(0,0,0,0.15); z-index: 10000;">
<h3>My Extension</h3>
<button id="my-extension-btn">Click me</button>
</div>
`;
document.body.appendChild(container);
document.getElementById('my-extension-btn').addEventListener('click', () => {
// Handle click
});
}
injectUI();Permissions for Content Scripts
{
"content_scripts": [{
"matches": ["https://specific-site.com/*"],
"js": ["content.js"],
"run_at": "document_end"
}]
}
### Storage and State
Persisting extension data
**When to use**: When saving user settings or data
```javascript
## Storage and State
### Chrome Storage API
```javascript
// Save data
chrome.storage.local.set({ key: 'value' }, () => {
console.log('Saved');
});
// Get data
chrome.storage.local.get(['key'], (result) => {
console.log(result.key);
});
// Sync storage (syncs across devices)
chrome.storage.sync.set({ setting: true });
// Watch for changes
chrome.storage.onChanged.addListener((changes, area) => {
if (changes.key) {
console.log('key changed:', changes.key.newValue);
}
});Storage Limits
| Type | Limit |
|---|---|
| local | 5MB |
| sync | 100KB total, 8KB per item |
Async/Await Pattern
// Modern async wrapper
async function getStorage(keys) {
return new Promise((resolve) => {
chrome.storage.local.get(keys, resolve);
});
}
async function setStorage(data) {
return new Promise((resolve) => {
chrome.storage.local.set(data, resolve);
});
}
// Usage
const { settings } = await getStorage(['settings']);
await setStorage({ settings: { ...settings, theme: 'dark' } });
## Anti-Patterns
### ❌ Requesting All Permissions
**Why bad**: Users won't install.
Store may reject.
Security risk.
Bad reviews.
**Instead**: Request minimum needed.
Use optional permissions.
Explain why in description.
Request at time of use.
### ❌ Heavy Background Processing
**Why bad**: MV3 terminates idle workers.
Battery drain.
Browser slows down.
Users uninstall.
**Instead**: Keep background minimal.
Use alarms for periodic tasks.
Offload to content scripts.
Cache aggressively.
### ❌ Breaking on Updates
**Why bad**: Selectors change.
APIs change.
Angry users.
Bad reviews.
**Instead**: Use stable selectors.
Add error handling.
Monitor for breakage.
Update quickly when broken.
## Related Skills
Works well with: `frontend`, `micro-saas-launcher`, `personal-tool-builder`