Arrays

Question 1

Mutable

Answer 1

JS arrays are mutable - meaning the values they contain can be changed

Even if declared using const, values can be manipulated with .push() and .pop()

Question 2

.length

Answer 2

Indicates the number of elements the array contains

Question 3

Index

Answer 3

Array elements are indexed, starting with 0
Elements can be accessed by their index like this:

let array = [1, 2, 3]
array[0]; //1
array[1]; //2
array[3]; //3

Question 4

.push() method

Answer 4

Appends one or more elements to the end of an array and returns the new length

const list = [1, 2, 3, 4, 5];
list.push(6); // 6
list; // [1, 2, 3, 4, 5, 6]

Question 5

.pop() method

Answer 5

Removes the last element from an array and returns that element

const list = [1, 2, 3, 4, 5];
list.pop(); // 5
list; // [1, 2, 3, 4]

Question 6

.map()

Answer 6

Returns a new array with the results of calling a provided function on every element in the given array

const list = [1, 2, 3, 4];
list.map((el) => el * 2); // [2, 4, 6, 8]

Question 7

.filter()

Answer 7

Returns a new array with all elements that pass the test implemented by the provided function

const list = [1, 2, 3, 4];
list.filter((el) => el % 2 === 0); // [2, 4]

Question 8

.reduce()

Answer 8

Reduce the array to a single value. The value returned is stored in an accumulator (result/total)

const list = [1, 2, 3, 4, 5];
list.reduce((total, item) => total + item, 0); // 15

Question 9

.fill()

Answer 9

Fill the elements in an array with a static value

const list = [1, 2, 3, 4, 5];
list.fill(0); // [0, 0, 0, 0, 0]

Question 10

.find()

Answer 10

Returns the value of the first element in the array that satisfies the provided testing function, otherwise returns undefined

const list = [1, 2, 3, 4, 5];
list.find((el) => el === 3); // 3
list.find((el) => el === 6); // undefined

Question 11

.indexOf()

Answer 11

Returns the first index at which a given element can be found in the array, else returns -1

const list = [1, 2, 3, 4, 5];
list.indexOf(3); // 2
list.indexOf(6); // -1

Question 12

.includes()

Answer 12

Returns true if the given element is present in the array

const list = [1, 2, 3, 4, 5];
list.includes(3); // true
list.includes(6); // false

Question 13

.shift()

Answer 13

Removes the first element of the array and returns that element

const list = [1, 2, 3, 4, 5];
list.shift(); // 1
list; // [2, 3, 4, 5]