Top 100 JavaScript questions asked in interview?

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 Here’s a comprehensive list of 100 commonly asked JavaScript interview questions, covering a range of topics from basic to advanced concepts:

Basic Questions

  1. What is JavaScript?
  2. What are the data types supported in JavaScript?
  3. What is the difference between undefined and null?
  4. Explain variable hoisting in JavaScript.
  5. What are primitive and non-primitive data types?
  6. How does JavaScript handle type coercion?
  7. What is a closure?
  8. How do you create a function in JavaScript?
  9. What is the difference between == and ===?
  10. What is a callback function?
  11. What are arrow functions?
  12. Explain the this keyword in JavaScript.
  13. What is event bubbling and event capturing?
  14. What is the purpose of the bind, call, and apply methods?
  15. What are JavaScript templates (template literals)?
  16. What are the different types of scopes in JavaScript?
  17. Explain the concept of promises.
  18. What is the fetch API?
  19. How do you handle asynchronous code in JavaScript?
  20. What is an IIFE (Immediately Invoked Function Expression)?

Intermediate Questions

  1. What is a prototype in JavaScript?
  2. Explain the prototype chain.
  3. What are JavaScript modules?
  4. How do you create a class in JavaScript?
  5. What is the difference between let, const, and var?
  6. What are destructuring assignments?
  7. Explain the spread operator (...).
  8. What is the rest parameter?
  9. What is the difference between shallow copy and deep copy?
  10. How do you implement inheritance in JavaScript?
  11. What are higher-order functions?
  12. Explain the concept of currying.
  13. What is a service worker?
  14. What are Web Workers?
  15. What is the purpose of setTimeout and setInterval?
  16. What is debouncing and throttling?
  17. How do you create an object in JavaScript?
  18. What are the different ways to declare a function?
  19. What is the difference between a function expression and a function declaration?
  20. Explain the concept of event delegation.

Advanced Questions

  1. What is the difference between synchronous and asynchronous programming?
  2. What are decorators in JavaScript?
  3. Explain the async/await syntax.
  4. What is the purpose of the instanceof operator?
  5. How does garbage collection work in JavaScript?
  6. What are closures and how are they used?
  7. Explain the concept of the event loop.
  8. What is the difference between map, filter, and reduce?
  9. What is memoization?
  10. What are the built-in methods for arrays in JavaScript?
  11. How do you manage state in JavaScript applications?
  12. What is a memory leak in JavaScript, and how can you prevent it?
  13. What are JSON and JSONP?
  14. How do you manipulate the DOM using JavaScript?
  15. What is the localStorage and sessionStorage?
  16. What is the purpose of Object.freeze()?
  17. How do you create a singleton in JavaScript?
  18. Explain the difference between call and apply.
  19. What is the window object?
  20. What are symbol data types?

Practical Questions

  1. How do you reverse a string in JavaScript?
  2. How do you check if a string is a palindrome?
  3. Write a function to remove duplicates from an array.
  4. How do you sort an array of objects?
  5. Write a function to flatten a nested array.
  6. How can you merge two arrays without duplicates?
  7. Write a function to debounce another function.
  8. How do you check if an object is empty?
  9. How can you generate a random number between two values?
  10. Write a function to count the occurrences of elements in an array.

Concepts and Best Practices

  1. What is the difference between a shallow copy and a deep copy?
  2. How do you optimize performance in JavaScript applications?
  3. What are some best practices for writing clean JavaScript code?
  4. Explain the concept of functional programming.
  5. How can you handle errors in JavaScript?
  6. What is the difference between functional and object-oriented programming?
  7. How do you prevent XSS (Cross-Site Scripting) attacks?
  8. What are the common security risks in JavaScript?
  9. Explain the concept of polyfills.
  10. How do you use the this keyword in different contexts?

Real-World Application Questions

  1. How would you implement a basic routing mechanism in a single-page application?
  2. What is the significance of the DOMContentLoaded event?
  3. How can you implement lazy loading in JavaScript?
  4. What is Cross-Origin Resource Sharing (CORS)?
  5. How do you handle form validation in JavaScript?
  6. Explain the concept of microservices in relation to JavaScript applications.
  7. How do you implement real-time communication in a web application?
  8. What is the purpose of the Service Worker API?
  9. How can you create a responsive web design using JavaScript?
  10. How do you manage dependencies in a JavaScript project?

Miscellaneous Questions

  1. What are some common JavaScript design patterns?
  2. What is the difference between function.prototype.bind and function.prototype.call?
  3. What are some of the most significant features introduced in ES6?
  4. How do you test JavaScript code?
  5. What is the difference between the global scope and functional scope?
  6. How can you optimize the loading time of a JavaScript application?
  7. What are the benefits of using a task runner like Gulp or Grunt?
  8. How do you ensure compatibility across different browsers?
  9. What are the performance implications of using eval() in JavaScript?
  10. How can you implement a custom iterator in JavaScript?

Conclusion

These questions encompass a wide range of topics in JavaScript, from fundamental concepts to advanced features and practical applications. Preparing for these can give you a solid foundation for interviews and help demonstrate your expertise in JavaScript. Good luck!

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