L1- Array Methods

Question 1

What does forEach return?

Answer 1

Undefined

Question 2

Does .filter mutate?

Answer 2

no

Question 3

Does .filter use a callback method?

Answer 3

yes. It examinds the return from each iteration for truthiness.

Question 4

Does .map use a callback method?

Answer 4

Yes, whatever is return is push to a new array.

Question 5

Can the array returned from .map be shorter than the caller?

Answer 5

no, it will be the same length even if elements have to be undefined.
EVEN IF the callback function changes the length of the original. But then it still creates empty elements and counts toward the length

Question 6

Return true if at least 1 array element returns true from a callback function

SIMPLE LANGUAGE:
Do some of the array element follow this pattern?

Answer 6

myArray.some( )

Question 7

What’s the difference between .some() and .includes()?

Answer 7

.includes() is just a simple comparison of the argument and array values.
.some() lets you use a callback function to test whatever logic you want.

Question 8

What is the difference between each of these (what do they do and return?)

**WHat do they return if they don't find the thing they're looking for?

Array.prototype.includes()
Array.prototype.some( )
Array.prototype.every()
Array.prototype.filter() **(no elements return truthy value)
Array.prototype.find() ** 
Array.prototype.indexOf() **
Array.prototype.findIndex() **

Answer 8

.includes() - returns boolean based on whether the argument is within the array

.some( ) - iterates over each element and returns boolean if at least 1 iteration returns true based on a callback function
.every() - iterates over each element and returns boolean if all iterations return true based on a callback function

.filter() - iterates over each element and returns an array based on whether the elements evaluate to true based on a callback function
**empty array

.find() - returns the value of the first element that returns true from a callback function
**Undefined

.indexOf() returns the first index of an element that matches the argument
**-1
Array.prototype.findIndex() returns the index of the first element that returns true from a callback function
**-1

Question 9

What does
Array.prototype.some()
do?
and/or return?

Answer 9

“Are there some values in the array for which the given callback returns a truthy value?

tests whether at least one element in the array passes the test implemented by the provided function. It returns true if, in the array, it finds an element for which the provided function returns true; otherwise it returns false. It doesn’t modify the array.

// Arrow function
some((element) => { /* ... */ } )
some((element, index) => { /* ... */ } )
some((element, index, array) => { /* ... */ } )

// Callback function
some(callbackFn)
some(callbackFn, thisArg)

// Inline callback function
some(function(element) { /* ... */ })
some(function(element, index) { /* ... */ })
some(function(element, index, array){ /* ... */ })
some(function(element, index, array) { /* ... */ }, thisArg)

Question 10

What does
Array.prototype.every()
do?
and/or return?

Answer 10

The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.

// Arrow function
every((element) => { /* ... */ } )
every((element, index) => { /* ... */ } )
every((element, index, array) => { /* ... */ } )

// Callback function
every(callbackFn)
every(callbackFn, thisArg)

// Inline callback function
every(function(element) { /* ... */ })
every(function(element, index) { /* ... */ })
every(function(element, index, array){ /* ... */ })
every(function(element, index, array) { /* ... */ }, thisArg)

Question 11

Return true if all array elements returns true from a callback function

SIMPLE LANGUAGE:
Does every array element follow this pattern?

Answer 11

myArray.every()

Question 12

Returns the first element that returns true from a callback function.

SIMPLE LANGUAGE:
What’s the first element in the array that follows this pattern?

Answer 12

myArray.find( callback fuction)

Question 13

What does
Array.prototype.find()
do? and/or return?

Answer 13

The find() method returns the value of the first element in the provided array that satisfies the provided testing function. If no values satisfy the testing function, undefined is returned.

// Arrow function
find((element) => { /* ... */ } )
find((element, index) => { /* ... */ } )
find((element, index, array) => { /* ... */ } )

// Callback function
find(callbackFn)
find(callbackFn, thisArg)

// Inline callback function
find(function(element) { /* ... */ })
find(function(element, index) { /* ... */ })
find(function(element, index, array){ /* ... */ })
find(function(element, index, array) { /* ... */ }, thisArg)

Question 14

Returns the index of the first element in the array that satisfies the provided testing/callback function. Otherwise, it returns -1, indicating that no element passed the test.

Answer 14

Array.prototype.findIndex()

Question 15

What does:
Array.prototype.findIndex()
do and/or return?

Answer 15

Returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test.

Question 16

Reverse an array’s elements in place (mutating).

It also returns the reversed array

Answer 16

Array.prototype.reverse( )

Question 17

What does
Array.prototype.reverse()
do?
and/or return?

Answer 17

reverses an array in place. The first array element becomes the last, and the last array element becomes the first.
Mutates

Returns the reversed array

reverse()

Question 18

Reverese an array without mutating it

Answer 18

.slice().reverse()

Question 19

Return boolean if a string contains a specified substring
or
if an array element matches the entirety of the argument

SIMPLE LANGUAGE:
Does the array contain this entry?
Does the string contain this substring?

Answer 19

myStringOrArray.includes(‘value’)

Question 20

What’s the difference between .some() and .includes()?

Answer 20

.includes() is just a simple comparison of the argument and array values.
.some() lets you use a callback function to test whatever logic you want.

Question 21

What does
Array.prototype.includes()
do?
and/or return?

Answer 21

Determines whether an array includes a certain value among its entries, returning true or false as appropriate.

includes(searchElement)
includes(searchElement, fromIndex)

(There is also a String.prototype.includes())

Question 22

can .includes find NaN?

Answer 22

yes. it does not operate stricly on ===
But otherwise it’s the same.

Question 23

Replace an array’s elements between certain indexes with a SINGLE entry
Returns the changed array

SIMPLE LANGUAGE
Replace each consecutive elements with a 1 argument.

Answer 23

myArray.fill(replacement, start ind, end exclusive index)
mutates
[1, 2, 3, 4, 5, 6, 7, 8].fill(“apple”, 4, 6)
Expected output:
[ 1, 2, 3, 4, ‘apple’, ‘apple’, 7, 8 ]

Question 24

What does
Array.prototype.fill()
do?
and/or return?

Answer 24

Mutates and returns reference to array.

The fill() method changes all elements in an array to a static value, from a start index (default 0) to an end index (default array.length). It returns the modified array.

Mutates
Returns reference to original array

fill(value)
fill(value, start)
fill(value, start, end)

Question 25

What will this do: arr.fill(1, 1, 3);

Answer 25

replaces elements with first arg
Starts at second arg
Ends just before third arg

Question 26

What is returne from .map?
[‘ant’, ‘bear’].map(elem => {
if (elem.length > 3) {
return elem;
}
});

Answer 26

[ undefined, ‘bear’ ]

Question 27

What is returns from this?

[‘ant’, ‘bear’, ‘caterpillar’].pop().length;

Answer 27

11

Question 28

Write code to output something like this:
flintstonesObj; // { Fred: 0, Barney: 1, Wilma: 2, Betty: 3, Pebbles: 4, Bambam: 5 }

From
let flintstones = [“Fred”, “Barney”, “Wilma”, “Betty”, “Pebbles”, “Bambam”];

(you shouldn’t need Object.entries)

Answer 28

let flintstonesObj = {};

flintstones.forEach((name, index) => {
flintstonesObj[name] = index;
});

flintstonesObj; // { Fred: 0, Barney: 1, Wilma: 2, Betty: 3, Pebbles: 4, Bambam: 5 }

Question 29

Write code to get the sum of ages.
let ages = {
Herman: 32,
Lily: 30,
Grandpa: 5843,
Eddie: 10,
Marilyn: 22,
Spot: 237
};

Answer 29

let totalAges = 0;
Object.values(ages).forEach(age => totalAges += age);
totalAges; // => 6174

Question 30

Pick out the minimum age from our current Munster family object: (2 ways)

let ages = {
Herman: 32,
Lily: 30,
Grandpa: 5843,
Eddie: 10,
Marilyn: 22,
Spot: 237
};

Answer 30

let smallest = Object.values(ages).sort((a, b) => a - b)[0]
or
let agesArr = Object.values(ages);
Math.min(…agesArr); // => 10

Question 31

How do you input all of an arrays elements as arguments into a function?

Answer 31

…[array]

Question 32

Create an object that expresses the frequency with which each letter occurs in this string: (2 ways)
‘The apple orchard blah blah blah’
ex:
{ T: 1, h: 1, e: 2, F: 1, l: 1, … }

Answer 32

charsInStatement.forEach(char => {
result[char] = result[char] || 0;
result[char] += 1;
});

Question 33

Answer 33

D

Question 34

Answer 34

B

Question 35

What does .sort() return/mutate?

Answer 35

It mutates
Sorted alphabetically (return b > a)

Question 36

What does this return: console.log([‘asdf’].includes(‘asd’))

Answer 36

false

Question 37

What’s the difference between .indexOf() and .findIndex()

Answer 37

indexof is just looking for strict equality
find index is looking for the first element that returns true from a call back function

Question 38

Remove element from beginning, return that element

Answer 38

.shift

Question 39

Create new array, with the caller and argument(s) together in one array

Answer 39

.concat

Question 40

does .fill mutaet?

Answer 40

yes

Question 41

Does sort or reverse mutate?

Answer 41

both mutate

Question 43

What happens when you call .reduce on an empty array and an initial accumulator value of 0?

On an empty array with with no initial accumulator value

On an array with 1 element and no initial accumulator value

On an array with 2 or more elements and no initial accumulator value

Answer 43

returns 0

Type error

Whatever the value of that one element is
Because there is no previous iteration and no initial accumulator value, .reduce uses the first element as the return from the first iteration for some reason.

Same. Can have unexpected results. Always have an initial value.