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100 JavaScript Interview Questions and Answers

Web & Backend · 100 questions, each with a full written answer — free, no sign-up.

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Basic 20Intermediate 20Advanced 20Expert 20Guru 20

Basic

  1. What is JavaScript, and how is it different from Java?
  2. What are some key features of JavaScript?
  3. What are the data types in JavaScript?
  4. How do you declare a variable in JavaScript?
  5. What is the difference between "==" and "===" in JavaScript?
  6. What are the various types of loops in JavaScript, and how do they work?
  7. What is the purpose of a function in JavaScript?
  8. How do you write a simple function in JavaScript?
  9. What is the difference between "var," "let," and "const" when declaring a variable?
  10. How do you add a comment in JavaScript code?
  11. What are the different ways to create an object in JavaScript?
  12. What is an array, and how do you create one in JavaScript?
  13. What is the DOM (Document Object Model)?
  14. How do you select an HTML element using JavaScript?
  15. What are event listeners, and how do you use them in JavaScript?
  16. What is the difference between "null" and "undefined" in JavaScript?
  17. What is the purpose of JSON, and how does it relate to JavaScript?
  18. How do you concatenate strings in JavaScript?
  19. What are some common string methods in JavaScript?
  20. What is the difference between "function declaration" and "function expression"?

Intermediate

  1. What is a closure in JavaScript, and why is it useful?
  2. Can you explain the difference between "call," "apply," and "bind" methods in JavaScript?
  3. What are promises, and how do they work in JavaScript?
  4. What is the concept of hoisting in JavaScript?
  5. How does event delegation work in JavaScript, and why is it important?
  6. What is the prototype-based inheritance in JavaScript?
  7. What is the "this" keyword in JavaScript, and how does it behave in different contexts?
  8. Can you explain the differences between synchronous and asynchronous JavaScript code?
  9. What are arrow functions, and what advantages do they have over regular functions?
  10. How do you handle exceptions in JavaScript using try-catch blocks?
  11. What are the different ways to create a copy of an object in JavaScript?
  12. How do you handle asynchronous operations with async/await?
  13. What are JavaScript modules, and how do you import and export them?
  14. What is the purpose of template literals in JavaScript, and how do you use them?
  15. How does the event loop work in JavaScript?
  16. Can you explain the difference between a shallow copy and a deep copy of an object in JavaScript?
  17. What are some popular design patterns in JavaScript, and can you provide an example of one?
  18. How do you manipulate the DOM efficiently to reduce performance issues?
  19. What are higher-order functions, and why are they useful in JavaScript?
  20. Can you explain the concept of debouncing and throttling in JavaScript, and why they are important for performance optimization?

Advanced

  1. Can you explain the concept of "memoization" and provide an example of how it can be used in JavaScript?
  2. What is the IIFE (Immediately Invoked Function Expression) pattern, and why is it useful in JavaScript?
  3. How do you implement custom iterators using JavaScript’s iterator and generator functions?
  4. What are the key differences between Web Workers and Service Workers in JavaScript?
  5. What is the Shadow DOM, and how does it help with web component encapsulation?
  6. Can you explain the concept of "currying" in JavaScript and provide an example?
  7. How do you implement a custom "bind" function in JavaScript?
  8. What are the different ways to handle module loading in JavaScript, such as CommonJS, AMD, and ES6 Modules?
  9. How do you implement a pub-sub pattern in JavaScript?
  10. What is the role of JavaScript in server-side programming, and how does Node.js enable this?
  11. What are some performance optimization techniques for JavaScript applications?
  12. Can you explain the difference between microtask queue and macrotask queue in JavaScript?
  13. How do you implement a polyfill for a specific JavaScript feature?
  14. What is the concept of "strict mode" in JavaScript, and why is it important?
  15. What are the differences between virtual DOM and real DOM, and how do libraries like React utilize virtual DOM?
  16. Can you explain the purpose and usage of JavaScript Proxy objects?
  17. What is the importance of CORS (Cross-Origin Resource Sharing), and how does it relate to JavaScript?
  18. What is the role of JavaScript in Progressive Web Apps (PWAs)?
  19. How do you implement a custom "new" operator in JavaScript to create objects?
  20. What are JavaScript decorators, and how do you use them?

Expert

  1. Can you explain the differences between Observables and Promises in handling asynchronous operations?
  2. What are some techniques for optimizing JavaScript garbage collection to improve application performance?
  3. How do you handle large datasets efficiently in JavaScript, and what data structures or algorithms can help?
  4. How do you implement code splitting in a JavaScript application to improve its performance?
  5. What are some best practices for optimizing JavaScript code execution in a browser environment?
  6. How do you manage application state effectively in large-scale JavaScript applications?
  7. What are some of the security concerns related to JavaScript, and how do you mitigate them?
  8. Can you explain the use of JavaScript in WebAssembly and how it can enhance web applications’ performance?
  9. How do you manage memory leaks in JavaScript applications?
  10. How do you implement a custom "require" function to load modules dynamically in a Node.js application?
  11. What are the considerations for choosing between server-side rendering and client-side rendering in a JavaScript application?
  12. How do you implement an efficient search algorithm in JavaScript, such as a trie or a suffix tree, for large text datasets?
  13. How do you build a JavaScript application with offline capabilities and data synchronization for when connectivity is restored?
  14. Can you explain the concept of "tree shaking" in JavaScript and how it helps optimize code bundling?
  15. How do you implement a custom event system in JavaScript that allows for event registration, deregistration, and event propagation?
  16. What are the various ways to handle internationalization (i18n) and localization (l10n) in JavaScript applications?
  17. How do you ensure the accessibility of a JavaScript application following Web Content Accessibility Guidelines (WCAG)?
  18. Can you explain the role of JavaScript in serverless architectures and how it can be implemented using platforms like AWS Lambda or Azure Functions?
  19. How do you implement real-time communication in JavaScript applications using technologies like WebSockets or WebRTC?
  20. Can you discuss some emerging JavaScript technologies, libraries, or frameworks, and explain their benefits and potential use cases?

Guru

  1. Can you discuss the inner workings of the JavaScript engine, such as V8 or SpiderMonkey, and how they optimize JavaScript code execution?
  2. What are some advanced techniques for optimizing the performance of single-page applications (SPAs) in JavaScript?
  3. How do you ensure scalability and high availability in large-scale JavaScript applications, both on the client and server sides?
  4. How do you design and implement a custom JavaScript framework or library, and what are some key considerations and challenges?
  5. Can you explain the process of implementing advanced animations or interactive experiences in JavaScript, including performance optimizations and best practices?
  6. How do you profile and optimize the JavaScript code execution on both the CPU and GPU for highly performant applications?
  7. What are some advanced concurrency patterns in JavaScript for managing complex asynchronous operations or parallel computing?
  8. Can you discuss the role of JavaScript in the Internet of Things (IoT) ecosystem and the challenges of running JavaScript on resource-constrained devices?
  9. How do you build and maintain a highly modular, testable, and maintainable JavaScript codebase for large-scale applications?
  10. Can you explain the process of implementing end-to-end encryption in JavaScript applications and discuss potential security risks and mitigations?
  11. How do you design and implement real-time collaborative editing features in JavaScript applications, such as operational transformation or conflict-free replicated data types (CRDTs)?
  12. Can you discuss the role of JavaScript in the development and implementation of machine learning and artificial intelligence algorithms in the browser or on the server-side with Node.js?
  13. How do you implement advanced JavaScript performance monitoring and profiling tools to identify bottlenecks and optimize resource usage?
  14. Can you explain the process of implementing a custom garbage collector in a JavaScript environment to optimize memory management?
  15. How do you design and implement a JavaScript application with advanced accessibility features, such as support for screen readers or other assistive technologies?
  16. Can you discuss the process of implementing a custom JavaScript transpiler or interpreter for a specific use case or platform?
  17. What are the most significant changes or advancements in the ECMAScript specification over the past few years, and how have they impacted the JavaScript ecosystem?
  18. Can you explain the process of implementing advanced data visualization techniques in JavaScript, such as real-time, interactive data visualization or geospatial mapping?
  19. How do you optimize JavaScript applications for low-power or low-bandwidth environments, such as mobile devices or rural areas with limited connectivity?
  20. Can you discuss some cutting-edge JavaScript research, innovations, or developments and explain their potential implications for the future of web development?
📕 Buy this interview preparation book: 100 JavaScript questions & answers — PDF + EPUB for $5
Reading is step one. Saying it out loud is the interview. Our AI interviewer calls your phone and runs a realistic JavaScript interview — then scores it.
📞 Practice JavaScript — free 15 min
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