4.Arrays.md

100 JS Questions (Arrays)

Q1. What are Arrays in JS? How to get, add & remove elements from arrays?

An array is a data type that allows you to store multiple values in a single variable.

Get	Add	Remove	Modify	Others
indexOf()	push()	pop()	map()	join()
find()	concat()	shift()	forEach()	length
filter()		splice()	sort()	reduce()
slice()			reverse()	some()
				every()

Q2. What is the indexOf() method of an Array?

IndexOf() method gets the index of a specified element in the array.

//Example array
const array = [1,2,3,4,5];

let a = array.indexOf(3);
console.log(a);
//Output: 2

Q3. What is the difference between find() and filter() methods of an Array? V.IMP.

find()

find() method get the first element that satisfies a condition.

// Example array
const array = [1, 2, 3, 4, 5];

// Using find() to find the first even number
let c = array.find(num => num % 2 === 0);
console.log(c); // Output: 2

filter()

filter() method get an array of elements that satisfies a condition.

const array = [1, 2, 3, 4, 5];

// Using filter() to find all even numbers
let d = array.filter(num => num % 2 === 0);
console.log(d); // Output: [2, 4]

Q4. What is the slice() method of an Array?

slice()

slice() method get a subset of the array from start index to end index end not included.

const array = ["a", "b", "c", "d","e"];

let e = array.slice(1,4)
console.log(e)

//Output: ['b','c','d']

Q5. What is the difference between push() and concat() methods of an Array?

push()

push() will modify the original array itself.

let array1 = [1, 2];

// Using push()
array1.push(3, 4);
console.log(array1); 
// Output: [1, 2, 3, 4]

concat()

concat() method will create the neww array and not modify the original array.

let array2 = [5, 6];

// Using concat()
let array3 = array2.concat(7, 8);
console.log(array3); // Output: [5, 6, 7, 8]

console.log(array2); 
//original array is not modified
// Output: [5, 6]

Q6. What is the difference between pop() and shift() methods of an Array?

pop()

pop() will remove the last element of the array

// Using pop()
let arr1 = [1, 2, 3, 4];
let popped = arr1.pop();
console.log(popped); // Output: 4
console.log(arr1); // Output: [1, 2, 3]

shift()

shift() will remove the first element of the array

// Using shift()
let arr2 = [1, 2, 3, 4];
let shifted = arr2.shift();
console.log(shifted); // Output: 1
console.log(arr2); // Output: [2, 3, 4]

Q7. What is the splice() method of an Array? V.IMP.

The splice() method is used to add, remove or replace elements in an array.

array.splice(startIndex, deleteCount, ...itemsToAdd);

let letters = ['a', 'b', 'c'];

// Add 'x' and 'y' at index 1
letters.splice(1, 0, 'x', 'y');
console.log(letters);
// Output: ['a', 'x', 'y', 'b', 'c']

// Removes 1 element starting from index 1
letters.splice(1, 1);
console.log(letters);
// Output: ['a', 'y', 'b', 'c']

// Replaces the element at index 2 with 'q'
letters.splice(2, 1, 'q');
console.log(letters);
// Output: ['a', 'y', 'q', 'c']

Q8. What is the difference between the slice() and splice() methods?

The slice() method is used to get a subset of the array from the start index to the end index (end not included).

The splice() method is used to add, remove, or replace elements in an array.

Q9. What is the difference map() and forEach() array methods? V.IMP.

map()

The map() method is used when you want to modify each element of an array and create a new array with the modified values.

// Using map()
let arr1 = [1, 2, 3];
let mapArray = arr1.map((e) => e * 2);
console.log(mapArray); // Output: [2, 4, 6]

forEach()

The forEach() method is used when you want to perform some operation on each element of an array without creating a new array.

// Using forEach()
let arr2 = [1, 2, 3];
arr2.forEach((e) => {
  console.log(e * 2);
});
// Does not return anything
// Output: 2 4 6

console.log(arr2); // Output: [1, 2, 3]

Q10. How to sort and reverse an array?

Array can be sorted or reversed by using sort() and reverse() methos of array.

let array = ['c', 'e', 'a', 't'];

// Sort the array
array.sort();
console.log(array);
// Output: ['a', 'c', 'e', 't']

// Reverse the array
array.reverse();
console.log(array);
// Output: ['t', 'e', 'c', 'a']

Q11. What is Array Destructuring in JS? V.IMP.

Array destructuring allows you to extract elements from an array and assign them to individual variables in a single statement.

Array destructuring is introduced in ECMAScript 6 (ES6).

// Example array
const fruits = ['apple', 'banana', 'orange'];

// Array destructuring
const [firstFruit, secondFruit, thirdFruit] = fruits;

// Output
console.log(firstFruit); // Output: "apple"
console.log(secondFruit); // Output: "banana"
console.log(thirdFruit); // Output: "orange"

Q12. What are array-like objects In JS? V.IMP.

Array-like objects are objects that have indexed elements and a length property, similar to arrays but they may not have all the methods of arrays like push(), pop() & others.

Types of array like object

• arguments
• Strings
• HTML collections

sum(1, 2, 3);

// Arguments Object
function sum() {
  console.log(arguments); // Output: [1, 2, 3]
  console.log(arguments.length); // Output: 3
  console.log(arguments[0]); // Output: 1
}

// String
const str = "Hello";
console.log(str); // Output: Hello
console.log(str.length); // Output: 5
console.log(str[0]); // Output: H

// Accessing HTML collection
var boxes = document.getElementsByClassName('box');
// Accessing elements in HTML collection using index
console.log(boxes[0]);
// Accessing length property of HTML collection
console.log(boxes.length);

Q13. How to convert an array-like object into an array?

Methods to convert an array-like object into an array:

• Array.from()
• Spread Syntax(...)
• Array.prototype.slice.call()

// Example array-like object
var arrayLike = {0: 'a', 1: 'b', 2: 'c', length: 3};

// Using Array.from()
var array1 = Array.from(arrayLike);
console.log(array1);
// Output: ['a', 'b', 'c']

// Using spread syntax (...)
var array2 = [...arrayLike];
console.log(array2);
// Output: ['a', 'b', 'c']

// Using Array.prototype.slice.call()
var array3 = Array.prototype.slice.call(arrayLike);
console.log(array3);
// Output: ['a', 'b', 'c']