In a frontend-heavy High-Level Design (HLD) interview, the goal is to show how you approach building scalable, maintainable, and performant user-facing systems. Focus on the following key pillars:

• Performance: How fast does it feel to the user? (load time, interaction speed)

• Scalability: How does the system handle growth — more users, more features, and more developers?

• Maintainability: How easy is it to debug, update, and extend? (code quality, architecture, documentation)

• User Experience (UX): Does the architecture enable a seamless and accessible user experience?

• Developer Experience (DX): How easily can teams build, test, and ship new features?

🏗️ Core Frontend Architecture

Large-scale frontend systems are structured for modularity, independent deployment, and team autonomy.

Microfrontends

Break down a large monolithic frontend into smaller, independently deployable applications.

Why:
This approach allows autonomous teams, independent release cycles, and even mixed tech stacks (for example, one microfrontend in React, another in Vue).

How:

• Module Federation (Webpack 5, Vite): Enables independent builds that share code and dependencies at runtime.

• Iframes: Simple to implement but can hurt SEO, routing, and communication between apps.

• Web Components: Framework-agnostic and great for creating reusable UI blocks.

Trade-offs:
Microfrontends increase operational complexity and can lead to code duplication or inconsistent UX if not managed carefully.

State Management

How you manage data across different parts of your app defines scalability and maintainability.

• Local State (useState): Great for component-specific data.

• Context API: Useful for passing state down without prop drilling. However, high-frequency updates can cause re-renders.

• External Libraries:

  • Zustand: Minimal boilerplate and simple hook-based API.

  • Redux: Predictable, structured, and supported by great dev tools, but more verbose.

• State Machines (XState): Ideal for managing complex states (e.g., multi-step forms, video players) with explicit transitions.

Design Systems

A design system provides reusable UI components guided by consistent design and accessibility standards.
It ensures UI consistency and accelerates development across teams.

💡 Common Frontend System Design Prompts

Example 1: Design a Newsfeed (like Facebook or X)

Key challenge: Achieving smooth infinite scrolling with high performance.

Architecture:

• Data Fetching: The client requests the first page of post IDs; the server returns the list.

• Hydration: The client fetches detailed data only for posts currently visible in the viewport.

• Virtualization: Use react-window or react-virtualized to render only visible items, keeping the DOM light.

• Infinite Scroll: Trigger new data fetches via IntersectionObserver when reaching the end of the list.

• Real-time Updates: Use WebSockets or SSE to push new posts live.

Example 2: Design a Typeahead Search (like Google Search)

Key challenge: Deliver fast, relevant suggestions while avoiding backend overload.

Architecture:

• Debouncing: Wait ~300ms after typing stops before firing an API call.

• Caching:

  • Client-side: Store previous queries (e.g., “reac”) in a Map or Trie.

  • Server/CDN: Cache popular queries at the edge.

• Cancellation: Use AbortController to cancel outdated API requests.

• API Design: Keep responses small — text + ID only.

🌐 The Network Layer

APIs

• REST: The classic approach — define contracts with OpenAPI/Swagger, use plural nouns (e.g., GET /users).

• GraphQL: Lets clients fetch exactly what they need, avoiding over/under-fetching.
  Trade-offs: Setup complexity and tricky caching compared to REST.

HTTP Versions

• HTTP/1.1: Suffers from head-of-line blocking.

• HTTP/2: Adds multiplexing — multiple requests share one TCP connection.

• HTTP/3 (QUIC): Runs over UDP, removing TCP-level blocking entirely.

Real-Time Communication

• WebSockets: Two-way, great for chat or collaboration tools.

• SSE: One-way (server → client), ideal for feeds and notifications.

• WebRTC: Peer-to-peer audio/video with minimal server load.

🔒 Security Essentials

• SSL/TLS: Establishes encrypted HTTPS connections.

• Cookies:

  • HttpOnly: Prevents JavaScript access.

  • Secure: Only over HTTPS.

  • SameSite: Controls cross-origin cookie behavior (Strict, Lax, None).

🖥️ Browser Fundamentals & Performance

Critical Rendering Path

Steps the browser takes to paint pixels:

1. Parse HTML → DOM

2. Parse CSS → CSSOM

3. Combine → Render Tree

4. Layout → Paint → Composite

Optimize by: Minimizing DOM nodes, deferring non-critical JS/CSS, and avoiding layout thrashing.

Event Loop

The JS runtime mechanism that handles async operations.
Long-running tasks block the main thread — freeze avoided via async/await, microtasks, and breaking up work.

Browser Storage

• LocalStorage: Persistent (~10MB).

• SessionStorage: Temporary per-tab (~5MB).

• Cookies: Small (~4KB) and sent with every HTTP request.

Cache Busting

Use hashed filenames (e.g., style.a1b2c3.css) for reliable cache invalidation.

🛠️ Modern Tooling & Frameworks

Vite

Blazing-fast dev bundler using native ES modules — instant start and hot reload.
Uses Rollup for optimized production builds.

Next.js

Full-stack React framework with SSR, SSG, and ISR.
Why it matters: Lets you choose rendering strategies that impact SEO and performance.

React 19 Highlights

• React Compiler: Automatic memoization, reducing need for manual optimization.

• Actions: Simplifies async data mutations (like form handling).

• use Hook: Enables reading promises and contexts directly in render.

📈 Engineering Excellence & Operations

AI in Development

• Auto-generate PR summaries and test cases.

• Use AI-powered static analysis for security.

• Generate docs automatically from comments.

Testing Layers

• Unit: Small, isolated tests (Jest, Vitest).

• Integration: Component interaction tests.

• E2E: Real user journey simulation (Cypress, Playwright).

Observability & Analytics

• Analytics: Tracks user behavior (e.g., Amplitude).

• Observability: Monitors system health (e.g., Sentry, Datadog) via logs, metrics, and traces.

Accessibility (a11y)

Use semantic HTML, keyboard navigation, and alt text. Accessibility is non-negotiable.

Feature Flags

Use tools like LaunchDarkly to release features safely and gradually.
They decouple deployment from release, enabling A/B tests and rollback switches.

Progressive Web Apps (PWAs)

Use service workers for offline caching and web app manifests for installable experiences.

✅ Key Takeaway:
A strong frontend architecture isn’t about flashy tech — it’s about clear boundaries, consistent patterns, and decisions that balance performance, maintainability, and user experience at scale.