101

Question 1

Describe the .forEach() , method

Answer 1

forEach() is an array method

It calls a provided callbackFn function once for each element in an array in ascending-index order. Unlike map(), forEach() always returns undefined

forEach((element) => { /* … / })
forEach((element, index) => { / … / })
forEach((element, index, array) => { / … */ })

Question 2

Describe the .map() , method

Answer 2

map() is an array method

that returns a new array with each element being the result of the callback function

Question 3

Explain the following code:

const kvArray = [
{ key: 1, value: 10 },
{ key: 2, value: 20 },
{ key: 3, value: 30 },
];

const reformattedArray = kvArray.map(({ key, value }) => ({ [key]: value }));

console.log(reformattedArray); // [{ 1: 10 }, { 2: 20 }, { 3: 30 }]
console.log(kvArray);
// [
// { key: 1, value: 10 },
// { key: 2, value: 20 },
// { key: 3, value: 30 }
// ]

Answer 3

the map() function is taking in destructured { key, value } as an argument

const reformattedArray = kvArray.map(({ key, value }) => ({ [key]: value }));

Question 4

What does .split() do ?

Answer 4

split() is a String method that returns an array of strings based on a separator argument

const str = ‘The quick brown fox jumps over the lazy dog.’;

const words = str.split(‘ ‘);
console.log(words[3]);
// Expected output: “fox”

const chars = str.split(‘’);
console.log(chars[8]);
// Expected output: “k”

const strCopy = str.split();
console.log(strCopy);
// Expected output: Array [“The quick brown fox jumps over the lazy dog.”]

Question 5

What is logged?

const str = ‘The quick brown fox jumps over the lazy dog.’;

const words = str.split(‘ ‘); <= space

console.log(words);

Answer 5

[“The”, “quick”, “brown”, “fox”, “jumps”, “over”, “the”, “lazy”, “dog.”]

Question 6

What is logged?

const str = ‘quickly’;

const chars = str.split(‘ ‘); <= empty string, no space

console.log(chars);

Answer 6

[“q”, “u”, “i”, “c”, “k”, “l”, “y”]

Question 7

What does join() do?

Answer 7

join() is an array method that creates and returns a new string by concatenating all of the elements in an array, separated by commas or a specified separator string.

const elements = [‘Fire’, ‘Air’, ‘Water’];

console.log(elements.join());
// Expected output: “Fire,Air,Water” <= includes commas

console.log(elements.join(‘’)); <= empty string
// Expected output: “FireAirWater” <= no commas

console.log(elements.join(‘-‘));
// Expected output: “Fire-Air-Water”

Question 8

What type of method is .join()

Answer 8

array method

Question 9

what is the sort() method ?

Answer 9

array method

The sort() method sorts the elements of an array in place and returns the reference to the same array, now sorted

Question 10

Rearrange the array into ascending order:

const array = [1, 30, 4, 21, 100000];

Answer 10

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

console.log(array); <= mutates array!
// [1, 4, 21, 30, 100000]

Question 11

Rearrange the array into descending order:

const array = [1, 30, 4, 21, 100000];

Answer 11

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

console.log(array); <= mutates array!
// [100000, 30, 21, 4, 1]

Question 12

const words = [‘spray’, ‘limit’, ‘elite’, ‘exuberant’, ‘destruction’, ‘present’];

How can you create a new list containing only words more than 6 characters long?

Answer 12

The filter() method creates a shallow copy of a portion of a given array, with elements that pass the test implemented by the provided function

const result = words.filter(word => word.length > 6);

console.log(result);
// Expected output: Array [“exuberant”, “destruction”, “present”]

Question 13

what does the Array.includes() return ?

Answer 13

Boolean => true or false

the includes() method determines whether an array includes a certain value among its entries, returning true or false as appropriate.

includes(searchElement)

Example:
const pets = [‘cat’, ‘dog’, ‘bat’];

console.log(pets.includes(‘cat’));
// Expected output: true