Title:Interview Questions on Javascript – Top 50+ Most Asked in 2025

Description:Interview Questions on Javascript: Prepare with the top 50+ javascript interview questions for freshers in 2025. Get simple answers, coding examples, and expert tips for javascript interviews and placements.

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Interview Questions on Javascript – Top 50+ Most Asked in 2025

Interview Questions on Javascript are essential for anyone preparing for developer roles in 2025. This blog covers the most asked javascript interview questions for freshers, entry-level jobs, and college placements. Each question includes a clear, concise answer and practical coding examples to help you ace your next interview.

1. What is JavaScript and why is it used?

JavaScript is a high-level, interpreted scripting language that enables interactive web experiences. It’s used for dynamic content, form validation, animations, and server-side development (Node.js). JavaScript is essential for both front-end and back-end development.

2. What are the different data types present in JavaScript?

JavaScript supports:

Primitive: Number, String, Boolean, Undefined, Null, Symbol, BigInt
Non-Primitive: Object (Array, Function, Date, etc.)
Understanding these helps you write robust code and avoid bugs.

3. Explain the difference between `let`, `const`, and `var`.

var: Function-scoped, can be re-declared/updated.
let: Block-scoped, can be updated but not re-declared in the same scope.
const: Block-scoped, cannot be updated or re-declared.
Use let and const for modern, predictable code.

4. What is hoisting in JavaScript?

Hoisting is JavaScript’s default behavior of moving declarations to the top of the scope. var is hoisted and initialized as undefined; let and const are hoisted but not initialized. Function declarations are fully hoisted.

5. What is the difference between `==` and `===`?

== compares values after type coercion, while === compares both value and type.
Example: 5 == '5' is true, but 5 === '5' is false.

6. What is a closure in JavaScript?

A closure is a function with access to its own scope, the outer function’s scope, and the global scope. Closures enable data privacy and stateful functions.

7. Explain event bubbling and event capturing.

Event Bubbling: Event propagates from the innermost element outward.
Event Capturing: Event propagates from the outermost element inward.
Use event.stopPropagation() to control propagation.

8. What is the DOM?

DOM (Document Object Model) is a programming interface for HTML/XML. It represents the page as a tree, allowing JavaScript to manipulate content and structure dynamically.

9. What is the BOM?

BOM (Browser Object Model) lets you interact with the browser outside the document, including window, navigator, location, history, and screen.

10. How do you create an object in JavaScript?

Use object literals, constructors, or Object.create():js

let obj = { name: "Alice", age: 25 };

11. What is the purpose of `this` keyword?

this refers to the object from which the function was called. In global scope, it refers to the global object (window in browsers).

12. What are arrow functions and how do they differ from regular functions?

Arrow functions have a shorter syntax and don’t have their own this, arguments, or super. Ideal for callbacks and non-method functions.js

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

13. Explain the difference between `null` and `undefined`.

null: Assigned value representing no value.
undefined: Variable declared but not assigned a value.

14. How do you check if a variable is an array?

Use Array.isArray(variable).

15. What are template literals in JavaScript?

Template literals allow embedded expressions and multi-line strings using backticks:js

let name = "Bob";

console.log(`Hello, ${name}!`);

16. What is the use of `isNaN()` function?

Checks if a value is NaN (Not a Number). Returns true if not a number, false otherwise.

17. How do you handle errors in JavaScript?

Use try...catch...finally blocks:js

try {

  // code

} catch (error) {

  // handle error

} finally {

  // always runs

}

18. What is a callback function?

A function passed as an argument to another function, executed after the parent function completes.

19. What is async/await in JavaScript?

async and await simplify working with promises, making asynchronous code look synchronous.js

async function fetchData() {

  let response = await fetch(url);

  let data = await response.json();

}

20. What is a promise in JavaScript?

A promise represents the eventual completion (or failure) of an asynchronous operation. States: pending, fulfilled, or rejected.

21. What is the event loop in JavaScript?

The event loop allows JavaScript to perform non-blocking operations by offloading tasks and executing callbacks from the message queue.

22. What are map, filter, and reduce methods?

map: Creates a new array by applying a function to each element.
filter: Creates a new array with elements that pass a test.
reduce: Reduces the array to a single value.

23. What is the difference between synchronous and asynchronous code?

Synchronous code runs sequentially, blocking further execution. Asynchronous code allows other operations to run while waiting for a task to finish.

24. How do you prevent default action of an event?

Use event.preventDefault().

25. What are JavaScript modules?

Reusable code pieces imported/exported between files using import and export.

26. What is JSON and how do you parse it?

JSON (JavaScript Object Notation) is a lightweight data format. Use JSON.parse() and JSON.stringify().

27. What is the difference between `forEach()` and `map()`?

forEach() executes a function for each array element but returns undefined, while map() returns a new array.

28. How do you deep clone an object in JavaScript?

Use JSON.parse(JSON.stringify(obj)) for simple objects, or structuredClone for complex objects.

29. What is a generator function?

A generator function returns an iterator and can pause/resume execution using yield.js

function* gen() {

  yield 1;

  yield 2;

}

30. What is a WeakMap and WeakSet?

Collections allowing garbage collection of keys (objects) when there are no other references.

31. What is strict mode in JavaScript?

'use strict'; enforces stricter parsing and error handling.

32. What is currying in JavaScript?

Transforms a function with multiple arguments into a sequence of functions, each taking a single argument.js

function add(a) {

  return function(b) {

    return a + b;

  }

}

33. How does JavaScript handle memory management?

Automatic garbage collection reclaims memory from objects no longer in use.

34. How do you detect the operating system on the client machine?

Use navigator.platform.

35. What is the use of the `delete` operator?

Removes a property from an object.

36. What are pop-up boxes available in JavaScript?

alert()
confirm()
prompt()

37. How do you create a function in JavaScript?js

function greet(name) {

  return "Hello, " + name;

}

38. Can you assign an anonymous function to a variable?

Yes:js

let sum = function(a, b) { return a + b; };

39. What is the difference between shallow and deep copy?

Shallow copy copies property references; deep copy duplicates all nested objects.

40. What is the output of `3 + 2 + "7"`?

The result is "57" (3 + 2 is 5, then "5" + "7" is string concatenation).

41. What is a runtime environment in JavaScript?

Where JavaScript code executes, providing access to objects, libraries, and APIs (e.g., browsers, Node.js).

42. What is the role of the `debugger` keyword?

Pauses code execution for inspection in developer tools.

43. How do you check if a variable is undefined?

Use typeof variable === "undefined".

44. What is the purpose of `addEventListener()`?

Attaches an event handler to an element without overwriting existing handlers.

45. What is event delegation?

Uses event bubbling to handle events at a parent element, improving performance.

46. What is the spread operator?

Expands arrays or objects into individual elements.js

let arr = [1, 2, 3];

let newArr = [...arr, 4, 5];

47. What is the difference between `call`, `apply`, and `bind`?

call: Invokes a function with a given this and arguments.
apply: Arguments passed as an array.
bind: Returns a new function with bound this.

48. How do you handle exceptions in JavaScript?

Use try, catch, finally, and throw.

49. What is the use of `setTimeout` and `setInterval`?

setTimeout: Executes a function after a delay.
setInterval: Executes a function repeatedly at intervals.

50. How do you optimize JavaScript code for performance?

Minimize DOM access
Use local variables
Debounce/throttle events
Efficient algorithms
Minify and bundle code

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FAQs

Q1. What are the best resources to prepare for JavaScript interviews?

A: Use w3schools , Interview Bit for practice and coding challenges.

Q2. What should I focus on for JavaScript interviews as a fresher?

A: Master basics (variables, loops, functions), DOM manipulation, event handling, ES6 features, and common coding patterns.