Web Development - HTML, CSS & JavaScript

🚀 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]

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