JavaScript Interview Preparation Playground

A comprehensive collection of JavaScript interview questions and their implementations, organized by categories and difficulty levels.

📚 Overview

This repository contains 40+ commonly asked JavaScript interview questions with working implementations. Perfect for preparing for frontend developer positions at companies like Google, Amazon, Microsoft, and startups.

🎯 What's Inside

📖 QUESTIONNAIRE.md - Complete Question Bank

A comprehensive guide with:

• Detailed problem statements
• Key concepts for each question
• Difficulty ratings
• Category-wise organization
• Quick reference guide

📁 Repository Structure

javascript-playground/
├── async-patterns/          # Async programming challenges
│   ├── asyncSequence.js     # Parallel vs Series execution
│   ├── FetchWithLimit.js    # Rate-limited API calls
│   ├── mapLimit.js          # Concurrent async operations
│   └── PromiseSchedulor.js  # Promise orchestration with pause/resume
│
├── patterns/                # Design Patterns
│   ├── circuitBreaker.js    # Circuit Breaker pattern
│   ├── singleton.js         # Singleton implementation
│   ├── pub-sub.js          # Event Emitter/Observer
│   ├── proxy.js            # Proxy pattern
│   └── resourcePool.js     # Resource pooling
│
├── utilities/              # Utility implementations
│   ├── currencyConverter.js # Graph-based currency conversion
│   ├── customCookie.js      # Custom cookie handler
│   ├── customSetTimeout.js  # setTimeout implementation
│   ├── mutationObserver.js  # DOM observation
│   └── pipe.js             # Function composition
│
├── polyfills/              # JavaScript polyfills
│   └── index.js            # Promise, map, reduce, call, bind, etc.
│
├── lld/                    # Low-Level Design
│   ├── fileSystem.js       # In-memory file system
│   └── orderManagement.js  # E-commerce order system
│
├── interview-questions/    # Company-specific questions
│   ├── rateLimiter.js      # API rate limiting
│   ├── middleware-thoughtspot.js # Middleware pattern
│   ├── tekion.js           # Tekion interview problems
│   └── tekion2.js
│
├── debounce-throttle/      # Performance optimization
│   └── index.js            # Debounce & Throttle
│
├── promises/               # Promise deep dive
│   ├── CustomPromise.js    # Full Promise implementation
│   └── index.js            # Promise concepts & examples
│
├── outputs/                # Output-based questions
│   └── index.js            # Event loop, closures, etc.
│
├── this/                   # 'this' keyword concepts
│   └── index.js            # All 'this' binding scenarios
│
├── dom/                    # DOM manipulation
│   └── index.js            # DOM APIs
│
├── system-design/          # System design concepts
│   └── normalization/      # State normalization
│
└── typescript/             # TypeScript examples
    └── src/

🚀 Getting Started

Prerequisites

• Node.js (v14+)
• Modern web browser (for DOM examples)

Running Examples

1. Node.js examples:

node async-patterns/PromiseSchedulor.js
node patterns/circuitBreaker.js

2. Browser examples:

# Open HTML files in browser
open debounce-throttle/index.html
open promises/index.html

📊 Question Categories

🔄 Async Patterns (5 questions)

• Promise scheduling with pause/resume
• Rate-limited fetch
• Concurrent operation limiting
• Series vs Parallel execution

🎨 Design Patterns (5 questions)

• Circuit Breaker
• Singleton
• Pub-Sub (Event Emitter)
• Proxy
• Resource Pool

🔧 Polyfills (10+ questions)

• Promise, Promise.all, Promise.allSettled
• call, bind, apply
• map, reduce, forEach, flat
• Object.flatten

🏗️ Low-Level Design (4 questions)

• File System
• Order Management System
• Rate Limiter
• Middleware System

💻 Output-Based (6+ questions)

• Event loop & execution order
• Closures & scope
• Promise chains
• Array.map with parseInt
• setTimeout in loops

📘 Concepts (3 topics)

• this keyword (all scenarios)
• Promises (deep dive)
• DOM manipulation

🎓 Difficulty Levels

⭐ Easy (Good for Beginners)

• Debounce/Throttle
• Array polyfills
• DOM manipulation
• Basic Promise usage
• Singleton pattern

⭐⭐ Medium (Common Interview Questions)

• Custom Promise implementation
• Circuit Breaker
• Rate Limiter
• mapLimit
• Curry/Compose
• Pub-Sub pattern

⭐⭐⭐ Hard (Senior Level)

• Promise Scheduler with pause/resume
• Resource Pool
• File System Design
• Currency Converter with graph traversal
• Middleware implementation

📝 How to Use This Repository

For Interview Prep:

1. Read QUESTIONNAIRE.md for problem statements
2. Try solving without looking at code
3. Compare your solution with implementation
4. Understand the key concepts

For Quick Revision:

1. Use QUESTIONNAIRE.md as a checklist
2. Review output-based questions
3. Practice explaining concepts

For Deep Learning:

1. Read and understand each implementation
2. Run the code and see outputs
3. Modify code to test edge cases
4. Add your own test cases

🎯 Interview Tips

What Interviewers Look For:

• Problem-solving approach: Think out loud
• Code quality: Clean, readable code
• Edge cases: Consider boundary conditions
• Time/Space complexity: Analyze your solution
• Communication: Explain your thought process

Common Topics:

• ✅ Promises & Async/Await
• ✅ Closures & Scope
• ✅ Event Loop & Execution Context
• ✅ Array/Object methods
• ✅ this keyword
• ✅ Design Patterns
• ✅ Performance Optimization
• ✅ System Design basics

📚 Key Concepts Covered

JavaScript Fundamentals

• Execution Context
• Hoisting
• Closures
• Prototypes
• this binding

Async JavaScript

• Promises
• Async/Await
• Event Loop
• Microtasks vs Macrotasks
• Concurrent execution

Design Patterns

• Singleton
• Observer (Pub-Sub)
• Circuit Breaker
• Proxy
• Factory

Data Structures & Algorithms

• Graphs (Currency Converter)
• Trees (File System)
• Queues (Rate Limiter, Middleware)
• BFS/DFS

Performance

• Debouncing
• Throttling
• Memoization
• Resource Pooling

🏢 Company-Specific Questions

This repository includes actual questions asked at:

• Tekion
• Thoughtspot
• And other top companies

🔗 Related Topics Not Covered

This is a JavaScript-focused repository. For complete interview prep, also study:

• React/Vue/Angular (if required)
• TypeScript
• Testing (Jest, Testing Library)
• Build tools (Webpack, Vite)
• Git & Version Control
• System Design (for senior roles)

📈 Progress Tracking

Use this checklist to track your progress:

Async Patterns:

• asyncSequence
• FetchWithLimit
• mapLimit
• PromiseSchedulor

Design Patterns:

• Circuit Breaker
• Singleton
• Pub-Sub
• Proxy
• Resource Pool

Polyfills:

• Promise
• Promise.all/allSettled
• call/bind/apply
• map/reduce/forEach
• Array.flat
• Object.flatten

LLD:

• File System
• Order Management
• Rate Limiter
• Middleware

Concepts:

• this keyword
• Promises
• Event Loop
• Closures

🤝 Contributing

Feel free to:

• Add more questions
• Improve implementations
• Fix bugs
• Add test cases
• Improve documentation

📄 License

This repository is for educational purposes. Feel free to use for interview preparation.

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🌟 Star this repo if it helped you!

Good luck with your interviews! 🚀

Pro Tip: Don't just memorize solutions. Understand the concepts and practice explaining them in simple terms.