JavaScript Interview Questions & Answers – Part 2: DOM, Types & Advanced Topics

Continue your JavaScript interview preparation with Part 2 of our Q&A series. This set of 25 curated questions dives into DOM & Browser APIs, JavaScript types and coercion, advanced concepts like closures and hoisting, and various miscellaneous topics to help you build deeper understanding and tackle tricky interview problems.

📦 DOM & Browser APIs

1. What is the DOM?
Document Object Model – tree representation of HTML

2. How to select DOM elements?

document.getElementById("id"), querySelector(".class")

3. How to add/remove event listeners?

element.addEventListener('click', handler);

4. What is event delegation?
Event handling at parent level to reduce listeners.
5. What are localStorage and sessionStorage?
Store key-value data in browser; localStorage persists, sessionStorage clears on close.

🧪 Types & Coercion

6. What is type coercion?
JS automatically converts types when needed (e.g., '5' + 1 → '51')
7. How to check the type of a variable?
```
typeof variable
```
8. How to convert a string to a number?
```
Number("5"), parseInt("5"), +"5"
```
9. Falsy values in JavaScript?
false, 0, "", null, undefined, NaN,
10. What is NaN?
“Not a Number” – result of invalid numeric operations.

🛠️ Advanced Topics

11. What is a generator function?
```
function* gen() { yield 1; yield 2; }
```

12. What are modules in JS?

export const a = 5; import {a} from './file';

13. Difference between deep freeze and shallow freeze?
Object.freeze makes top-level immutable; not nested objects.
14. What is memoization?
Caching results of function calls to optimize performance.
15. What are higher-order functions?
Functions that take or return other functions.

📚 Miscellaneous

16. What is debouncing and throttling?
Debounce: delays function until inactivity
Throttle: limits function execution rate
17. What is the difference between map, filter, and reduce?
map transforms, filter selects, reduce accumulates
18. What is JSON and how do you parse it?
```
JSON.parse(jsonStr), JSON.stringify(obj)
```
19. What is the difference between forEach and map?
map returns new array, forEach doesn’t.
20.What is the Temporal Dead Zone (TDZ)?
Time between declaration and initialization of let / const where accessing it throws error.
21. What is optional chaining (?.)
```
let name = user?.profile?.name;
```
22. What are template literals?
```
`Hello ${name}`
```
23.What is the difference between Array.of() and Array()?
Array.of(3) creates [3], Array(3) creates empty of length 3.
24. What is a symbol in JavaScript?
A unique, immutable primitive used as object keys.

25. What is the use of Object.keys(), Object.values(),Object.entries() ?

Object.keys(obj) // ['a']
Object.values(obj) // [1]
Object.entries(obj) // [['a', 1]]

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