browser-extension-builder
Frontend & Expérience UX"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."
Documentation
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
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{
"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": ["
"js": ["content/content.js"]
}],
"background": {
"service_worker": "background/service-worker.js"
},
"options_page": "options/options.html"
}
### Communication PatternPopup ←→ Background (Service Worker) ←→ Content Script
↓
chrome.storage
Content Scripts
Code that runs on web pages
When to use: When modifying or reading page content
## Content Scripts
### Basic Content Script// 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 = `
box-shadow: 0 4px 12px rgba(0,0,0,0.15); z-index: 10000;">
My Extension
`;
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
## Storage and State
### Chrome Storage API// 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
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