1. What is Hoisting in JavaScript?

Answer: Hoisting is JavaScript’s default behaviour of moving declarations (not initializations) to the top of their scope before code execution.
Only var declarations are hoisted. let & const are also hoisted but stay in Temporal Dead Zone (TDZ).

// Example
console.log(a); // undefined
var a = 10;

console.log(b); // ReferenceError: Cannot access 'b' before initialization
let b = 20;

2. What is the difference between var, let and const?

Answer:

• var → function-scoped, hoisted, can be re-declared & updated
• let → block-scoped, hoisted (TDZ), cannot be re-declared, can be updated
• const → block-scoped, hoisted (TDZ), cannot be re-declared nor re-assigned (but object properties can be mutated)

3. Explain this keyword with all possible cases.

Answer:

• Global context → window (non-strict) / undefined (strict)
• Object method → refers to the object
• Simple function → undefined in strict mode
• Arrow function → inherits this from enclosing lexical scope
• call / apply / bind → explicitly set
• Event listener → refers to the DOM element

4. What is Closure? Give a practical example.

Answer: A closure is a function that remembers its outer variables even after the outer function has returned.

function createCounter() {
  let count = 0;
  return function() {
    return ++count;
  }
}
const counter = createCounter();
console.log(counter()); // 1
console.log(counter()); // 2

5. Explain Event Loop in simple words.

Answer: Event Loop has one job: check if Call Stack is empty → if yes, push the first task from Callback Queue (or Microtask Queue) is pushed to Call Stack.

Priority: Microtask Queue (Promises, MutationObserver) > Callback Queue (setTimeout, DOM events).

6. What will be the output?

console.log('1');
setTimeout(() => console.log('2'), 0);
Promise.resolve().then(() => console.log('3'));
console.log('4');

Answer: 1 → 4 → 3 → 2

7. Difference between == and ===?

== does type coercion, === checks value and type both (strict equality).

8. What is Prototype Inheritance?

Every object in JS has a hidden [[Prototype]] property that points to another object (or null). This chain is used for property lookup.

9. Explain async/await vs Promises.

async/await is syntactic sugar over Promises – makes asynchronous code look synchronous and easier to read.

10. What are Higher-Order Functions?

Functions that take other functions as arguments or return a function (map, filter, reduce, etc.).

11–50 Quick List (Full answers available in downloadable PDF below)

11. What is Debouncing and Throttling?
12. Explain Call, Apply and Bind with example
13. What is Currying?
14. Difference between null and undefined
15. What are Promises? States?
16. Promise.all vs Promise.allSettled vs Promise.race
17. What is Event Bubbling and Capturing?
18. How to stop propagation?
19. What is event.target vs event.currentTarget?
20. Explain fetch API and how to handle errors
21. What are Modules in JavaScript (ES6 import/export)?
22. Difference between module.exports and export default
23. What is the use of strict mode?
24. Explain IIFE with example
25. What is Temporal Dead Zone (TDZ)?
26. Explain Spread vs Rest operator
27. What are Arrow functions limitations?
28. Difference between forEach and map
29. What is NaN and how to check it?
30. Explain Object.freeze() vs Object.seal()
31. What are Generators?
32. Explain Symbol data type
33. What is Proxy and Reflect?
34. Difference between delete and undefined
35. What is Memoization?
36. How does Garbage Collection work in JS?
37. What is Optional Chaining (?.) and Nullish Coalescing (??)?
38. Explain BigInt
39. What are WeakMap and WeakSet?
40. Difference between localStorage and sessionStorage
41. How to deep clone an object?
42. What is Array.flat() and Array.flatMap()?
43. Explain Promise.any() and Promise.withResolvers()
44. What are Top-Level Awaits?
45. How to handle memory leaks in JavaScript?
46. What is the difference between slice() and splice()?
47. Explain reduce() with real-world example
48. What is event delegation> and why use it?
49. How does JSON.stringify and JSON.parse work with circular references?
50. What are Service Workers?
51. Explain CORS and how to fix it
52. What is Tree Shaking?
53. Difference between typeof and instanceof
54. How to detect dark mode in JavaScript?
55. What is the purpose of Object.hasOwn() (new in ES2022)?
56. Explain Private class fields (#)
57. What are JavaScript Design Patterns you know?
58. How would you implement inheritance in ES6 classes?
59. Explain the most tricky question you faced in interview? (Bonus 😄)