---
name: tech-leads-club/best-practices
source: https://app.decimal.ai/s/tech-leads-club-best-practices@1/SKILL.md
source_sha256: 47ffc88e663f
---
# Best practices
Modern web development standards based on Lighthouse best practices audits. Covers security, browser compatibility, and code quality patterns.
## Security
### HTTPS everywhere
**Enforce HTTPS:**
```html
```
**HSTS Header:**
```
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
```
### Content Security Policy (CSP)
```html
```
**CSP Header (recommended):**
```
Content-Security-Policy:
default-src 'self';
script-src 'self' 'nonce-abc123' https://trusted.com;
style-src 'self' 'nonce-abc123';
img-src 'self' data: https:;
connect-src 'self' https://api.example.com;
frame-ancestors 'self';
base-uri 'self';
form-action 'self';
```
**Using nonces for inline scripts:**
```html
```
### Security headers
```
# Prevent clickjacking
X-Frame-Options: DENY
# Prevent MIME type sniffing
X-Content-Type-Options: nosniff
# Enable XSS filter (legacy browsers)
X-XSS-Protection: 1; mode=block
# Control referrer information
Referrer-Policy: strict-origin-when-cross-origin
# Permissions policy (formerly Feature-Policy)
Permissions-Policy: geolocation=(), microphone=(), camera=()
```
### No vulnerable libraries
```bash
# Check for vulnerabilities
npm audit
yarn audit
# Auto-fix when possible
npm audit fix
# Check specific package
npm ls lodash
```
**Keep dependencies updated:**
```json
// package.json
{
"scripts": {
"audit": "npm audit --audit-level=moderate",
"update": "npm update && npm audit fix"
}
}
```
**Known vulnerable patterns to avoid:**
```javascript
// ❌ Prototype pollution vulnerable patterns
Object.assign(target, userInput)
_.merge(target, userInput)
// ✅ Safer alternatives
const safeData = JSON.parse(JSON.stringify(userInput))
```
### Input sanitization
```javascript
// ❌ XSS vulnerable
element.innerHTML = userInput
document.write(userInput)
// ✅ Safe text content
element.textContent = userInput
// ✅ If HTML needed, sanitize
import DOMPurify from 'dompurify'
element.innerHTML = DOMPurify.sanitize(userInput)
```
### Secure cookies
```javascript
// ❌ Insecure cookie
document.cookie = "session=abc123";
// ✅ Secure cookie (server-side)
Set-Cookie: session=abc123; Secure; HttpOnly; SameSite=Strict; Path=/
```
---
## Browser compatibility
### Doctype declaration
```html
Page
Page
```
### Character encoding
```html
Page
Page
```
### Viewport meta tag
```html
Page
Page
```
### Feature detection
```javascript
// ❌ Browser detection (brittle)
if (navigator.userAgent.includes('Chrome')) {
// Chrome-specific code
}
// ✅ Feature detection
if ('IntersectionObserver' in window) {
// Use IntersectionObserver
} else {
// Fallback
}
// ✅ Using @supports in CSS
@supports (display: grid) {
.container {
display: grid;
}
}
@supports not (display: grid) {
.container {
display: flex;
}
}
```
### Polyfills (when needed)
```html
```
---
## Deprecated APIs
### Avoid these
```javascript
// ❌ document.write (blocks parsing)
document.write('');
// ✅ Dynamic script loading
const script = document.createElement('script');
script.src = '...';
document.head.appendChild(script);
// ❌ Synchronous XHR (blocks main thread)
const xhr = new XMLHttpRequest();
xhr.open('GET', url, false); // false = synchronous
// ✅ Async fetch
const response = await fetch(url);
// ❌ Application Cache (deprecated)
// ✅ Service Workers
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/sw.js');
}
```
### Event listener passive
```javascript
// ❌ Non-passive touch/wheel (may block scrolling)
element.addEventListener('touchstart', handler)
element.addEventListener('wheel', handler)
// ✅ Passive listeners (allows smooth scrolling)
element.addEventListener('touchstart', handler, { passive: true })
element.addEventListener('wheel', handler, { passive: true })
// ✅ If you need preventDefault, be explicit
element.addEventListener('touchstart', handler, { passive: false })
```
---
## Console & errors
### No console errors
```javascript
// ❌ Errors in production
console.log('Debug info') // Remove in production
throw new Error('Unhandled') // Catch all errors
// ✅ Proper error handling
try {
riskyOperation()
} catch (error) {
// Log to error tracking service
errorTracker.captureException(error)
// Show user-friendly message
showErrorMessage('Something went wrong. Please try again.')
}
```
### Error boundaries (React)
```jsx
class ErrorBoundary extends React.Component {
state = { hasError: false }
static getDerivedStateFromError(error) {
return { hasError: true }
}
componentDidCatch(error, info) {
errorTracker.captureException(error, { extra: info })
}
render() {
if (this.state.hasError) {
return
}
return this.props.children
}
}
// Usage
;
```
### Global error handler
```javascript
// Catch unhandled errors
window.addEventListener('error', (event) => {
errorTracker.captureException(event.error)
})
// Catch unhandled promise rejections
window.addEventListener('unhandledrejection', (event) => {
errorTracker.captureException(event.reason)
})
```
---
## Source maps
### Production configuration
```javascript
// ❌ Source maps exposed in production
// webpack.config.js
module.exports = {
devtool: 'source-map', // Exposes source code
}
// ✅ Hidden source maps (uploaded to error tracker)
module.exports = {
devtool: 'hidden-source-map',
}
// ✅ Or no source maps in production
module.exports = {
devtool: process.env.NODE_ENV === 'production' ? false : 'source-map',
}
```
---
## Performance best practices
### Avoid blocking patterns
```javascript
// ❌ Blocking script
// ✅ Deferred script
// ❌ Blocking CSS import
@import url('other-styles.css');
// ✅ Link tags (parallel loading)
```
### Efficient event handlers
```javascript
// ❌ Handler on every element
items.forEach((item) => {
item.addEventListener('click', handleClick)
})
// ✅ Event delegation
container.addEventListener('click', (e) => {
if (e.target.matches('.item')) {
handleClick(e)
}
})
```
### Memory management
```javascript
// ❌ Memory leak (never removed)
const handler = () => {
/* ... */
}
window.addEventListener('resize', handler)
// ✅ Cleanup when done
const handler = () => {
/* ... */
}
window.addEventListener('resize', handler)
// Later, when component unmounts:
window.removeEventListener('resize', handler)
// ✅ Using AbortController
const controller = new AbortController()
window.addEventListener('resize', handler, { signal: controller.signal })
// Cleanup:
controller.abort()
```
---
## Code quality
### Valid HTML
```html
Click
```
### Semantic HTML
```html
Headline
```
### Image aspect ratios
```html
```
---
## Permissions & privacy
### Request permissions properly
```javascript
// ❌ Request on page load (bad UX, often denied)
navigator.geolocation.getCurrentPosition(success, error)
// ✅ Request in context, after user action
findNearbyButton.addEventListener('click', async () => {
// Explain why you need it
if (await showPermissionExplanation()) {
navigator.geolocation.getCurrentPosition(success, error)
}
})
```
### Permissions policy
```html
```
---
## Audit checklist
### Security (critical)
- [ ] HTTPS enabled, no mixed content
- [ ] No vulnerable dependencies (`npm audit`)
- [ ] CSP headers configured
- [ ] Security headers present
- [ ] No exposed source maps
### Compatibility
- [ ] Valid HTML5 doctype
- [ ] Charset declared first in head
- [ ] Viewport meta tag present
- [ ] No deprecated APIs used
- [ ] Passive event listeners for scroll/touch
### Code quality
- [ ] No console errors
- [ ] Valid HTML (no duplicate IDs)
- [ ] Semantic HTML elements used
- [ ] Proper error handling
- [ ] Memory cleanup in components
### UX
- [ ] No intrusive interstitials
- [ ] Permission requests in context
- [ ] Clear error messages
- [ ] Appropriate image aspect ratios
## Tools
| Tool | Purpose |
| -------------------------------------------------- | -------------------------- |
| `npm audit` | Dependency vulnerabilities |
| [SecurityHeaders.com](https://securityheaders.com) | Header analysis |
| [W3C Validator](https://validator.w3.org) | HTML validation |
| Lighthouse | Best practices audit |
| [Observatory](https://observatory.mozilla.org) | Security scan |
## References
- [MDN Web Security](https://developer.mozilla.org/en-US/docs/Web/Security)
- [OWASP Top 10](https://owasp.org/www-project-top-ten/)
- [Web Quality Audit](../web-quality-audit/SKILL.md)