System Design Interview Cheatsheet
Your quick reference guide for acing frontend system design interviews
The RADIO Framework
Requirements
Clarify functional and non-functional requirements
- Ask clarifying questions
- Define scope boundaries
- Identify constraints
- Set success metrics
Architecture
Design high-level component structure
- Component hierarchy & breakdown
- Component responsibilities
- System layers (UI, Business, Data)
- Data flow & communication patterns
Trade-offs
Discuss pros/cons of different approaches
- Compare alternatives
- Performance vs complexity
- Scalability considerations
- Team & timeline factors
Interface
Define API contracts and data models
- Data structures (Post, User, etc.)
- API endpoints & contracts
- Component prop interfaces
- State management approach
Optimization
Performance and scalability improvements
- Code splitting
- Lazy loading
- Caching strategies
- Bundle optimization
What Interviewers Are Looking For
Communication & Collaboration
- •Clear articulation of thoughts and decisions
- •Asking clarifying questions upfront
- •Discussing trade-offs openly
- •Incorporating interviewer feedback
- •Thinking out loud during problem-solving
Technical Depth
- •Understanding of core web technologies (HTML, CSS, JS)
- •Knowledge of modern frameworks and libraries
- •Awareness of browser APIs and limitations
- •Understanding of network protocols and APIs
- •Familiarity with performance optimization techniques
Problem-Solving Approach
- •Structured thinking (using frameworks like RADIO)
- •Breaking down complex problems into manageable parts
- •Identifying and handling edge cases
- •Considering scalability from the start
- •Balancing perfection with pragmatism
Design Trade-offs
- •Articulating pros and cons of different approaches
- •Understanding when to optimize vs. keep it simple
- •Knowing when to use libraries vs. custom solutions
- •Balancing performance, maintainability, and complexity
- •Considering team skill level and project timeline
User-Centric Thinking
- •Considering accessibility (WCAG compliance)
- •Optimizing for different devices and network conditions
- •Focusing on core user workflows
- •Planning for error states and edge cases
- •Thinking about international users (i18n, RTL support)
Common Mistakes to Avoid
Jumping into code without clarifying requirements
Why it's bad: Leads to solving the wrong problem or missing critical constraints
Do this instead: Spend 5-10 minutes asking clarifying questions and defining scope
Being too quiet or not explaining your thought process
Why it's bad: Interviewer can't assess your problem-solving skills or provide guidance
Do this instead: Think out loud, explain your reasoning, and discuss trade-offs
Going too deep too quickly without high-level design
Why it's bad: Wastes time on details before understanding the big picture
Do this instead: Start with architecture overview, then dive into specific components
Not asking whether to go breadth or depth
Why it's bad: You might spend 30 minutes on one component when breadth was expected
Do this instead: Explicitly ask: 'Should I go broad covering all components, or deep into one area?'
Ignoring non-functional requirements (performance, accessibility)
Why it's bad: Real-world systems must handle scale, errors, and diverse users
Do this instead: Proactively discuss scalability, error handling, and accessibility
Using buzzwords without understanding them
Why it's bad: Interviewer will probe deeper and expose lack of knowledge
Do this instead: Only mention technologies you can explain and justify
Not considering mobile or different network conditions
Why it's bad: Shows lack of real-world experience with diverse users
Do this instead: Discuss responsive design, progressive enhancement, offline support
Over-engineering or under-engineering the solution
Why it's bad: Either wastes time on unnecessary complexity or misses key requirements
Do this instead: Match complexity to the problem scope and ask for feedback
Not discussing trade-offs of your decisions
Why it's bad: Interviewer wants to see you understand pros/cons of choices
Do this instead: For each major decision, briefly explain why you chose it over alternatives
Forgetting about the user experience
Why it's bad: Frontend is all about creating great user experiences
Do this instead: Discuss loading states, error messages, accessibility, and UX patterns
Key Questions to Ask Your Interviewer
At the Start (Requirements Phase)
- Q:"What are the core features we need to support? What can we defer?"
- Q:"Who are the target users? What devices and network conditions?"
- Q:"What's the expected scale? How many users, requests, data volume?"
- Q:"Are there any specific technical constraints or requirements (browser support, frameworks)?"
- Q:"What does success look like for this system?"
During Design (Scoping Phase)
- Q:"Should I focus on breadth (covering all components) or depth (diving deep into specific areas)?"
- Q:"Which component is most critical? Should I start there?"
- Q:"Is [specific technology/pattern] acceptable to use here?"
- Q:"Do you want me to discuss [X] now or can we come back to it?"
Throughout (Validation)
- Q:"Does this approach make sense? Would you like me to explore alternatives?"
- Q:"Am I on the right track? Should I adjust my focus?"
- Q:"Is there anything specific you'd like me to elaborate on?"
- Q:"How much time do we have left? Should I prioritize differently?"
Time Management (45-60 min interview)
Requirements Gathering
5-10 minutes- •Ask clarifying questions
- •Define scope and constraints
- •Confirm understanding with interviewer
- •Decide on breadth vs. depth approach
High-Level Architecture
10-15 minutes- •Draw component diagram
- •Explain data flow
- •Identify key components and responsibilities
- •Discuss component interactions
Deep Dive
15-20 minutes- •Pick 1-2 critical components to elaborate
- •Discuss data models and interfaces
- •Explain API contracts
- •Address edge cases and error handling
Optimization & Trade-offs
10-15 minutes- •Discuss performance optimizations
- •Talk about scalability considerations
- •Address accessibility and UX
- •Mention monitoring and debugging
Q&A and Wrap-up
5 minutes- •Answer interviewer's follow-up questions
- •Summarize key decisions
- •Mention what you'd do with more time
- •Ask for feedback
RADIO Framework Deep Dive
Architecture (A) - What Goes Here?
- •Component Hierarchy: Draw the component tree (FeedContainer → PostList → PostCard → MediaViewer)
- •Component Responsibilities: What each component does (PostCard handles display, LikeButton handles interactions)
- •System Layers: Presentation, Business Logic, Data/API, Caching layers
- •Communication Patterns: How components talk (props down, events up, context, etc.)
Interface (I) - What Goes Here?
- •Data Models: TypeScript interfaces for Post, User, Comment, FeedState
- •API Contracts: Endpoint definitions, request/response formats, error handling
- •Component Interfaces: Props, callbacks, event signatures for each component
- •State Management: Choose approach (Redux, Zustand, React Query) and define state shape
⚡ Quick Tips for Success
Start Simple, Then Iterate
Begin with a basic solution that works, then add complexity as needed.
Draw Diagrams
Visual representations help both you and the interviewer understand the system.
Use Real Examples
Reference actual products (Instagram, Google Docs) to clarify your points.
Admit What You Don't Know
It's better to be honest than to fake knowledge. Offer to discuss what you do know.
Discuss Patterns, Not Just Libraries
Explain the reducer pattern before mentioning Redux. Show you understand concepts.
Check Your Pace
Periodically ask if you should continue on this path or shift focus.
Remember
System design interviews are conversations, not exams. The interviewer wants you to succeed and is there to guide you.
- There's no single "correct" answer—show your thinking process
- Ask questions early and often—it shows engagement and thoroughness
- Focus on communication and collaboration as much as technical skills