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Frontend System Design Essentials

A practical guide to designing scalable, reliable, and maintainable frontend systems.

Juntao Qiu

A practical guide to designing scalable and maintainable frontends. Learn how to think beyond components, model data effectively, shape APIs around real access patterns, and apply modern techniques like pagination, caching, virtualization, and real-time updates. Perfect for engineers preparing for system design interviews or leveling up in complex frontend projects.

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About

About

About the Book

Modern frontend applications look simple on the surface, but behind every smooth user experience is a system of data flows, rendering strategies, caching layers, pagination models, and real-time updates. This book breaks down those complexities and helps you understand how to design frontends that scale — not just in performance, but in clarity, maintainability, and user experience.

Frontend System Design Essentials introduces a structured approach for thinking about complex UI architecture through the CCDAO framework. You’ll learn how to analyse product requirements, model data effectively, design APIs that support real-world access patterns, and apply techniques like virtualization, optimistic updates, pagination, caching, and performance optimization.

Rather than presenting abstract theories, the book uses concrete case studies — typeahead search, feed lists, modals, and more — to show how system design decisions play out in practice. Each chapter builds on the last, helping you form a mental model you can use in interviews or in your day-to-day work as a senior engineer.

If you want to grow beyond “just building UI” and start designing frontends that hold up under real-world pressure, this book gives you the tools, patterns, and clarity to do it well.

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Computers and ProgrammingReactWeb Development

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Author

About the Author

Juntao Qiu

Hi, I'm Juntao, and I help developers write better code through clear, practical guidance.

I am deeply committed to contributing to both the online and offline developer communities, sharing insights and knowledge gleaned from my experiences in various projects. Among my contributions are several publications focused on Web Development. Notably, my latest works include "React Anti-Patterns" (2024), "Test-Driven Development with React (2nd)" (2023) and "Maintainable React" (2022).

Additionally, I host a YouTube channel (@icodeit.juntao), where I delve into Clean Code practices and Refactoring techniques, aiming to assist developers in honing their coding skills.

Contents

Table of Contents

Preface

  1. A High-Level View of Frontend Work
  2. Book Structure and Outline
  3. How to Make the Most of This Book
  4. Summary

Chapter 1 — Evolving from Components to Systems

  1. Analyzing how simple requests turn complex
  2. Part I — How a small component grows
  3. Zooming out to the system perspective
  4. Developing system design thinking
  5. A note on CCDAO and the interview lens
  6. Summary — The invisible work

Chapter 2 — Introducing the CCDAO Framework

  1. Collecting information for requirement clarity
  2. Designing modular component structures
  3. Modeling data for predictability and efficiency
  4. Designing stable API contracts
  5. Implementing optimization strategies early
  6. Applying CCDAO in an Interview
  7. Why It Matters Beyond Interviews

Chapter 3 — Applying CCDAO: Designing a Typeahead Search Box

  1. 1. Collect Information
  2. 2. Component Structure
  3. 3. Data Modeling
  4. 4. API Design
  5. 5. Optimization Strategies
  6. Closing

Chapter 4 — Applying CCDAO: Designing a Scalable Feed List

  1. 1. Collect Information
  2. 2. Component Structure
  3. 3. Data Modeling
  4. 4. API Design
  5. 5. Optimization Strategies
  6. Closing

Chapter 5 — Data Modeling: Understanding the Domain and the UI

  1. Defining the domain clearly through business rules
  2. Designing models for specific UI consumption patterns
  3. From Domain to Design

Chapter 6 — Case Study: Implementing Sidebar Entitlements

  1. Step 1 — The Starting Point: Logic in the UI
  2. Step 2 — When the Rules Multiply
  3. Step 3 — Moving Business Logic to the Backend
  4. Step 4 — Where Frontend Logic Still Belongs
  5. Step 5 — Tailoring APIs with GraphQL or a BFF
  6. Lessons from the Sidebar

Chapter 7 — Setting Up the Project Environment

  1. Why We Use a Starter Project
  2. Prerequisites
  3. Getting the Project Running
  4. Useful Scripts
  5. Configuring the Mock API with MSW
  6. Mock Endpoints Provided
  7. Data Shapes You’ll Work With
  8. Project Structure (High-Level)
  9. Troubleshooting
  10. What Comes Next

Chapter 8 — Implementing Data Normalization

  1. Why Normalisation Matters
  2. A Real Example: Inconsistent User Data
  3. Establishing a consistent source of truth
  4. Transforming nested board payloads into flat tables
  5. Holding the normalized store in React Context
  6. Hydrating data during the rendering process
  7. Summary

Chapter 9 — Drawing Inspiration from Backend Databases

  1. A Familiar Pattern From Databases
  2. How Databases Reconstruct Data
  3. The Frontend Equivalent
  4. Why This Matters
  5. A Good Place to Pause

Chapter 10 — Managing Requests and Data Fetching

  1. Showing Real Assignees in the UI
  2. Extending the Backend API
  3. Updating the Board on Selection
  4. Understanding Race Conditions in Search
  5. Fixing Race Conditions with Request Cancellation
  6. Reducing Request Volume with Debouncing and Throttling
  7. Summary

Chapter 11 — Implementing Pagination Strategies

  1. Demonstrating Pagination in UserSelect
  2. The Pagination Strategies
  3. A Simple Users Table
  4. Offset Pagination
  5. Cursor Pagination
  6. Choosing the Right Strategy for UserSelect
  7. UI Patterns for Pagination
  8. Summary

Chapter 12 — Migrating to an Express Backend

  1. A Brief Introduction to Express
  2. Setting up a standalone mock API server
  3. Proxying Frontend Requests to The Express Server
  4. Summary

Chapter 13 — Implementing Server-Side Rendering

  1. Rendering strategies
  2. How SSR works
  3. Coordinating hydration between server and client
  4. Creating the client entry (hydration)
  5. Creating the server entry (rendering on the server)
  6. Creating separate entry points
  7. Configuring Vite to build dual entries
  8. Generating HTML on the server
  9. Bringing it together in the Express SSR route
  10. Why consistency matters
  11. From SSR to SPA behaviour
  12. Summary

Case Study — Understanding Bundlers Through Code Splitting

  1. What Problem Are We Solving?
  2. What a Bundler Actually Does
  3. Tree Shaking: Removing Unused Exports
  4. Code Splitting: Deferring Code Until It’s Needed
  5. A Subtle Caveat: Tree Shaking Stops at Dynamic Boundaries
  6. What This Case Study Teaches Us
  7. Summary

Chapter 14 – Implementing Code Splitting and Lazy Loading

  1. Why Lazy Loading Helps
  2. The Building Blocks of Lazy Loading
  3. Implementing Lazy Loading in Our Board Application
  4. Another Example: Lazy Loading the List View
  5. Understanding the Build Output After Code Splitting
  6. What Happens at Runtime
  7. How Code Splitting Changes the Bundle
  8. Summary

Chapter 15 – Implementing Data Prefetching

  1. How Prefetch Works Conceptually
  2. The Prefetch Function Inside QueryProvider
  3. Exposing Prefetch Through a Hook
  4. Prefetching Users in Card.tsx
  5. How Prefetch and useQuery Work Together
  6. The Full Runtime Experience
  7. When Prefetch Helps
  8. When Prefetch Doesn’t Help
  9. Summary

Chapter 16 – Implementing Optimistic Updates

  1. Creating a New Card (Server-First)
  2. Introducing Optimistic Updates
  3. Adding a Card Optimistically
  4. Deleting a Card Optimistically
  5. Things to Keep in Mind
  6. Summary

Chapter 17 – Exploring Real-time Update Strategies

  1. Polling
  2. Server-Sent Events (SSE)
  3. WebSockets
  4. Scaling Considerations
  5. Summary

Chapter 18 – Implementing Real-time Updates with SSE

  1. The pub-sub pattern
  2. Using the pub-sub pattern on the server
  3. Using the event emitter with SSE
  4. Emitting events from the update API
  5. Frontend: consuming SSE with EventSource
  6. Demonstration and behaviour
  7. Summary

Case Study — Implementing WebSockets for Real-time Updates

  1. Server Side — Setting Up WebSockets
  2. Client Side — Receiving WebSocket Events
  3. A Real-World Bug: Duplicate Cards
  4. Making the State Update Idempotent
  5. Summary

Chapter 19 — Optimizing Performance with HTTP Caching

  1. Why caching matters
  2. Cache-Control — how long a response stays fresh
  3. ETag — detecting whether content changed
  4. Last-Modified — timestamp-based validation
  5. Vary — preventing incorrect caching
  6. Putting it all together: a realistic request flow
  7. Summary

Chapter 20 — Handling Runtime Errors in React

  1. A quick comparison to try/catch
  2. Understanding Error Boundaries
  3. Wrapping a fragile component
  4. Cascading error boundaries in a column
  5. Application-level fallback
  6. Summary

Chapter 21 — Designing for Accessibility

  1. Why we need to consider accessibility at all
  2. Accessibility-first system design principles
  3. Accessibility checks as part of the testing architecture
  4. Semantic HTML in real components
  5. Keyboard navigation as a system concern
  6. A more complex example: accessible drag and drop
  7. My typical accessibility workflow
  8. Summary

Chapter 22 — Implementing Performance Monitoring

  1. Why performance monitoring matters
  2. Understanding bundle size in practice
  3. Making performance limits explicit with budgets
  4. Measuring real user experience with Web Vitals
  5. Performance as a continuous system concern
  6. Summary

The Architectural Roadmap: From Components to Systems

  1. Module 1: The System Design Mindset (Chapters 1–4)
  2. Module 2: Building the Data Foundation (Chapters 5–9)
  3. Module 3: Managing Data Flow and Connectivity (Chapters 10–12)
  4. Module 4: Rendering and Performance (Chapters 13–15)
  5. Module 5: Mutations, Real-Time Sync, and Resilience (Chapters 16–22)
  6. Designing System Step By Step

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