Whether you're aiming for a front-end role, a full-stack position, or just looking to strengthen your core JavaScript skills, this guide covers the most important JavaScript interview topics to help you ace any technical interview in 2025.

🔹 Core JavaScript Fundamentals

  1. Data Types – Primitive vs Reference, typeof, dynamic typing.
  2. Variables – var, let, and const: differences and best practices.
  3. Hoisting – Function and variable hoisting behavior.
  4. Scopes – Global, local, block-level, lexical scope.
  5. Closures – Function inside function, data encapsulation.
  6. Functions – Declarations, expressions, arrow functions, IIFE.
  7. This Keyword – How this behaves in different contexts.
  8. Strict Mode – 'use strict' and its implications.
  9. ES6+ Features – Spread, rest, destructuring, template literals, default parameters.
  10. Truthy & Falsy Values – Conditional evaluations.

🔹 Advanced JavaScript Concepts

  1. Promises & Async/Await – Asynchronous JavaScript, chaining.
  2. Event Loop & Call Stack – Execution context, microtasks, and macrotasks.
  3. Prototypes & Inheritance – Prototype chain, __proto__, Object.create().
  4. Event Delegation – Efficient event handling.
  5. Debounce & Throttle – Optimizing event-heavy operations.
  6. Currying & Partial Application – Functional programming.
  7. Memoization – Caching function results.
  8. Modules – ES Modules (import/export) vs CommonJS (require/module.exports).
  9. Garbage Collection – How JS manages memory.
  10. WeakMap & WeakSet – Memory-sensitive collections.

🔹 DOM Manipulation & Events

  1. DOM API – getElementById, querySelector, createElement, etc.
  2. Event Listeners – addEventListener, bubbling vs capturing.
  3. Event Object – target, currentTarget, stopPropagation.
  4. Forms & Input Handling – Validation, form submission.
  5. Virtual DOM – Conceptual difference from real DOM (React-focused).

🔹 Error Handling

  1. try-catch-finally – Standard error handling.
  2. Custom Errors – Creating your own error types.
  3. Promise Rejection Handling – .catch(), async error handling.

🔹 Object-Oriented JavaScript

  1. Constructor Functions
  2. ES6 Classes – class, constructor, extends, super
  3. Encapsulation & Inheritance
  4. Static vs Instance Methods

🔹 Functional Programming in JS

  1. Pure Functions
  2. Higher-Order Functions – map, filter, reduce
  3. Immutability – Spread operator, Object.assign()
  4. Composition & Chaining

🔹 Browser APIs & Tools

  1. LocalStorage & SessionStorage
  2. Fetch API – REST calls using fetch()
  3. WebSockets Basics
  4. Timers – setTimeout, setInterval
  5. Geolocation API, History API, etc.

🔹 JavaScript in the Ecosystem

  1. Node.js Basics – Modules, NPM, event-driven architecture.
  2. Package Managers – npm, yarn
  3. Bundlers & Transpilers – Webpack, Babel
  4. Linting & Formatting – ESLint, Prettier
  5. Testing – Jest, Mocha, Chai

🔹 Security in JavaScript

  1. XSS and CSRF – Prevention techniques
  2. Content Security Policy (CSP)
  3. CORS – Cross-Origin Resource Sharing
  4. Sanitizing Inputs – DOMPurify or manual methods

🔹 Common Interview Tasks

  1. Deep Clone vs Shallow Clone – Spread, JSON, structuredClone()
  2. Debounce/Throttle Implementation
  3. Custom bind, call, apply Functions
  4. Array Manipulations – Flatten, chunk, deduplicate
  5. Create a Custom Promise Implementation
  6. Polyfills – Promise, map, bind

Final Tips

  • Build real projects to master concepts.
  • Practice DSA in JavaScript to crack coding rounds.
  • Stay up to date with the latest ECMAScript features.
  • Contribute to open-source or read source code on GitHub.

JavaScript is vast, but focused preparation can help you master the essentials. Bookmark this post and start practicing today!