JavaScript Interview Questions And Answer Built-in JavaScript Methods – Part 4

JavaScript Interview Questions And Answer Built-in JavaScript Methods – Part 4. Explore essential JavaScript topics including built-in methods, data structures (Map, Set), JSON, local storage, and ES6 modules. A must-read guide for frontend and full-stack developer interviews.

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🧰 JavaScript Interview Questions And Answer Built-in JavaScript Methods

46. What are some commonly used Array methods in JavaScript?

• map(): Creates a new array by transforming elements.
• filter(): Filters elements based on a condition.
• reduce(): Reduces array to a single value.
• forEach(): Executes a function on each element.

const nums = [1, 2, 3, 4];
const doubled = nums.map(n => n * 2); // [2, 4, 6, 8]

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47. What are important String methods in JavaScript?

• charAt()
• includes()
• indexOf()
• slice()
• split()
• replace()
• toUpperCase(), toLowerCase()

const str = "JavaScript";
console.log(str.includes("Script")); // true

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48. What are Object utility methods?

• Object.keys()
• Object.values()
• Object.entries()
• Object.assign()
• Object.freeze()

const obj = { a: 1, b: 2 };
console.log(Object.entries(obj)); // [['a', 1], ['b', 2]]

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🧠 JavaScript Data Structures

49. What is a Map in JavaScript?

Answer:
Map holds key-value pairs and remembers the original insertion order of the keys. Keys can be of any type.

const map = new Map();
map.set("name", "Alice");
map.set(123, "ID");

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50. What is a Set in JavaScript?

Answer:
Set is a collection of unique values. Duplicate values are automatically removed.

const set = new Set([1, 2, 2, 3]);
console.log(set); // Set { 1, 2, 3 }

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51. Difference between Map and Object?

Feature	Map	Object
Key Types	Any (object, primitive)	Only string or symbol
Order	Ordered	Unordered
Performance	Better for frequent additions/removals	General usage

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52. What is the difference between Set and Array?

Feature	Set	Array
Duplicates	No	Yes
Order	Yes	Yes
Access	No index-based access	Index-based access

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📦 JSON and Local Storage

53. What is JSON in JavaScript?

Answer:
JSON (JavaScript Object Notation) is a format to store and exchange data.

const obj = { name: "John", age: 30 };
const jsonStr = JSON.stringify(obj);
const parsed = JSON.parse(jsonStr);

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54. What is localStorage in JavaScript?

Answer:
localStorage allows storing key-value pairs in the browser with no expiration.

localStorage.setItem("name", "Alice");
console.log(localStorage.getItem("name")); // Alice

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55. Difference between localStorage and sessionStorage?

Feature	localStorage	sessionStorage
Expiration	Never (manual removal)	When tab is closed
Scope	Entire origin	Per tab/session

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📂 JavaScript Modules

56. What are ES6 modules in JavaScript?

Answer:
ES6 modules allow you to break up your code into separate files.

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

// main.js
import { add } from './utils.js';

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57. What is the difference between export default and named export?

// Default export
export default function greet() {}

// Named export
export const sayHi = () => {};

• Default export: Only one per file.
• Named export: Multiple per file.

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58. Can you use CommonJS and ES6 modules together?

Answer:
Not directly. CommonJS (require) is used in Node.js; ES6 modules (import) in modern browsers. Tools like Babel or Webpack can transpile between them.

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🔧 Best Practices and Coding Standards

59. What are some JavaScript best practices?

• Use let and const instead of var.
• Avoid polluting global scope.
• Use arrow functions for callbacks.
• Use descriptive variable names.
• Write pure functions when possible.
• Avoid deep nesting.
• Use === instead of ==.

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60. Why should you avoid global variables in JavaScript?

Answer:
Global variables can lead to naming collisions and hard-to-track bugs, especially in larger apps.

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61. What is the use of strict mode in JavaScript?

"use strict";

function test() {
  x = 10; // ReferenceError in strict mode
}

It prevents the use of undeclared variables and other unsafe practices.

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JavaScript Interview Questions And Answer Built-in JavaScript Methods – Part 4

🧠 Real-World Coding Questions

62. Reverse a string in JavaScript.

function reverse(str) {
  return str.split("").reverse().join("");
}

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63. Check if a string is a palindrome.

function isPalindrome(str) {
  return str === str.split("").reverse().join("");
}

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64. Find the maximum number in an array.

const arr = [5, 1, 9];
console.log(Math.max(...arr));

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65. Flatten a nested array.

const nested = [1, [2, [3, [4]]]];
console.log(nested.flat(Infinity)); // [1, 2, 3, 4]

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JavaScript OOP Interview Questions Master Object-Oriented Programming, Functional Programming, DOM, and Event Handling part 3

✅ Up Next: Part 5

In the final part (Part 5), we’ll cover:

• Asynchronous JavaScript (Promises, async/await, callbacks)
• Event loop and microtask queue
• JavaScript memory management
• JavaScript security practices
• JavaScript interview tips and preparation checklist