methods: filter (), map (), reduce ()

Question 1

What does Array.filter do?

Answer 1

The filter() method creates a shallow copy of a portion of a given array, filtered down to just the elements from the given array that pass the test implemented by the provided function.

it seems to loop through all elements of an array

e.g.,
const numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];

const names = [
‘Ada’,
‘Hedy’,
‘Jean’,
‘Grace’,
‘Evelyn’,
‘Joan’,
‘Elizabeth’,
‘Janese’,
‘Donna’
];

const evenNumbers = numbers.filter(number => number % 2 === 0);
console.log(evenNumbers);

const overFive = numbers.filter(number => number > 5);
console.log(overFive);

const startWithE = names.filter(name => name[0] === ‘E’);
console.log(startWithE);

const haveD = names.filter(name => name.includes(‘d’) || name.includes(‘D’));
console.log(haveD);

Question 2

What should the array.filter callback function return?

Answer 2

it should return conditionals

Question 3

What is Array.filter useful for?

Answer 3

filtering with a method instead of filtering using loops

Question 4

what does array.map( ) do?

Answer 4

The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.

e.g.

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];

const languages = [
‘Hypertext Markup Language’,
‘Cascading Style Sheets’,
‘ECMAScript’,
‘PHP Hypertext Preprocessor’,
‘Structured Query Language’
];

const doubled = numbers.map(x => x * 2);
console.log(doubled);

const prices = numbers.map(x => ‘$’ + x.toFixed(2));
console.log(prices);

const upperCased = languages.map(x => x.toUpperCase());
console.log(upperCased);

const firstLetters = languages.map(x => x[0]);
console.log(firstLetters);

Question 5

What should the array.map callback function return?

Answer 5

a full, new, copied array, but with some operations performed on each element in the array

demo from mdn:

const array1 = [1, 4, 9, 16];

// Pass a function to map
const map1 = array1.map(x => x * 2);

console.log(map1);
// Expected output: Array [2, 8, 18, 32]

Question 6

What is Array.map useful for?

Answer 6

Useful for manipulating all elements in an array at the same time, so long as they all have the same operations performed on them

Question 7

What does Array.reduce do?

Answer 7

Combining the elements of an array into a single value

Question 8

What action should the callback function perform?

Answer 8

examine element, do computation on it, update accumulated value

Question 9

What should the callback function return?

Answer 9

returns accumulated value

Question 10

What is Array.reduce useful for?

Question 11

What is a directory?

Answer 11

a folder!

Question 12

What is a relative file path?

Answer 12

one on your system, which you can change, and it starts from the current working directory
-it never starts with a slash (but can be ‘./’ )

Question 13

What is an absolute file path?

Answer 13

online, you cannot change, and it starts from
-it always starts with a slash

Question 14

What module does Node.js include for manipulating the file system?

Question 15

What method is available in the node:fs module for reading data from a file?

Question 16

what is current directory syntax?

Answer 16

it is “.”

. means current direcltory

. /x means “look in current directory for file x