exist? □ Check if window.__INITIAL_STATE__ or similar SSR data is missing □ Add error boundary (React) or errorCaptured (Vue) to catch silent throws □ Verify VITE_* / NEXT_PUBLIC_* env vars are set in production build ``` **Quick fixes:** ```jsx // React: Add error boundary at root class ErrorBoundary extends React.Component { state = { error: null } componentDidCatch(error) { this.setState({ error }) } render() { if (this.state.error) return

{this.state.error.message}

return this.props.children } } ``` ```js // Vite SPA: fix 404 on refresh — configure server fallback // vite.config.ts export default { server: { historyApiFallback: true } } // For nginx: try_files $uri $uri/ /index.html; ``` --- #### ⚠️ JS Errors **Common causes:** - `Cannot read properties of undefined/null` - Module not found / import path wrong - Async race condition (component unmounted before fetch resolves) - Third-party script conflict - TypeScript compiled to wrong target **Checklist:** ``` □ Read the full stack trace — find YOUR file, not node_modules □ Check if the error is in an async callback (add try/catch) □ Verify all imports resolve (check tsconfig paths, aliases) □ Check if optional chaining (?.) is needed □ Look for useEffect cleanup missing (React) ``` **Quick fixes:** ```js // Safe optional chaining const name = user?.profile?.name ?? 'Anonymous' // React: cancel async on unmount useEffect(() => { let cancelled = false fetchData().then(data => { if (!cancelled) setData(data) }) return () => { cancelled = true } }, []) ``` --- #### 📦 Resource Loading Failures **Common causes:** - Wrong `publicPath` / `base` in build config - CORS headers missing on CDN/API - Content Security Policy blocking scripts - Asset hash mismatch after deploy (stale cache) - Font/image path wrong in CSS **Checklist:** ``` □ Network tab: check exact URL being requested vs actual file location □ Check response headers for CORS: Access-Control-Allow-Origin □ Check browser console for CSP violations □ Verify base URL in vite.config.ts / next.config.js / webpack output.publicPath □ Hard refresh (Cmd+Shift+R) to rule out cache ``` **Quick fixes:** ```ts // vite.config.ts — fix base path for subdirectory deploy export default defineConfig({ base: '/my-app/' }) // next.config.js — fix asset prefix module.exports = { assetPrefix: 'https://cdn.example.com' } ``` ```nginx # nginx CORS for fonts/assets location ~* \.(woff2?|ttf|eot|svg)$ { add_header Access-Control-Allow-Origin *; } ``` --- #### ⚛️ React / Vue Hydration Errors **Common causes:** - Server HTML doesn't match client render (date, random ID, window check) - Using `typeof window !== 'undefined'` incorrectly in render - Third-party component not SSR-safe - Mismatched whitespace in HTML **React checklist:** ``` □ Error: "Hydration failed because the initial UI does not match" □ Find component that reads browser-only APIs (localStorage, window, Date.now()) □ Wrap browser-only code in useEffect or dynamic import with ssr: false ``` **Quick fixes:** ```jsx // Next.js: disable SSR for a component import dynamic from 'next/dynamic' const BrowserOnlyChart = dynamic(() => import('./Chart'), { ssr: false }) // React 18: suppress hydration warning for intentional mismatch {new Date().toLocaleString()} ``` **Vue / Nuxt checklist:** ``` □ Error: "Hydration node mismatch" □ Use wrapper for browser-only components □ Avoid v-if based on window/document in SSR context ``` ```vue ``` --- #### 🧩 Browser Extension Popup Not Showing **Common causes:** - `popup.html` not declared in `manifest.json` - Content Security Policy blocking inline scripts in popup - JS error in popup script crashes before render - Manifest V3: service worker not registered - Hot reload dev server URL used instead of extension URL **Checklist:** ``` □ Check manifest.json → action.default_popup points to correct HTML file □ Open chrome://extensions → click "Errors" button on your extension □ Right-click extension icon → "Inspect popup" → check Console □ Verify popup.html has ``` --- #### 🎨 CSS Layout Issues **Common causes:** - Flexbox/Grid child overflow not handled - `z-index` not working (stacking context issue) - `position: absolute` escaping wrong parent - `100vh` broken on mobile (address bar) - Margin collapse unexpected behavior - CSS specificity conflict **Checklist:** ``` □ Open DevTools → Elements → Computed styles: check display, position, overflow □ Use DevTools Layout panel to visualize flex/grid □ Check if z-index parent has position set (required for z-index to work) □ Check if overflow: hidden on parent clips child □ Use outline: 1px solid red on elements to debug box model ``` **Quick fixes:** ```css /* Fix z-index not working — parent needs a stacking context */ .parent { position: relative; /* or absolute/fixed/sticky */ z-index: 0; } /* Fix 100vh on mobile */ .full-height { height: 100vh; height: 100dvh; /* dynamic viewport height */ } /* Fix flex child overflow */ .flex-child { min-width: 0; /* allows text truncation inside flex */ overflow: hidden; text-overflow: ellipsis; white-space: nowrap; } /* Debug layout quickly */ * { outline: 1px solid rgba(255, 0, 0, 0.2); } ``` --- ## Step 3 — Provide Fix After diagnosis, provide: 1. **Root cause** — one clear sentence 2. **Fix** — minimal code change with before/after if applicable 3. **Verification** — how to confirm the fix worked 4. **Prevention** — one tip to avoid recurrence --- ## Usage Examples ``` /frontend-doctor my Next.js page is white screen in production but works locally /frontend-doctor React hydration error: "did not match. Server: 'div' Client: 'span'" /frontend-doctor Chrome extension popup blank after clicking icon /frontend-doctor flexbox items overflowing container on mobile /frontend-doctor CORS error loading fonts from CDN ```