Day-43 Interview Questions and Answers (Frontend + Node.js + JavaScript)

1. What is the difference between a function declaration and function expression?

• Function Declaration: Declared with the function keyword and hoisted.

  function greet() { console.log('Hello'); }

• Function Expression: Assigned to a variable; not hoisted.

  const greet = function() { console.log('Hello'); }

2. How does setTimeout work?

• It runs a function after a specified delay asynchronously.
• Uses the event loop: the function is placed in a task queue after the delay.

3. How do you select elements from the DOM?

• document.getElementById('id')
• document.querySelector('selector')
• document.getElementsByClassName('class')
• document.querySelectorAll('selector')

4. What is the purpose of async and await in JavaScript?

• Handle asynchronous operations in a synchronous way.
• Helps avoid callback hell.

async function fetchData() {
  const data = await fetch('url');
}

5. What is a callback function?

• A function passed as an argument to another function to be executed later.

setTimeout(() => { console.log('Hello'); }, 1000);

6. What is Node.js and how does it work?

• Node.js is a JavaScript runtime built on Chrome's V8 engine.
• Uses non-blocking I/O and an event-driven model to handle multiple connections with a single thread.

7. For Node.js, why does Google use the V8 engine?

• V8 is fast, compiles JavaScript to native machine code.
• Provides high performance and efficient memory management.

8. Why is Node.js single-threaded?

• Node uses a single-threaded event loop for scalability.
• Handles multiple requests asynchronously using callbacks, promises, and async/await.

9. Can you access DOM in Node.js?

• ❌ No, DOM is a browser-specific API. Node.js runs on the server.

10. How to create a simple HTTP server in Node.js?

const http = require('http');
const server = http.createServer((req, res) => {
  res.end('Hello World');
});
server.listen(3000);

11. What is the difference between synchronous and asynchronous functions?

• Synchronous: Blocks the execution until the task completes.
• Asynchronous: Doesn't block. Tasks like API calls, timers are handled in the background.

12. What are the different types of HTTP requests?

• GET: Retrieve data.
• POST: Send data.
• PUT: Update entire resource.
• PATCH: Update part of a resource.
• DELETE: Delete resource.
• OPTIONS, HEAD

13. Explain this keyword.

• In object: Refers to the object itself.
• In function (non-strict): Refers to the global object.
• In strict mode or arrow functions: this is undefined or inherits from the parent scope.

14. What is the difference between undefined and null?

• undefined: Variable declared but not assigned.
• null: Intentional assignment to represent no value.

15. What is the difference between == and === in JavaScript?

• ==: Compares values with type coercion.
• ===: Strict comparison without type coercion.

16. What are truthy and falsy values?

• Falsy: false, 0, '', null, undefined, NaN
• Truthy: Anything that is not falsy.

17. What is the difference between global and local scope?

• Global Scope: Available anywhere in the code.
• Local Scope: Available only within functions or blocks.

18. How to optimize a webpage for performance?

• Compress images.
• Minify CSS, JS.
• Use CDN.
• Lazy load assets.
• Implement caching.
• Remove unused code.

19. How to handle CORS in JavaScript?

• Set headers like:

Access-Control-Allow-Origin: '*'

• Or handle via backend or use proxies.

20. What is a closure and how does it work?

• A function that remembers its outer scope, even after the outer function has finished.

function outer() {
  let count = 0;
  return function inner() { count++; }
}

21. Explain CSS positions: relative, absolute, fixed.

• Relative: Positioned relative to itself.
• Absolute: Positioned relative to the nearest positioned ancestor.
• Fixed: Sticks relative to the viewport.

22. What is a media query and how does it work?

• Used to make responsive designs based on device size.

@media (max-width: 600px) {
  body { background: red; }
}

23. What is SPA (Single Page Application)?

• Loads a single HTML page.
• Updates content dynamically without refreshing the entire page.

24. Client-side vs Server-side Rendering

• Client-side: Rendering happens in the browser.
• Server-side: Rendering happens on the server; better SEO and faster initial load.

25. Explain npm and yarn.

• Both are JavaScript package managers.
• Yarn is generally faster with better caching and offline capabilities.

26. How do you manage version control and collaboration using Git?

• Branching (git branch)
• Merging (git merge)
• Pull requests (collaborative review)
• Resolving conflicts and version tracking.

27. What are build tools like Vite or Webpack used for?

• Bundling, transpiling, and optimizing front-end code for development and production.

28. What is the difference between development and production environments?

• Development: Includes debugging, detailed logs, source maps.
• Production: Optimized, minified, no source maps, faster.

29. How do you debug a frontend application?

• Use browser DevTools (Console, Network, Sources).
• Add breakpoints.
• Use console.log() for quick checks.

30. Difference between id and class in HTML?

• id: Unique identifier (#id), used once per page.
• class: Reusable (.class), used for styling and selecting multiple elements.

31. How to use the CSS box model?

• Content → Padding → Border → Margin
• Controls layout spacing in CSS.

32. Difference between arrow function and traditional function.

• Arrow functions: Don't have their own this, arguments.
• Shorter syntax.

const add = (a, b) => a + b;

33. Explain em, rem, and px in CSS.

• px: Absolute pixels.
• em: Relative to parent font size.
• rem: Relative to root (html) font size.

34. What are template literals and why are they used?

• String literals using backticks for interpolation.

`Hello, ${name}`

• Supports multi-line strings and embedding expressions.

35. Explain var, let, and const.

• var: Function-scoped, hoisted.
• let: Block-scoped, not hoisted.
• const: Block-scoped, cannot be reassigned.

36. Explain synchronous and asynchronous in JavaScript.

• Synchronous: Runs line by line, blocks execution.
• Asynchronous: Runs in the background, non-blocking.

37. How does the event loop work in JavaScript?

• Executes synchronous code first.
• Moves asynchronous tasks (like setTimeout, promises) to the callback/task queue.
• Processes them when the call stack is empty.

Conclusion

This numbered format gives a clean overview of the most important frontend, JavaScript, and Node.js concepts for interviews. You can directly use this for your blog or study notes.