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.

❤️ Double Tap For Part 5

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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()

Quick Memory Trick:

push/pop = END, unshift/shift = START

❤️ Double Tap For Part 6

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🚀 JavaScript Interview Questions with Answers — Part 6

51. How do you sort arrays?

The sort() method is used to sort the elements of an array.

Sorting Strings

const fruits = ["Banana", "Apple", "Mango"];
fruits.sort();
console.log(fruits);

Output:

["Apple", "Banana", "Mango"]

Sorting Numbers

By default, sort() converts elements to strings, so a comparison function should be used for numbers.

const numbers = [10, 5, 20, 2];
numbers.sort((a, b) => a - b);
console.log(numbers);

Output:

[2, 5, 10, 20]

Descending Order:

numbers.sort((a, b) => b - a);

Interview Tip:

sort() mutates the original array.

52. What is array destructuring?

Array destructuring allows you to extract values from an array and assign them to variables.

Example:

const numbers = [10, 20, 30];
const [a, b, c] = numbers;
console.log(a);
console.log(b);
console.log(c);

Output:

10

20

30

Skipping Values:

const numbers = [10, 20, 30];
const [first, , third] = numbers;
console.log(first, third);

Output:

10 30

Default Values:

const numbers = [10];
const [a, b = 20] = numbers;
console.log(a, b);

Output:

10 20

53. What are Sets?

A Set is a collection of unique values.

Duplicate values are automatically removed.

Example:

const numbers = new Set([1, 2, 2, 3, 3]);
console.log(numbers);

The Set contains:

{1, 2, 3}

Common Methods:

const numbers = new Set();
numbers.add(10);
numbers.add(20);
console.log(numbers.has(10));
numbers.delete(20);

Convert Set to Array:

const arr = [...numbers];

54. What are Maps?

A Map is a collection of key-value pairs.

Unlike regular objects, a Map can use different data types as keys.

Example:

const users = new Map();
users.set(1, "Deepak");
users.set(2, "John");
console.log(users.get(1));

Output:

Deepak

Common Methods:

users.set(key, value);
users.get(key);
users.has(key);
users.delete(key);
users.clear();

Example:

console.log(users.has(2));

Output:

true

Map vs Object:

Map

• Any value can be a key

• Has size property

• Built-in methods

• Designed for key-value collections

Object

• Keys are primarily strings/symbols

• No built-in size

• Different object APIs

• General-purpose objects

55. What are Symbols?

Symbol is a primitive data type used to create unique identifiers.

Example:

const id1 = Symbol("id");
const id2 = Symbol("id");
console.log(id1 === id2);

Output:

false

Even though both have the same description, each Symbol is unique.

Using Symbol as an Object Property:

const id = Symbol("id");
const user = {
    name: "Deepak",
    [id]: 101
};
console.log(user[id]);

Common Use:

Symbols are useful when you need unique property keys that are unlikely to conflict with other properties.

56. What are generators?

Generators are special functions that can pause and resume execution.

They are created using function* and use the yield keyword.

Example: