Frontend System Design — Senior Engineer Interview Prep

A comprehensive, hands-on learning project for mastering Frontend System Design concepts through practical implementation. Every module covers real-world patterns with Vanilla JS, React/Next.js examples, production best practices, and senior-level interview Q&A.

Goal: One repo to rule them all — your personal tutor, checklist, and interview reference. No need to revisit course materials.

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📦 Project Structure

sysdes/
├── modules/
│   ├── communication/        # Client-server communication patterns
│   ├── security/             # Frontend security patterns & defenses
│   ├── testing/              # Testing strategies & frameworks
│   ├── performance/          # Performance monitoring & optimization
│   ├── database-caching/     # Client-side storage & caching
│   ├── logging-monitoring/   # Telemetry, alerting, debugging
│   ├── accessibility/        # WCAG, ARIA, keyboard, screen readers
│   ├── offline-support/      # Service Workers & PWAs
│   └── networking/           # (Placeholder — covered in course)
├── shared/
│   ├── server/               # Shared server utilities
│   └── client/               # Shared client utilities
└── README.md                 # ← You are here

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🗂️ Modules Overview

1. 📡 Communication

Client-server communication patterns with full server + client implementations.

Topic	Server	Vanilla JS	React
Short Polling	✅	✅	✅
Long Polling	✅	✅	✅
WebSockets	✅	✅	✅
Server-Sent Events (SSE)	✅	✅	✅
WebHooks	✅	✅	✅

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2. 🔒 Security

Frontend security patterns with vulnerable ❌ vs secure ✅ examples.

Topic	Description
XSS (Cross-Site Scripting)	DOM/Reflected/Stored XSS, sanitization, CSP
iFrame Protection	X-Frame-Options, CSP frame-ancestors, sandbox
Security Headers	CSP, HSTS, X-Content-Type-Options, Referrer-Policy
CORS	Preflight, whitelisted origins, credentials
CSRF	Token-based, SameSite cookies, HMAC signatures
+ Input Validation, SRI, HTTPS, Feature Policy, SSRF, SSJI, Dependency Security

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3. 🧪 Testing

Testing strategies, frameworks, and patterns for production apps.

Topic	Description
Unit & Integration Testing	Vitest, React Testing Library, mocking
E2E & Automation Testing	Playwright, page objects, CI setup
A/B Testing	Feature flags, experiment design, statistical significance
Performance Testing	Lighthouse CI, Core Web Vitals, budgets
TDD	Red-Green-Refactor cycle
Security Testing	OWASP, automated scanning, penetration testing

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4. ⚡ Performance

Performance monitoring, tools, and optimization techniques.

Topic	Description
Performance Monitoring	Core Web Vitals (LCP, FID, CLS, INP, TTFB)
Performance Tools	Lighthouse, DevTools, WebPageTest
Network Optimization	Critical Rendering Path, Resource Hinting, HTTP Caching, Service Worker Caching, Compression, HTTP/2+3
Rendering Patterns	SSR, SSG, ISR, CSR, Streaming SSR
Build Optimization	Tree shaking, code splitting, bundle analysis

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5. 💾 Database & Caching

Client-side storage, caching strategies, and state management.

Topic	Description
Local Storage	Persistent key-value, TTL wrapper, 5MB limit
Session Storage	Tab-scoped, form drafts, wizard state
Cookie Storage	HttpOnly, Secure, SameSite, auth tokens
IndexedDB	Structured data, offline storage, async API
Normalization	Flat state shape, entity relationships
HTTP Caching	Cache-Control, ETag, stale-while-revalidate
Service Worker Caching	Cache First, Network First, SWR strategies
API Caching	SWR, React Query, request deduplication
State Management	Context API, Zustand, Redux Toolkit

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6. 📊 Logging & Monitoring

Production observability — errors, performance, and debugging.

Topic	Description
Telemetry	Error tracking, CWV performance, event tracking, sampling
Alerting	Threshold rules, cooldowns, webhook/beacon channels
Fixing / Debugging	Structured logging, source maps, breadcrumbs, error context

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7. ♿ Accessibility

WCAG 2.1 compliance, ARIA, keyboard navigation, and screen reader support.

Topic	Vanilla JS	React/Next.js
Keyboard Accessibility	✅ interactive HTML	✅ hooks + components
Screen Reader (ARIA)	✅ interactive HTML	✅ hooks + components
Focus Management	✅ interactive HTML	✅ hooks + components
Color Contrast	✅ JS utilities	✅ hooks + components
Accessibility Tools	✅ audit scripts	✅ axe + Playwright
Fixing Accessibility	✅ before/after	✅ components

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8. 📴 Offline Support

Service Workers, PWAs, caching strategies, and offline-first architecture.

Topic	Vanilla JS	React/Next.js
Service Workers	✅ Full SW + registration	✅ hooks (useServiceWorker, useOnlineStatus, useOfflineQueue)
PWAs	✅ interactive HTML	✅ hooks (usePWAInstall, useDisplayMode, usePushNotifications)

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📝 Interview Guides

Every module includes a dedicated INTERVIEW_GUIDE.md with:

• ✅ Quick-fire Q&A (senior level)
• ✅ Scenario-based questions ("Design an offline-first app...")
• ✅ Common mistakes and how to fix them
• ✅ System design trade-off discussions

Module	Interview Guide
Testing	INTERVIEW_GUIDE.md
Performance	INTERVIEW_GUIDE.md
Database & Caching	INTERVIEW_GUIDE.md
Logging & Monitoring	INTERVIEW_GUIDE.md
Accessibility	INTERVIEW_GUIDE.md
Offline Support	INTERVIEW_GUIDE.md

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🚀 Getting Started

# Clone and navigate
cd sysdes/

# Pick any module/topic and follow its README
cd modules/communication/websockets/
cat README.md

# For runnable servers (Communication module)
cd server && npm install && npm start

# For React clients
cd client-react && npm install && npm start

# Vanilla JS examples — open .html files directly in browser
open modules/accessibility/keyboard-accessibility/vanilla-examples.html

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🎯 How to Use This Repo

As a Learning Tool

Navigate to any module → read the README → explore examples → modify and experiment.

As an Interview Prep Checklist

1. Open the module's INTERVIEW_GUIDE.md
2. Try answering each question before reading the answer
3. Review the code examples for practical implementation details
4. Focus on trade-offs and "when to use what"

As a Quick Reference

Each topic README includes:

• ✅ DO's and ❌ DON'Ts
• Common mistakes tables
• Production checklists
• Code examples (both Vanilla JS and React/Next.js)

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📐 Approach & Philosophy

Principle	How
Practical	Every concept has runnable code — not just theory
Senior-level	Covers production concerns, trade-offs, and edge cases
Both perspectives	Vanilla JS for fundamentals + React/Next.js for real-world
Interview-ready	Every module has an interview guide with scenario questions
Self-contained	No external courses needed — this repo IS the reference

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📈 Progress

Module	Status	Topics
Communication	✅ Complete	5
Security	✅ Complete	12+
Testing	✅ Complete	6
Performance	✅ Complete	5
Database & Caching	✅ Complete	9
Logging & Monitoring	✅ Complete	3
Accessibility	✅ Complete	6
Offline Support	✅ Complete	2
Networking	⏭️ Skipped	—