🚀 Must-Know JavaScript & Node.js Topics for Interviews in 2025

If you're preparing for a software engineering interview—whether for a startup or a top tech company—it's crucial to have a solid understanding of JavaScript and Node.js fundamentals. Having gone through several interviews and worked extensively in full-stack development, I’ve compiled a refined list of core concepts that are commonly asked and essential for building real-world applications.

This list is structured to give you a roadmap for preparation, especially if you're targeting backend roles or full-stack positions using JavaScript.

🧠 Core JavaScript Concepts

• let, const, var – When and why to use them
• Data Types and Type Conversion
• Hoisting – How variables/functions are moved in memory
• Scope – Block, function, and global
• Closures – Functions with preserved data
• Lexical Environment & Execution Context
• Call Stack – Behind the scenes of JS execution
• Synchronous vs Asynchronous behavior
• Event Loop, Microtasks, Macrotasks
• Callback Functions, Promises, and Async/Await
• setTimeout, setInterval
• Higher Order Functions
• map, filter, reduce, forEach
• Pure Functions and Side Effects
• Spread and Rest Operators
• Object and Array Destructuring
• Optional Chaining and Nullish Coalescing
• Shallow Copy vs Deep Copy
• this keyword (and arrow function differences)
• Call, Apply, Bind
• Prototypes and Inheritance
• Classes and Constructor Functions
• Currying, Memoization, Debouncing, Throttling
• JSON methods: stringify and parse
• Call by Value vs Call by Reference
• Error Handling: try / catch / finally

🛠️ Node.js & Backend Essentials

• Node.js Architecture – Event-driven, non-blocking I/O
• Single-threaded nature & handling concurrency
• Built-in modules: fs, http, path, etc.
• CommonJS vs ES Modules
• NPM and package.json
• Express.js – Basic setup and routing
• CRUD operations with Express
• HTTP Methods – GET, POST, PUT, PATCH, DELETE
• Middleware and Error Handling Middleware
• Async/Await with Express routes
• Connecting Node.js with databases (MongoDB / SQL)
• Using dotenv for environment configs
• CORS – Cross-origin issues and how to solve them
• Schema Design and Database Models
• Pagination, Filtering, Sorting in APIs
• MVC Architecture
• Authentication – JWT (JSON Web Tokens)
• Password hashing with Bcrypt
• API Security Basics – Rate Limiting, Helmet, etc.
• File Upload with Multer

🧵 Advanced Node.js Topics

• Worker Threads & Multithreading in Node.js
• Clustering & Load Balancing
• Event Emitters
• Streams & Buffers
• Child Processes

🧪 Testing & Deployment Basics

• Unit Testing with Jest or Mocha
• API Testing with Postman
• Introduction to CI/CD
• Deploying apps on platforms like Render, Railway, or Vercel

💡 Bonus Interview Questions

• Difference between == and ===
• Why console.log(a) before declaration shows undefined?
• Real-life use case of closures
• Deep clone vs shallow clone methods
• Difference between PUT and PATCH
• Callback hell and how to fix it
• How does the event loop handle async operations?
• Is JavaScript multithreaded?
• What happens under the hood with async/await?
• Garbage collection and memory management

✅ Final Thoughts

This list isn't just for clearing interviews—it's a roadmap to becoming a confident JavaScript and Node.js developer. If you’re just getting started or looking to brush up, tackling these topics one by one will greatly boost your confidence and technical depth.

Whether you're building projects, preparing for interviews, or just sharpening your skills—mastering these fundamentals is non-negotiable.

💬 Let me know in the comments if you'd like a detailed breakdown or learning path for any of these topics!

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