🚀 JavaScript Interview Questions with Answers — Part 2

11. What is the typeof operator?

The typeof operator is used to determine the data type of a value.

Example:

console.log(typeof "Hello");   // "string"
console.log(typeof 100);       // "number"
console.log(typeof true);      // "boolean"
console.log(typeof undefined); // "undefined"

Important Interview Point:

console.log(typeof null);

Output: "object"

This is a historical behavior in JavaScript.

12. What are template literals?

Template literals are strings created using backticks ``.

They allow you to easily embed variables and expressions using ${}.

Example:

const name = "Ajay";
const age = 25;

console.log(`My name is ${name} and I am ${age} years old.`);

Output: My name is Ajay and I am 25 years old.

Benefits:

Easy string interpolation

Supports multi-line strings

Allows expressions inside strings

13. What are JavaScript operators?

Operators are symbols used to perform operations on values.

Common Types:

Arithmetic: + - * / % **

Comparison: == === != !== > < >= <=

Logical: && || !

Assignment: = += -= *= /=

Example:

const a = 10;
const b = 5;
console.log(a + b);  // 15
console.log(a > b);  // true

14. What is the ternary operator?

The ternary operator is a short way of writing an if...else statement.

Syntax:

condition ? valueIfTrue : valueIfFalse;

Example:

const age = 20;
const result = age >= 18 ? "Adult" : "Minor";
console.log(result);

Output: Adult

Equivalent if...else:

if (age >= 18) {
    result = "Adult";
} else {
    result = "Minor";
}

Interview Tip: Use ternary operators for simple conditions. Avoid deeply nested ternaries because they reduce readability.

15. What is variable hoisting?

Hoisting is JavaScript's behavior of processing declarations before executing the code in their scope.

With var:

console.log(x);
var x = 10;

Output: undefined

The declaration is hoisted, but the assignment happens later.

With let and const:

console.log(x);
let x = 10;

This results in a ReferenceError.

let and const are hoisted but remain in the Temporal Dead Zone (TDZ) until their declaration is reached.

Function declarations are also hoisted:

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

16. What is scope in JavaScript?

Scope determines where a variable can be accessed in a program.

Example:

function test() {
    let message = "Hello";
    console.log(message);
}
test();

message is accessible inside the function but not outside it.

Main Types:

Global Scope

Function Scope

Block Scope

Module Scope

Understanding scope is essential for closures and avoiding variable conflicts.

17. What are global, function, and block scope?

Global Scope

A variable declared outside functions or blocks can generally be accessed throughout the script.

let name = "Deepak";
function greet() {
    console.log(name);
}

Function Scope

Variables declared with var inside a function are accessible throughout that function.

function test() {
    var age = 25;
    console.log(age);
}

age

cannot be accessed outside

test()

.

Block Scope

let

and

const

are block-scoped.

if (true) {
    let x = 10;
    const y = 20;
    console.log(x, y);
}

x

and

y

cannot be accessed outside the

if

block.

18. What is strict mode ("use strict")?

Strict mode enables a stricter set of JavaScript rules and helps catch certain programming mistakes.

Example:

"use strict";
x = 10;

This produces a

ReferenceError

because

x

was not declared. 

Without strict mode, older JavaScript behavior could create a global variable in some situations.

Benefits:

Catches common mistakes

Prevents accidental global variables

Makes some unsafe operations throw errors

Helps write cleaner code

19. What are comments in JavaScript?

Comments are text ignored by the JavaScript engine. They are used to explain code or temporarily disable code.

Single-Line Comment:

// This is a comment
console.log("Hello");

Multi-Line Comment:

/*
   This is a
   multi-line comment
*/
console.log("Hello");

Why Use Comments?

✅ Explain complex logic

✅ Improve code readability

✅ Help other developers understand the code

✅ Document important decisions

20. What are JavaScript modules?

Modules allow you to split JavaScript code into separate, reusable files.

They help organize large applications and prevent unnecessary global variables.

Export:

// math.js
export function add(a, b) {
    return a + b;
}

Import:

// app.js
import { add } from "./math.js";
console.log(add(10, 20));

Types of Exports:

Named exports

Default exports

Default Export:

export default function greet() {
    console.log("Hello");
}

Important Interview Point:

ES Modules use

import

and

export

. They are the standard module system for modern JavaScript.

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🚀 JavaScript Interview Questions with Answers — Part 4

31. How do you create objects in JavaScript?

There are several ways to create objects in JavaScript.

1. Object Literal

const user = {
    name: "John",
    age: 25
};

2. new Object()

const user = new Object();
user.name = "Frey";
user.age = 35;

3. Constructor Function

function User(name, age) {
    this.name = name;
    this.age = age;
}
const user = new User("John", 25);

4. Class

class User {
    constructor(name, age) {
        this.name = name;
        this.age = age;
    }
}
const user = new User("John", 25);

Interview Tip:

Object literals are usually preferred for simple objects, while classes or constructor functions are useful when creating many similar objects.

32. What is object destructuring?

Object destructuring allows you to extract properties from an object and store them in variables.

Example:

const user = {
    name: "John",
    age: 25,
    city: "New York"
};
const { name, age } = user;
console.log(name);
console.log(age);

Output:

John

25

Rename Variables:

const { name: userName } = user;
console.log(userName);

Default Value:

const { country = "India" } = user;
console.log(country);

33. What is the spread operator (...)?

The spread operator expands the elements of an iterable or properties of an object.

Array Example:

const numbers = [1, 2, 3];
const newNumbers = [...numbers, 4, 5];
console.log(newNumbers);

Output:

[1, 2, 3, 4, 5]

Object Example:

const user = {
    name: "John",
    age: 25
};

const updatedUser = {
    ...user,
    city: "New York"
};

Common Uses:

✅ Copy arrays

✅ Merge arrays

✅ Copy objects

✅ Merge objects

✅ Pass values to functions

34. What is the rest operator?

The rest operator (...) collects multiple values into a single array or object.

Example:

function sum(...numbers) {
    return numbers.reduce((total, num) => total + num, 0);
}
console.log(sum(10, 20, 30));

Output: 60

Here, ...numbers collects all arguments into an array.

Important Interview Point:

The same ... syntax has different purposes:

Spread → expands values: const arr2 = [...arr1];

Rest → collects values: function test(...args) {}

35. What are default parameters?

Default parameters allow you to provide a default value when an argument is not passed or is undefined.

Example:

function greet(name = "Guest") {
    console.log(`Hello ${name}`);
}
greet();

Output: Hello Guest

If a value is provided: greet("Deepika"); → Hello Deepika

Multiple Defaults:

function createUser(name = "Guest", age = 18) {
    console.log(name, age);
}

36. What is optional chaining (?. )

Optional chaining allows you to safely access nested properties without throwing an error when an intermediate value is null or undefined.

Without Optional Chaining:

const user = {};
console.log(user.address.city); // Error

With Optional Chaining:

console.log(user.address?.city); // undefined

Example with Function: user.getName?.();

The function is called only if getName exists and is callable.

Common Use: Very useful when working with API responses where some properties may be missing.

37. What is nullish coalescing (??)

The nullish coalescing operator returns the right-hand value when the left-hand value is null or undefined.

Example:

const username = null;
console.log(username ?? "Guest");

Output: Guest

Important Difference From ||

|| considers all falsy values: console.log(0 || 100); → 100

?? only checks null and undefined: console.log(0 ?? 100); → 0

Interview Tip: Use ?? when 0, false, or "" are valid values that should not be replaced.

38. What are object methods?

An object method is a function stored as a property of an object.

Example:

const user = {
    name: "Deepak",
    greet() {
        console.log(`Hello ${this.name}`);
    }
};
user.greet();

Output: Hello Deepak

Another Example:

const calculator = {
    add(a, b) { return a + b; },
    multiply(a, b) { return a * b; }
};
console.log(calculator.add(10, 20));

39. What is method chaining?

Method chaining means calling multiple methods one after another on the same object or result.

Example:

const result = "javascript"
   .toUpperCase()
   .split("")
   .reverse()
   .join("");
console.log(result);

Output: TPIRCSAVAJ

Array Example:

const result = [1, 2, 3, 4, 5]
   .filter(num => num % 2 === 0)
   .map(num => num * 10);
console.log(result);

Output: [20,40]

Common Uses: Array processing, String manipulation, Promise chains, Libraries such as jQuery

40. What is object freezing and sealing?

JavaScript provides Object.freeze() and Object.seal() to restrict modifications to objects.

Object.freeze()

Prevents: Adding properties, Removing properties, Changing existing properties

const user = { name: "Deepak", age: 25 };
Object.freeze(user);
user.age = 30;
user.city = "Pune";
console.log(user); // unchanged

Object.seal()

Prevents: Adding properties, Removing properties

But existing properties can still be modified.

const user = { name: "Deepak", age: 25 };
Object.seal(user);
user.age = 30;
console.log(user.age); // 30

Key Difference:

Object.freeze(): Cannot add, delete, or modify properties

Object.seal(): Cannot add or delete properties, but can modify existing ones

🔥 Interview Tip: Both methods are shallow — nested objects can still be modified unless they are separately frozen/sealed.

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🚀 JavaScript Interview Questions with Answers — Part 5

41. How do arrays work in JavaScript?

An array is an ordered collection of values. JS arrays can hold different data types and use zero-based indexing.

Example:

const items = ["Apple", 25, true];
console.log(items[0]); // Apple
console.log(items.length); // 3

Important Points:

• Index starts at 0

• Arrays are objects in JavaScript

• Arrays can grow or shrink dynamically

• Arrays can contain mixed data types

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

Both iterate over an array, but used differently.

map()

Creates and returns a new array.

const numbers = [1, 2, 3];
const doubled = numbers.map(num => num * 2); // [2, 4, 6]

forEach()

Executes a function for each element but does not return a new array.

numbers.forEach(num => console.log(num * 2));

Key Difference:

• map(): Returns a new array. Used for transformation. Can be chained.

• forEach(): Returns undefined. Used for side effects.

43. What is filter()?

Creates a new array with elements that pass a condition. Original array is not modified.

Example:

const numbers = [1, 2, 3, 4, 5];
const evenNumbers = numbers.filter(num => num % 2 === 0); // [2, 4]

44. What is reduce()?

Processes an array and produces a single accumulated value.

Example:

const numbers = [10, 20, 30];
const total = numbers.reduce((sum, num) => sum + num, 0); // 60

Common Uses: Calculate totals, averages, count items, group data, build objects.

Interview Tip: Understand the accumulator and current value arguments.

45. What is find()?

Returns the first element that satisfies a condition. Returns undefined if none match.

Example:

const numbers = [10, 20, 30, 40];
const result = numbers.find(num => num > 20); // 30

find() vs filter(): find = first match, filter = all matches.

46. What is findIndex()?

Returns the index of the first element that satisfies a condition. Returns -1 if none match.

Example:

const numbers = [10, 20, 30, 40];
const index = numbers.findIndex(num => num > 20); // 2

47. What is some()?

Checks if at least one element satisfies a condition. Returns Boolean.

Example:

const numbers = [1, 3, 5, 8];
const result = numbers.some(num => num % 2 === 0); // true

48. What is every()?

Checks if all elements satisfy a condition. Returns Boolean.

Example:

const numbers = [2, 4, 6, 8];
const result = numbers.every(num => num % 2 === 0); // true

some() vs every(): some = at least one, every = all.

49. What is the difference between slice() and splice()?

slice()

Returns a portion without modifying the original.

const numbers = [1, 2, 3, 4, 5];
const result = numbers.slice(1, 4); // [2, 3, 4]

splice()

Adds, removes, or replaces elements and modifies the original.

const numbers = [1, 2, 3, 4, 5];
numbers.splice(1, 2); // removes 2 elements at index 1
console.log(numbers); // [1, 4, 5]

Key Difference:

• slice(): Does not modify original. Extracts elements. Returns copied portion.

• splice(): Modifies original. Adds/removes/replaces. Returns removed elements.

50. What are push(), pop(), shift(), and unshift()?

Methods that modify arrays.

• push(): Add to END → arr.push(3)

• pop(): Remove from END → arr.pop()

• unshift(): Add to START → arr.unshift(0)

• shift(): Remove from START → arr.shift()

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push/pop = END, unshift/shift = START

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